Functional properties of sweet potato flour and its role in product development: a review

ABSTRACT

Producing food products from sweet potato flour are very feasible in worldwide due to wide availability, natural color, high-energy, low-protein, good biological activity in the human diet and low cost, as a result, become a key ingredient for the production of new products in the current global habitation. Sweet potato flours can be used for imparting desired properties, nutritional value, antioxidants, and natural color to processed foods and also used as thickeners and gelling agents. However, the paucity of information regarding the functional properties of the sweet potato flour greatly limits its exploitation. This review attempts at gathering data available on the functional properties of sweet potato flour, highlighting their unique properties and potential field of applications. A significant variation was observed in functional properties upon the variety and some processing methods. Also, the differences in the size of the granules and the various contents of starch, protein and amylose influenced functional characteristics such as viscosity, pasting temperature, gelatinization temperature, swelling power and solubility. The modified flours using steam and blanching are useful in food preparations to reduce paste thickness in food products such as confectioneries, as well as in pharmaceuticals industry as a drug binder and disintegrant as they exhibited high bulk density. Low-paste viscosity of the acetylated flour makes these flour to be used in formulations that require a high volume of solids. As thickening agents, enzymes modified that have high viscosity paste are good. The use of modified flours in food products would guarantee a suitable amount of digestible starch. This information can also be used to design protocols for the food processing industry, aiming at consumer’s needs, such as diabetics and obese people who may benefit from low digestibility of starch. The range of characters and food developments observed makes the sweet potato flour amenable to different applications in food industries based on their properties. An awareness of their potential uses can help in large-scale cultivation of these crops and the production of flour from them.

KEYWORDS:

• Sweet potato flour
• functional properties
• product development

Introduction

Root and tuber crops are global crops that yield starchy roots, corms, rhizomes, stems, and tubers in hot and humid regions. Sweet potato is a tuber of tropical and subtropical environments that can be easily grown with great adaptability, resistance, and low cost of production by both small and large scale farmers.^[1–3] It is produced for consumption and provides energy to more than 100 million people worldwide.^[4] The consumption of the sweet potato has always been allied with good health and boosted human nutrition as it contributes to calories, protein, vitamins and minerals. According to Jangchud et al.^[5] and Mais and Brennan,^[6] its normally eaten as raw, boiled, fried, steamed or further processed into various foods such as snacks, frozen and canned food. In recent years, there has been an increasing interest in sweet potato due to its eminent health assistances. The consumption of free radicals which are available in sweet potato has been associated with the prevention of cancer, aging, and degenerative diseases such as atherosclerosis .^[7]

The number of researchers has estimated that the production of staple crops in many temperate regions where today’s crops are grown can fall in the coming years due to the rise in the global climate. As a result, these crops can become limited and more costly as an upshot of the increasingly growing population. It has therefore been suggested to look for less costly sources, such as sweet potatoes, as a replacement for producing food items of comparable quality. Sweet potatoes are underutilized tubers that have the potential to bring a lot of prosperity to the producers. The production and consumption of sweet potato have risen significantly.^[8,9] Although sweet potato had a positive contribution and cheaper than other crops, up to now, far too little attention has been paid to it.

Sweet potatoes are voluminous and wasted unless cured.^[10] Moreover, production is very seasonal in most countries, resulting in considerable variation in quantity and quality, and associated with market price fluctuations. During peak harvest supply exceeds the market demand resulting in low prices and ensuing reduced return to the growers.^[4] Notwithstanding its nutritional, financial side and health status, production of sweet potatoes are embarrassed by postharvest losses, which limits the bulks of good quality yield getting users. This limits both the growers and the nation’s economic advantages. The processing and transformation of sweet potato into a staple form is suggested as an alternative to the difficulties associated with storage and transportations. It provides income for farmers and processors and in effect increases new national niches for exports.^[2,10] Furthermore, developing pleasing processed goods from sweet potatoes would play a significant role in raising awareness of the crop’s potential.^[2,11]

Flour is the main ingredient in the production of many foods and is a staple in many countries diets.^[12–14] The economic and political issue of the countries is influenced by the availability and adequate supply of flour.^[14] However, as specified earlier, sweet potato roots unused goes together with nutrient loss. For that reason, the renovation of sweet potato into flour can play a vital role in addressing the question of losses and used for the production of a variety of foodstuffs^{. [5,13]} It can be produced after the tubers are cleaned, peeled, sliced, dried and milled into a fine powder.^[15–18] Flours can be stored for a long time and used for the production of diverse foods.^[3,15] As well, it can serve as a source of minerals, energy and can add natural sweetness, color, and flavor to processed foods.^[18] In the food sector, sweet potato will produce a variety of products, including baked goods, hurts, cookies and bakery, doughnuts, breakfast foods, noodles such as styles of pasta, sauces, and other brewing beverage.^[6,18,19] Individuals who ache from celiac illness and gluten intolerance are looking for wheat-free products. Sweet potato flours partially swap cereal flours, which display benefits for people diagnosed with celiac disease.^[14]

Sweet potato flour, plays a pivotal role in the preparation of various food items, which can boost consumer nutritional and health status.^[20,21] Recently, the use of sweet potato flours as an ingredient for functional foods has garnered significant interest. However, if the flour is to be integrated into other food items, it will require high-quality sweet potato flour.^[15,21] Many factors can influence the quality of sweet potato flours and the acceptability of developed products. Variety, processing steps, processing methods such as parboiling, blanching, drying techniques, peeling, pretreatments and drying temperatures are among the factors documented.^{[5,18,19,22,23]} Yet, most studies have concentrated on the production of new foods with sweet potato flour.^[15,18] To the best of our knowledge, limited information is available regarding the functional properties and potential applications of sweet potato flours. However, the use of sweet potato flours are highly dependent on its physicochemical and functional properties that are highly influenced by the processing methods used. Thus, it is important to understand the effects of those parameters on the flour properties,^[10,22] to encourage sweet potato flours in the various formulations for the production of food products.^[21]

The functional properties reflect the complex linkage between the composition, molecular conformation of the structure and the physicochemical characteristics of food components.^[24–26] Such properties decide the production and use of food ingredients for different foods, and also regulate the processing and storage of these items.^[23,27,28] Similarly, it’s essential to assess and predict, the actions of new proteins, fat, fiber and carbohydrates in certain systems and whether they can be used for stimulation or replacement of traditional protein.^[25,28] Therefore, in the future a controlled method for producing and developing sweet potato flours based food products should be used to address the challenges and their associated solutions during food design, formulation, processing, and storage. This is the key to ensure that a stable and appealing food is produced from sweet potato flour.

Generally, the industrial application of sweet potato flour depends on its functional properties.^[22] This is due to its varied vital role in deciding the level of sweet potato flours used in the formulation and development of new food products. However, there is no comprehensive literature information regarding the functional characteristics of the flours produced from sweet potato tubers. Thus, this review focuses on the functional properties of sweet potato flour and its role in product development.

Sweet potato flour

There are currently numerous items in the baking industry such as sugar-free, fat-free, low calorie, zero cholesterol, and high in fiber and protein that health-conscious customers want.^[29] Also, a range of artificial sweeteners that are nontoxic, non-caloric, and sweeter than sucrose used in food processing instead of sugar.^[29] Sweet potato flour has become a key solution for the production of new products in the current global habitation. It has a high-energy, low-protein, and has been reported to be of good biological value.^[18,23] It has been found to contain a high source of β-carotene.^[18] Imperative content of nutrients that found in sweet potato flours plays a pivotal role in sustaining main food products developments and also can make it to be used as an ingredient in the numerous food formulations. Therefore, it can partially replace wheat flour in baked goods^[18] and can be effectively utilized if its functional properties are properly identified.

It has been claimed that the protein content of agricultural products varies with environmental location. Different researchers reported the protein content in sweet potato flours between 1.0 to 14.2%,^[18] 1.9 to 2.6%^[5] and 2.4 to 2.9%.^[3] Except for lysine, tyrosine, and isoleucine that are limited in one or more of the different cultivars, all other amino acids are available in abundance and so, the use of sweet potato flour in conjunction with other protein-rich ingredients has been suggested.^[18] The sweet potato flour contains the gross energy value of 17.50 to 18.73 MJ/kg.^[30] This shows sweet potato flour is a good source of energy. The major components of sweet potato flours are carbohydrates, making up 84.6% and 94.8%^[18] and 74.55–90.92%.^[23]

Starch is the primary biochemical energy source in the human diet. The total starch content varies between 57 to 90% ,^[18] 55.76 to 83.65%,^[23] and 36.6 to 75.0%.^[5] Sweet potato flour contains a fiber content of 3.20 to 3.46.^[30] From healthy diet point of view, foods with high fiber contents are acceptable.^[31] It has several valuable effects associated with its indigestibility in the small intestine. Sweet potato flours contain 10.06 to 21.26%, 0.46 to 29.41 μg/mg, 1.19 to 32.14 μg/mg, 4.90 to 113.18 μg/mg, and 22.39 to 125.46 μg/mg of amylose, fructose, glucose, sucrose, and total sugar, respectively.^[23] Amylose and amylopectin contents of sweet potato flour which ranged from 15.3 to 31.2% and 68.8–84.7%, respectively are reported by Aina et al.^[32] The chemical properties of sweet potato flour as affected by the variety and the processing method is given Table 1. The reducing sugars in sweet potato flour that play a vital role in the fermentation of dough in bread-making have been reported to range from 1.2 to 2.8 (Table 2).^[3]

Table 1. Chemical properties of sweet potato flour as affected by the variety and the processing method reported by different authors.

Variety	Processing method	Starch%	Amylose%	Fructose (µg/mg)	Glucose (µg/mg)	Sucrose (µg/mg)	Total sugar (µg/mg)	References
Ugandan Yellow-fleshed	soak in water	70.30	14.88	3.49	2.35	33.00	38.83	Fetuga et al.^[Citation10]
	parboil	59.86	16.79	0.28	8.47	27.92	36.67
American Orange-fleshed	soak in water	60.93	18.92	16.24	16.10	201.24	233.58
	parboil	41.25	12.23	9.11	21.69	142.05	172.85
Nigerian Yellow-fleshed	soak in water	59.74	17.63	5.71	13.86	19.31	38.87
	parboil	60.27	13.78	4.77	1.94	5.70	12.41
Local white-fleshed, local yellow-fleshed, Arrowtip, TIS 87/0087, TIS 2531 OP-1-13, TIS 8250, Shaba, CIP Tanzania, 199034.1 and Ex-Oyunga	soaked in water for 90 min, soaked in 0.5% potassium metabisulphite solution for 30 min followed by soaking in water for 45 min, blanched at 70°C for 5 min and control	55.76 to 83.65	10.06 to 21.26	0.46 to 29.41	1.19 to 32.14	4.90 to 113.18	22.39 to 125.46	Olatunde et al^[Citation23]

Table 2. Chemical composition of sweet potato flour as affected by the variety and the processing method reported by different authors.

Variety	Processing method	Moisture (%)	Proteins (%)	Ash (%)	Reducing sugars (%)	Fat (%)	Carbohydrate (%)	References
White	None	8 ± 0.06	2.9 ± 0.04	1.7 ± 0.1	1.9 ± 0.2	0.9 ± 0.2	84.6 ± 0.4	Ndangui et al.^[Citation3]
	Pretreated with CaCl2	7.8 ± 0.6	2.8 ± 0.1	3 ± 0.2	2.8 ± 0.2	0.8 ± 0.2	82.4 ± 0.1
	Blanched	7.3 ± 0.05	2.4 ± 0.1	1.2 ± 0.1	1.2 ± 0.2	0.6 ± 0.2	86.6 ± 0.3
Red	None	3.98 ± 0.08	6.56 ± 0.05	1.70 ± 0.06	NR	4.25 ± 0.14	172.85
	Parboiled	2.82 ± 0.11	5.88 ± 0.06	1.40 ± 0.03	NR	3.95 ± 0.03	38.87
White	None	3.58 ± 0.05	8.75 ± 0.06	3.10 ± 0.11	NR	6.55 ± 0.06	12.41
	Parboiled	4.28 ± 0.04	6.56 ± 0.14	3.00 ± 0.02	NR	4.10 ± 0.13		Osundahunsi et al.^[Citation33]
	Native	NR	6.670.1	1.070.06		1.070.07	73.070.6	Yadav et al.^[Citation34]
	Drum dried	NR	6.570.1	1.370.1		1.170.05	73.870.7
	Hot air dried	NR	6.370.1	1.170.07		1.170.06	73.670.7
Orange flesh	Control	7.76 ± 0.12	6.57 ± 0.32	2.70 ± 0.00	NR	25.66 ± 0.14	56.31 ± 2.11	Olubunmi et al.^[Citation11]
	Blanched	8.28 ± 0.08	3.14 ± 0.10	2.76 ± 0.30	NR	15.26 ± 0.86	67.34 ± 0.92
	Steamed	8.44 ± 0.00	3.95 ± 0.26	2.89 ± 0.19	NR	17.22 ± 0.39	64.32 ± 1.33
	Grilled	8.49 ± 0.61	3.29 ± 0.18	3.01 ± 0.01	NR	13.94 ± 1.30	68.45 ± 1.64
Orange	Unblanched	NR	1.9 ± 0.0	4.0 ± 0.0	2.6 ± 1.2	0.7 ± 0.0	NR	Jangchud et al.^[Citation5]
	Blanched	NR	2.6 ± 0.0	2.7 ± 0.1	4.3 ± 0.7	0.6 ± 0.1	NR
Purple	Unblanched	NR	1.9 ± 0.0	2.6 ± 0.1	1.5 ± 0.2	0.4 ± 0.2	NR
	Blanched	NR	2.2 ± 0.0	2.0 ± 0.1	3.8 ± 0.5	0.4 ± 0.1	NR

Abbreviation: NR, Not reported.

Sweet potato flours contains a substantial amounts of minerals such as 0.06 to 0.18% sodium, 0.76 to 1.22% potassium, 0.04 to 0.15% magnesium, 0.09 to 0.29% calcium, 0.07 to 0.19% phosphorus, 20.65 to 45.35 mg/kg iron, 18.85 to 33.75 mg/kg zinc, 3.60 to 8.50 mg/kg copper and 8.80 to 16.55 mg/kg manganese.^[23] Potassium was the major mineral present in the sweet potato flour.^[23] As such, it is a promising flour for supplementary claims in the nutraceutical and pharmaceutical industries. The average composition of sweet potato flour obtained from different treatments is given in Table 2.

Functional properties of sweet potato flour

The functional properties of sweet potato flours play an essential role in the manufacturing of food products. Such properties decide the production and use of sweet potato flours as a food ingredients for different foods, and also regulate the processing and storage of these items. For example, functional properties such as water absorption, oil absorption, and protein solubility affect the product’s texture and appearance.^[24,28]

According to Ojo et al.^[24] pasting properties are functional properties that are related to the ability of an item to act in a paste-like manner. It provides details on the production of dough and gas retention.^[14] Hence, the functional properties of flour are directly determined their end uses.^[11,15] Starch gelatinization is correlated with thermal properties.^[25] Thermal properties are a critical property used during the baking cycle to understand the phenomena.^[14] According to Kaur and Singh ,^[25] cooking time is a very significant part of the consistency of the cooking. This indicates that cooking time determines the cost of energy required.^[32] Overall, the functional properties of sweet potato flours describe the users’ desires and decide the suitability of the foodstuff for a given purpose. Processing methods such as physical and chemical modifications of sweet potato flours are sometimes necessary to overcome certain undesirable characteristics and make them suitable for specific end uses. However, processing methods are reported to affect some functional properties such as bulk density, water and oil absorption capacity, swelling, solubility and digestibility. In this review paper, the functional properties of sweet potato flour such as bulk density, swelling power, solubility, water and oil absorption capacity, pasting properties, gel consistency, sediment volume, in vitro digestibility, thermal and morphological properties are reported.

Bulk density

The bulk density indicates the relative volume of packaging material that plays an important role in the storage, transport, marketing and wet food processing industry.^[12,21,28] It also provides information on a product’s porosity that can affect package choice and design.^[15,21] The processing methods are reported to affect bulk density of the flours. Patria et al.^[35] studied the effect of steaming time and drying method on physically modified sweet potato flour and obtained an average bulk density of 0.46 g/mL to 0.48 g/mL. Their results identified steaming as an effective technique that can increases the bulk density of sweet potato flour, in turn, can add the value of the flour for different food formulation. The result has been explained based on the higher bulk density of modified flour due to the starch gelatinization process that occurs during the heating and the retrogradation process. Starch gelatinization lead to starch granules to burst, in effect amylose is out and re-binds with amylopectin at the edge of granules, making the particle size of granules larger as the gap between granules is larger.^[35] Besides, particle size and flour density are the variables that determined the bulk density.^[11,12,15]

Olubunmi et al.^[11] compared loose and packed density of sweet potato flours obtained from different processing methods. Only insignificant differences in the loose and packed density that ranged from 0.520 to 0.550 g/mL and 0.683 to 0.719 g/mL were noticed due to methods of processing (water blanching, steam blanching and grilling) of flour from orange flesh sweet potato variety. The loose bulk density is the lowest attainable density without compression.^[11] A value of 0.80 to 0.87 g/mL was reported by Ngoma et al.^[21] for chemically (sodium metabisulphite and citric acid) pretreated sweet potato flours. After chemical treatments a slight change in the bulk density of the flour was displayed, compared to the control, where the gelatinization process occurred, which increased the bulk density of the flours. Conversely, flours with high bulk density are not suitable in packaging and it is doubtful to result in cost savings as it will require more packaging materials.^[21]

High bulk density flours have been known to be good for mixing purposes, whereas those with lower bulk density can find better use in the infant food industry.^[28] On the other hand, the lower the value of bulk density, the higher the number of flour particles that can stay together^[11] that makes these flours valuable for food products required higher energy values.^[12] Also, offers better packaging advantage since more flour can be kept in a given space.^[36] The result found recommend that the steam and chemically modified flour can be used in the infant food formulations and food systems such as confectioneries, as well as in pharmaceuticals industry as a drug binder and disintegrant, whereas the unmodified flours can find use in food applications requiring high energy value.

Swelling power

Swelling power is a measure of starch hydration and used to show associative binding force within starch granules.^[12,15,37] According to Ojo et al.^[24] it is a function of starch’s capacity to swell and imbibe water. As the granules are heated above the initial gelatinization temperature, they swell as the hydrogen bonds are weakened, leading to drastic changes in the amorphous regions.^[38] It also indicated a water holding capacity of starch granules.^[12,38]

The swelling power of sweet potato flour from various varieties varies from 3.40 to 3.67 g/g were reported by Kusumayanti et al.^[39] They observed that the variety did not cause any significant difference in the swelling power. In a comparative study of twelve varieties from Ghana, Torte et al.^[4] have reported 4.6 to 5.9 g/g for swelling power of flour. The swelling power of the sweet potato flours obtained from different cultivars which had a value ranged from 5.7 to 23.5 g/g is reported by Aina et al.^[32]The size of starch granules, protein, amylose contents, and functional characteristics such as viscosity, gelatinization, and the temperature has been reported to significantly affect swelling power of flour.^[5,38]

Many researchers reported that the swelling power of sweet potato flour increased as the temperature increased.^[4,5,34] For instance, after drying the swelling power of sweet potato flour is increased, compared to the control, which increased as the temperature increased. (Figure 1).^[34] They noticed that the swelling power of drum dried flour is higher as compared to that of hot air-dried flour, which in turn is much higher than the native flour up to 70°C (Figure 1). This result is similar to that of Jangchud et al.^[5] who reported that the swelling power of sweet potato flour was increased as the temperature increased. High swelling power of flours is due to its higher amylopectin contents.^[39] The lower swelling power of native flour implies a greater degree of associative forces in the granules.^[34] Yadav et al.^[40] observed that the swelling power of acetylated and enzyme-modified flour increased with temperature. They observed that the acetylated and enzyme-modified flour had lower swelling power compared to native flours. The authors concluded that the high amylose content and the presence of stronger or a higher number of intermolecular bonds can reduce swelling.

Figure 1. Swelling power of sweet potato flour at different temperatures (-◊- native, – Δ – hot air-dried and … □ … drum dried). Reprinted from A. R. Yadav, M. Guha, R. N. Tharanathan, and R. S. Ramteke, ‘Changes in characteristics of sweet potato flour prepared by different drying techniques’, LWT, vol. 39, pp. 20–26, 2006, with kind permission of Swiss Society of Food Science and Technology, © 2004.

Jangchud et al.^[5] observed that blanching increased the swelling power of sweet potato flour up to 85°C. The influencing factor here might be the degradation of starch during blanching. These findings are in agreement with those reported by Ndangui et al.^[3] Swelling power was found to be also affected by variety and the processing method.^[10] Previous studies have reported that the botanical source affects swelling power. This effect is due to the differences in starch structure and morphology of amylose and amylopectin and the presence of salts, proteins.^[4,40] Also, the swelling power of the flours depends on the size of the particles.^[12,40] Swelling power can form the basis of the functional properties that determine their suitability in product development.^[34] It influences the characteristics of bakery product and flour which has lower swelling power cause the bakery product not swell.^[39,41] Starch damage is important in bread making applications, however, it takes more mechanical energy to disrupt the starch’s crystallized areas.^[3]

Bread making potential of flour is largely related to the gluten proteins.^[20] Gluten free products exhibits poor mouthfeel and flavor with low quality. However, tropical tubers flour do not have gluten that provides a stabilizing network for retention of gas during baking.^[39] So the preparation of gluten-free bakery products is considered a major challenge to researchers.^[29] A large number of crystallites, which improve granular stability by reducing the degree of granular swelling, decrease flour swelling potential.^[3] Lipids have been associated with restricting starch granule swelling.^[32] The flour prepared using blanching as pretreatments which have high swelling power can be partly incorporated with wheat flour for bread making.

Solubility

Solubility is proof of interaction between the amorphous and crystalline regions.^[34,39] It reflects the degree of dissolution during the starch swelling procedure.^[32] Sweet potato flour showed a wide range in the solubility and a noticeable relationship between the processing methods and the temperature was observed.

Solubility was found to increase with temperatures (Figure 2.).^[34] The values ranged from 23 g/100 g to 43.1 g/100 g up to 96°C, which indicates a high degree of variability. They concluded that the enormous difference between native and processed sweet potato flour in the solubility pattern appears to be the basis for differences in their functional properties. Besides, Yadav et al.^[40] have determined the solubility of acetylated and enzyme-modified sweet potato flour. The low value of solubility for acetylated flour is due to the substituent groups that made associative bonds stronger.^[40] They found that even though the solubility of acetylated and enzyme-modified flour increased as the temperature increased the solubility was low as compared to those of native flour. This result is similar to that of Jangchud et al.^[5] who reported that the solubility of sweet potato flour was increased as the temperature increased. The solubility of orange-fleshed sweet potato flour showed a very major difference between the processing methods (9.85 to 21.9%), the solubility of flour produced using grilling was much higher than those obtained using blanching and control flours.^[11] Similar observations were recorded for solubility of sweet potato flour up to 85°C.^[5] Hence, to increase or decrease solubility, these two processes (pretreatments and drying methods) must become combined.

Figure 2. Solubility pattern of sweet potato flour at different temperatures (-◊- native, – Δ – hot air-dried and … □ … drum dried) Reprinted from A. R. Yadav, M. Guha, R. N. Tharanathan, and R. S. Ramteke, ‘Changes in characteristics of sweet potato flour prepared by different drying techniques’, LWT, vol. 39, pp. 20–26, 2006, with kind permission of Swiss Society of Food Science and Technology, © 2004.

The varietal difference has been shown to influence the solubility of the sweet potato flours. For instance, Aina et al.^[32] found that the solubility of sweet potato flour obtained from different cultivars were 11.7 to 45.7%, in which the white-fleshed cultivars scored the highest value. Similarly, the swelling power of flour of twelve varieties of sweet potato showed the major difference between the varieties (3.4 to 9.7%), the swelling power of flour obtained from Bohye variety was much higher than other varieties.^[4] The high solubility values of sweet potato flour would be due to the high soluble sugar content and leaching of amylase from the starch.^[24,32] The low solubility value of the flour is maybe as a result of the low content of amylose.^[24]

Swelling power, inter-associative forces within the amorphous and crystalline realms, and the presence of other components are among the factors marking the flour’s solubility. The solubility of sweet potato flours from various varieties ranged from 8.61 to 9.57% with no significant difference due to variety are reported by Kusumayanti et al.^[39] This flours showed high solubility and would be useful for the production of bakery and confectionery products. The lower solubility of flour cause the bakery product to make not swell well.^[39] The enormous difference between native and processed sweet potato flour in the solubility pattern makes them usable for the preparation of various end products.^[34] Better solubility is an ideal choice for product formulations.^[19]

Water absorption capacity

Water absorption capacity describes flour water association ability under limited water supply.^[42,43] It is useful for assessing flour’s ability to take up water and swelling to increase food uniformity.^[11,21] These properties suggest the extent to which water can be added during the preparation of the dough.^[23,44] It is an important functional trait in food such as sausages custards and dough.^[24] Consequently, it is an important functional property for flours used in food preparation as it influences the functional and sensory properties of food.

Ngoma et al.^[21] reported the water absorption capacity of sweet potato flour pretreated with sodium metabisulphite and citric acid which ranged from 1.63 to 2.03 mL/g while that of the control flours was 1.44 mL/g. These result indicated that the pretreated flours had a higher water absorption capacity than control (untreated) flours and suggest that the sodium metabisulphite and citric acid treated flour would be useful in food systems such as bakery products. The influencing factor here might be the hydrophilic components such as polar or charged side chains, proteins, and carbohydrates being the main chemical components that increase the ability of water absorption.^[21,24] Water binding via flour is widely recognized as a feature of several parameters such as the size, shape, conformation properties, steric factors, physicochemical conditions, protein-associated lipids and carbohydrates, and the system’s thermodynamic properties.^[26]

The water absorption capacity ranged from 212.0 to 370.5% for native and steamed sweet potato flour was reported by Olubunmi et al.^[11] The steamed flour had the highest water absorption capacity. This indicates that this flour will retain large quantities of water during the preparation of food items such as gruels and thus become voluminous with low energy and nutrient density.^[11] Similarly, flour with high water absorption capacity may find application in the production of some baked products.^[12,41,42] It will also be useful in food systems that need water to be prepared such as meat sausage, bakery goods, dough, and processed cheese.^[21]

Fetuga et al.^[10] studied the effect of variety and processing method on functional properties of traditional sweet potato flour and reported 167 to 702.50% of water absorption capacity. They observed that the water absorption capacity of the flour was affected by both the variety and drying methods. An inverse result is reported by Sundahunsi et al.^[43] in which the variety did not show any effect on the water absorption capacity. The previous study has investigated that steaming time and drying methods significantly affects water absorption of modified sweet potato flour.^[35] The parboiling flour had the highest while that of native flour had the lowest water absorption capacity.^[10] This suggests that these flour would hold large amounts of water during preparation to food products such as gruels and thus become voluminous with low energy and nutrient density. Flours with high water absorption capacity are used for food systems to improve yield and consistency and give body to food.^[23,43] Starch is responsible for the behavior of the flours in the presence of water at high temperatures.^[45] The result obtained suggest that the native (untreated) flour would be useful in food systems such as production of thin meals which are also desirable for infant formulations.

Oil absorption capacity

Oil absorption is the capacity of absorbing oil through a dynamic mechanism of capillary attraction.^[45] It is an indication of the rate at which the protein binds to fat in food formulations^[46] and could be useful in food formulation where oil holding capacity is needed such as sausage and bakery products.^[42] Flour’s oil absorption ability is essential as it enhances mouth sensation and retains flavor.^[12] Also, fat increases the leavening power of the baking powder in the batter and improves the texture of the baked product.

There appear to be few studies on the oil absorption aspects of sweet potato flours. The oil absorption capacity of flour ranged from 9 to 12% and 94.90 to 106.7% for flour from red and white sweet potato and orange-fleshed sweet potato cultivars are reported by Sundahunsi et al.^[43] and Olubunmi et al.,^[11] respectively. These indicated that the flours have high oil absorption capacity and could find a useful application in bakery products like cake and cookies and in food formulation where oil holding capacity is needed such as sausage. It’s due to the presence of greater amounts of hydrophobic constituents in the flour.^[25] The higher oil absorption capacity of the flours suggested the presence of polar amino acids.^[12] Oil absorption capacity is attributed mainly to the physical entrapment of oils.^[11]

Pasting properties of sweet potato flour`

The efficiency of the flour in the production of baking and brewing is predicted by its pasting properties. It demonstrates the level of molecular degradation and the degree of viscosity and steadiness of the paste^[19,47] and comprises peak, hot paste, cold paste, breakdown and setback viscosities.^[48] The minimum temperature required for a cooking sample is termed as pasting temperature^[24–26] and it’s at which the first detectable increase in viscosity is measured.^[37,49]

Table 3 presents data found in the selected literature for pasting properties of sweet potato flour. As shown in the table and from selected papers, the reported pasting values vary due to the varieties and processing methods used. Pasting temperature of flour can have energy cost implications.^[23] Fetuga et al.^[10] reported a pasting temperature of 78.58°C for sweet potato flours prepared from different varieties and using different drying methods (Table 3). The pasting temperature ranged from 78.3 to 93°C are reported by Aina et al.^[32] for sweet potato flours obtained from different cultivars. The authors observed a noticeable effect of amylose content on pasting temperature. Pasting temperatures of flour produced from selected varieties were varied and white and yellow-fleshed varieties had higher pasting temperatures than the other varieties.^[22] Olatunde et al.,^[23] observed the pasting temperatures of flour from different sweet potato cultivars obtained from different pretreatments and drying methods to be 69.78 to 81.25°C. The Pasting temperature in twelve varieties of sweet potato varies from 79.23 to 84.05RVU for flour.^[4]

Table 3. Pasting properties of sweet potato flour as affected by variety and the processing method reported by different authors.

Variety	Processing method	Peak viscosity (RVU)	Trough (RVU)	Breakdown viscosity (RVU)	Final viscosity (RVU)	Setback viscosity (RVU)	Peak time (min)	Pasting temperature (°C	References
Ugandan Yellow-fleshed	Soak in water	82.69	47.75b	34.94	68.27	20.52	6.97	72.65	Fetuga et al.^[Citation10]
	Parboil	115.90	88.60 c	27.29	133.17	44.56	8.70	74.24
American Orange-fleshed	Soak in water	4.81	4.33a	0.48	6.23	1.90	4.30	na
	Parboil	167.65	119.90d	47.75	175.17d	55.27b	5.45	50.69
Nigerian Yellow-fleshed	Soak in water	161.50	89.67 c	71.21	155.56 c	68.60	8.06	77.91
	Parboil	231.67	139.96	92.25	220.38	80.13	7.60	78.58
Agriculture		90.6	40.5	50.2	62.7	22.3	3.7	78.4	Aina et al.^[Citation32]
Kizzy Red		76.0	29.9	48.3	45.5	17.7	3.7	78.3
Black Vine		26.9	5.2	21.7	70.0	1.8	3.6	90.4
CARDI		46.8	43.3	3.3	70.2	27.0	4.8	92.1
Lover’s Name		14.7	4.7	9.9	5.8	1.1	3.8	82.2
Sampson’s Lover’s Name		9.6	2.2	7.4	2.7	0.5	3.7	89.5
William’s White		54.1	35.7	18.5	53.8	18.6	3.7	93.0
Baptist White		53.4	37.2	16.3	57.8	20.6	3.6	89.5
Kizzy White		74.0	50.1	23.9	73.9	23.8	3.7	84.5
Indian Bay #1		55.6	15.4	39.1	23.6	8.1	3.6	85.4
CARDI K 84–7		39.0	12.4	26.3	19.6	7.2	3.7	90.8
Hubert Red		13.3	6.3	7.1	8.6	2.3	3.7	92.1
Big Red		13.7	2.3	11.5	2.7	0.4	3.3	84.2
CARDI Big Red		16.7	3.5	13.2	5.1	1.6	3.3	86.6
Hubert Red Devil		13.3	6.1	7.3	9.2	3.1	3.6	86.6
May Vine		25.1	15.7	9.3	25.4	9.7	3.8	90.8
CARDI RL 86		85.6	64.7	21.0	99.2	34.5	3.9	80.0
Williams Blue top		100.2	63.2	37.0	99.7	36.6	3.8	79.1
Williams Breakfall		49.4	33.2	16.2	57.3	24.1	3.8	90.8
Rasta		56.5	40.1	16.4	66.1	26.0	3.9	90.8
Indian Bay #2		37.0	29.7	7.3	56.3	26.0	3.7	89.5
Orange	Unblanched	103.9 ± 0.9	70.5 ± 0.1	33.4 ± 1.0	95.2 ± 2.4	–8.7 ± 2.6	4.2 ± 0.0	80.4 ± 0.3	Jangchud et al.^[Citation5]
	Blanched	39.1 ± 1.4	36.7 ± 1.2	2.4 ± 0.2	50.8 ± 1.9	11.8 ± 0.7	6.1 ± 0.4	86.4 ± 2.0
Purple	Unblanched	120.0 ± 2.3	71.4 ± 1.5	48.6 ± 2.4	104.4 ± 2.8	–15.6 ± 2.8	3.6 ± 0.0	74.0 ± 0.4
	Blanched	30.3 ± 8.2	26.5 ± 8.0	3.8 ± 0.5	49.6 ± 6.3	16.0 ± 2.6	6.9 ± 0.1	94.8 ± 0.4

Abbreviation: na, very low paste viscosities.

The pasting temperature of two varieties of sweet potato flour showed a major difference between the varieties (74.0 to 94.8°C), the pasting temperature of flour produced using blanching was much higher than those obtained without pretreatments (Table 3).^[5] The variation in pasting temperature is owed to the sugar content of flours, which enticements for moisture with starches, in effect reduces the swelling and viscosity while increasing the temperature of the pasting.^[32] The presence of starch in the flour that is highly resistance to swelling and rupturing can increase the pasting temperature.^[25] Similarly, pasting temperature is related to water-binding capacity.^[2] However, flours which have a higher pasting temperature may not be recommended for certain products due to the high cost of energy.^[24] Yadav et al.^[19] also observed that heat treatment influenced the pasting behavior of the flour. From those results, it can be concluded that it’s possible to develop different food products that require different pasting temperatures.

Peak time reflects the total amount of time flours take to reach their respective peak viscosity.^[36,45] For sweet potato flour, the values for peak time have been reported to be between 3.3 to 9.76 minutes.^{[5,10,22,23,32]} The blanched samples had the highest peak time, regardless of the variety (Table 3).^[5] The high peak time indicates longer time is required to attain peak viscosity.^[49] However, long peak time is not desired in baking industries as its related to dough development time, in turn, that is related to high energy input.^[20]

The viscosity of the breakdown is an indicator of the paste’s resistance to disintegration in response to heat and shear.^[24,32] It’s occurred as a result of holding slurries at high temperature and its difference between the peak viscosity and the trough viscosity.^[22] Fetuga et al.^[10] have examined the breakdown viscosity of three sweet potato varieties (Table 3). It ranged from 0.48 RVU for American orange-fleshed to 92.25RVU Nigerian yellow-fleshed compared to 34.94 RVU Ugandan yellow-fleshed varieties. The breakdown in viscosity was much lower compared to 11.00 to 125.33RVU in conformity with the results obtained by Olatunde et al.,^[23] but higher than that of Aina et al.^[32] who reported with the range of 3.3 to 50.2RVU.

The breakdown viscosity 602 to 2072cP was also reported by Nabubuya et al.^[22] for the flours of the sweet potato varieties. The changes in breakdown values of flour samples reflect changes in crystallinity, swelling and extent of amylose leaching.^[48] From Table 3 it can be seen that the blanched flours had the lowest value of breakdown viscosity. This might be due to the complete gelatinization of starch and the existence of swelling during blanching.^[5] A low breakdown value of the samples suggests the stability of starches under hot conditions.^[24,36] Thereby indicating its paste stability.^[25]

A high breakdown viscosity value indicates a lower ability of the sample to withstand heating and shear stress during cooking.^[49] The breakdown viscosity with the range 53.6 to 149.3RVU is reported by Tortoe et al.^[4] for the sweet potato flour from twelve varieties of Ghanaian sweet potatoes. The reduction in the breakdown viscosity of the samples indicates a significant breakdown of starches.^[49] The less stability of starch pastes commonly accompanied with a high value of breakdown.^[26] The ability to paste to withstand heating and shear stress is an important factor for many processes especially those requiring stable paste.^[49]

Setback viscosity is an index of retrogradation tendency of a paste prepared from a flour^[32,36,50] and its value is related to the amylose content and retrogradation of starch.^[6,20] Setback viscosity that ranged from 62 to 865 cP is reported by Nabubuya et al.^[22] for the flours of the sweet potato from different varieties. Fetuga et al.^[10] reported a range of 1.90 to 80.13 RVU for setback viscosity for sweet potato flours prepared from different varieties and using different drying methods (Table 3). Flours with high set back viscosity would not be suitable for pie fillings where retrogradation may cause syneresis and low-quality product.^[22] The setback viscosity with the range of 9.83 to 117.22 RVU was reported by Tortoe et al.^[4] for the sweet potato flour from twelve varieties of Ghanaian sweet potatoes. The flours showed high setback viscosity value, indicating a reasonable amount of reassociation of granules during cooling. Cultivars with the highest amount of amylose are supposed to have stiffer pastes and high setback viscosities.^[21]

Low setback values showed a lowered tendency for retrogradation.^[22,32,49] This was probably attributed to the high-fat content of the flours.^[42] The average set back viscosity of sweet potato flours between 0.4 to 36.6RVU were reported by Aina et al.^[32] for different cultivars. A low set back value indicates a noncohesive paste, which has many industrial implications.^[34] Olatunde et al.^[23] reported a range of 3.04 to 92.21 RVU of setback viscosity for sweet potato flours prepared from different varieties, pretreatments and drying methods. The authors observed a noticeable effect of amylose content on setback viscosity. Flours that have high setback value is desirable especially in bread making^[22,28,45] whereas those with low viscosity may be suitable for products requiring low gel strength and elasticity.^[22,51]

The peak viscosity is the maximum viscosity attained by gelatinized starch during heating in water or soon after the heating^[24,49] and indicative of ease of cooking a particular sample.^[41] A value of 826 to 3039cP was reported for the peak viscosity of sweet potato flour from different varieties ,^[22] while Aina et al.^[32] obtained 9.6 to 100.2RVU. Nabubuya et al.^[22] found a negative correlation between amylose content and peak viscosity. The extent of granule swelling is echoed by its peak value that indicates the viscid load likely to be met during mixing. Fetuga et al.^[10] reported a peak viscosity with a range of 4.81 to 231.67RVU for sweet potato flours prepared from different varieties and drying methods (Table 3). High peak viscosity is an indication of the suitability of the flours for products requiring high gel strength and elasticity.^[24] These demonstrated that the flours had highly pregelatinized.^[50]

Olatunde et al.^[23] reported a range of 24.50 to 260.92RVU of peak viscosity for sweet potato flours prepared from different varieties, pretreatments and drying methods. This flours showed a slightly higher value of peak viscosity. The peak viscosity of flours is correlated negatively with the amylose content of the starch in flours and often indicates the suitability of the flours in food processing.^[22] The peak viscosity with the range of 74 to 304 RVU was reported by Tortoe et al.^[4] for the sweet potato flour from twelve varieties of Ghanaian sweet potatoes. Those flours which have a high peak viscosity may be suitable for products requiring high gel strength and elasticity.^[22] The reduction in peak viscosity indicates a loose association between amylose and amylopectin.^[49] Moisture content can influence the peak viscosity of the flour due to the influence its starch degradation.

Trough viscosity (hot paste viscosity) is the minimum viscosity value in the constant temperature phase of the RVA profile measuring the ability of paste to withstand breakdown during cooling.^[24,45] Fetuga et al.^[10] reported a range of 4.33 to 139.96 trough viscosity for sweet potato flours prepared from different varieties and using different drying methods (Table 3). The trough viscosity that ranged from 826 to 3039cp for the flours of the sweet potato varieties reported by Nabubuya et al.^[22] The trough viscosity, decreased as more starch granules and amylose leach out into the solution.^[49]

Olatunde et al.^[23] reported a range of 7.08 to 145.83RVU of trough viscosity for sweet potato flours prepared from different varieties, pretreatments and drying methods. High trough values may represent low cooking losses and the superior eating quality.^[36] Avula^[48] reported that the native sweet potato flour has shown a very high peak for hot paste viscosity and increased rapidly with increase in temperature, it decreased during the holding period. Such flours used as thickeners.^[23] Avula^[48] also observed that the sweet potato flour produced by drum and hot-air driers showed high stability during heating and cooling processes and this flours have possible uses in products requiring sterilization such as baby food.

Final viscosity (cold paste viscosity) indicates the ability of a material to gel or paste after cooking or cooling in actual use^[41,44,49] and determines a particular starch-based food quality.^[24,44] Olatunde et al.^[23] reported a range of 10.21 to 225.50RVU final viscosity for sweet potato flours prepared from different varieties, pretreatments and drying methods. A high-value of final viscosity has been attributed to the aggregation of amylose and a low final viscosity indicates the resistance of the paste to shear stress during stirring.^[36]

The final viscosity of sweet potato flour from various varieties and at different processing methods has been collated by Fetuga et al.^[10] and the values vary considerably not only among varieties but also at different processing methods (Table 3). Nabubuya et al.^[22] studied the pasting properties of flours from ten sweet potato varieties. They observed the pasting behaviors of flours from all the sweet potato varieties were type A, that have high pasting peaks and rapid thinning. The final viscosity ranged between 35.8 to 296.1RVU were reported by Tortoe et al.^[4] for the sweet potato flour obtained from twelve varieties of Ghanaian sweet potatoes. They found varietal difference on the final viscosity of twelve sweet potato varieties. The viscosity at the cooling stage indicates the ability of the starch-based food to form a gel or paste after cooking and during cooling.^[45]

Reduction in viscosity is particularly important in the preparation of weaning and supplementary foods.^[34] Flours with low viscosity can be used in the product formulations to achieve desirable properties with high solids per unit volume.^[19] Other studies have concluded that low molecular weight carbohydrates might have contributed to reduced viscosity.^[21] The flours that have low viscosity can be used for infant food formulations due to their less water binding ability and more easily digested.^[21] Similarly, sweet potato flours that have low viscosity can be used in batters and breading for coating various foodstuff, in confectionery as binders and film formers, in dairy as texturizers.

The ability of a mixture to withstand heating and shear stress that is usually encountered during processing is an important factor for many processes especially those requiring stable pastes.^[45] Reduction in viscosity is particularly important in the preparation of weaning and supplementary foods from starchy raw materials.^[34] The flour pastes exhibited low viscosity with prolonged cooking indicating that these pastes should not be used in applications in which a prolonged period of high cooking is required.^[32] The cooking time has an energy cost implication.^[32]

The absence of peak and viscosity remaining constant or increasing during continued heating is ideal for good quality noodles.^[50] Reduction in viscosity is particularly important in the preparation of weaning and supplementary foods.^[19] When starch is heated in the presence of water and subsequently cooled, the disrupted amylose and amylopectin chains can gradually reassociate into a different ordered structure in a process termed retrogradation.^[24]These information can be utilized in the preparation of textured products and advantageous in the improvement of water barrier properties in edible films production.^[24] Therefore, with the increasing interest in the use of sweet potato flour in food product development, the availability of their chemical, functional, and pasting properties would lend itself as a processing protocol for the development of various value-addition food product.

Gel consistency

The gel-consistency test is a fast, easy, and sensitive indication of cold paste viscosity with which it correlates inversely.^[19] The lowest concentration of gelation can be defined as the lowest concentration of protein where gel remained in the inverted tube was used as an index of gelation ability.^[45] The degree of gelatinization is related to the amount of moisture.^[50]

In a study conducted by Avula,^[48] drum and hot air-dried sweet potato flour showed slightly higher compatibility with gel compared to enzyme-modified flour, while modified enzyme flour showed the lowest compatibility with gel. High gel sweet potato flour can, therefore, be useful in the production of products requiring greater strength per unit volume, such as dietary food specialties and formulations.^[48] Flours with high gel value can be useful in the development of products in which a higher solid content per unit volume is required, such as special diet foods, food formulations for children.^[19]

The higher gel consistency values of the flours may be attributed to their complete gelatinization.^[48] The low gel content of flours can be used as a composite food for curd formation or used as additives to other food materials that form a gel in food products such as sauce and pudding.^[21] Hence, the result obtained suggests that the flours obtained from drum and hot air-drying methods can be useful in the production of items requiring a higher solid content per unit volume, such as special diet foods, children’s food formulations while the enzyme and native flours can be used sauce and puddings.

Sediment volume

Sediment volume is a starch gelatinization index for refined starchy products.^[19,48] The data clearly and helpfully differentiates between different pre-cooked products.^[48] As a result of modifications, acetylated and enzyme-modified flours showed lower sediment volume as compared to native flours.^[48] The lower sediment volume values are attributed as a result of the change, to the existence of substituent groups and the formation of enzyme starch complex. The sediment amount of drum and hot air-dried sweet potato flour exhibits higher values than enzyme modified and acetylated flours were reported by Avula.^[48] It is also evident that the degree of gelatinization in drum-dried flour was significantly high, followed by hot air-dried flours. It is noted that, drum drying enhance sediment value mainly by reducing or destroying substituent groups and indicates the breakdown of starch into simple sugars.

In vitro digestibility

Digestibility can be used as an indicator of starch digestibility and saves the starch extraction stage.^[52] It’s also important for evaluating the nutritive value and industrial applications of the starch.^[37] Yadav et al.^[40] compared the in vitro digestibility of acetylated, enzyme-modified and native flours. They found that the in vitro digestibility was lower in the acetylated flour compared to those native and enzyme-modified flours. This has been amply illustrated in the effect of enzyme action during processing on the breakdown of starch into simple sugars.^[34] They also observed that in flour dried using hot air-dried was more digestible than drum dried and it’s as the result of instantaneous high hydrothermal treatment. Further research on applying drying methods in combinations with other techniques such as enzyme modification, different cooking temperatures, and different pretreatments for more digestion of food products developed from sweet potato flours should be taken up.

Cooking and drying increased the digestibility of flours compared to acetylation and enzyme modification.^[48] This could be concluded from the fact that flour obtained from the cooking and drying are digested easily. Although, there is not sufficient data regarding the digestibility of sweet potato flour, it is expected that flour obtained from drying and preheating treatments are digested easily. The fewer digestibility flours may function like dietary fiber and have therapeutic benefits such as blood glucose control in diabetes, or to aid in weight control.^[47] Other studies have concluded that restricted digestion is critical for infants, senior citizens having reduced digestive capacity and people with physical exhaustion, emotional stress or medical disorders leading to disturbed digestion.^[2] Hence, the acetylated flours can be used in functional foods formulations.

Thermal properties of sweet potato flour

The thermal properties are calculated using differential calorimeters, which are an important phenomenon in different food processing operations. The DSC has been extensively used to study the gelatinization or melting characteristics of crystalline starch types and it is useful to understand the phenomenon of starch gelatinization and retrogradation,^[37] in particular the temperature range of gelatinization, the structure of lipid amylose complexes, and retrograded starch.^[48] Therefore the quality and safety of food depend heavily on the entire history of temperature, condition, and water distribution in food.

The gelatinization endotherm obtained by DSC offers an overall measure of the progressive loss of long, medium and short order in crystallite starch granules, as they are heated slowly in excess water.^[48] The amount of heat required to gel the starch is called enthalpy.^[48] The temperature at which starch gelatinizes is given as T_onset (To), T_peak (Tp) and T_conclusion (Tc).^[48]

The data on the DSC of sweet potato flours are given in Table 4. Ndangui et al.^[3] have examined the DSC characteristics of sweet potato flour from different pretreatments and obtained considerable variation in all the parameters. The values for T_onset showed a range from 74.4°C to 77.4°C, highest and lowest being for the sweet potato flour obtained after being subjected to blanching and calcium chloride pretreatments, respectively (Table 4). Avula^[48] have compared the gelatinization temperature of flour from native, acetylated and enzyme-modified and observed that the enzyme-modified flour showed the highest value (Table 4). However, to swell the granules of such flour more heating is required.^[37] This showed that pretreated flour are more structurally stable and resistant to gelatinization.^[3,48] It’s maybe due to the interaction of calcium ions with the sweet potato tuber matrix, which in effect caused plasmolysis due to an osmotic process, leading to higher gelatinization for the former.^[3]

Table 4. DSC analyses of sweet potato flour reported by different authors.

Processing methods	To (°C)	Tp (°C)	Tc (°C)	ΔHGEL (J g-1)	References
None	75.8 ± 0.4^b	77.2 ± 0.1^a	84.4 ± 0.6^b	11.8 ± 0.5^b	Ndangui et al.^[Citation3]
Pretreated with CaCl2	77.9 ± 0.1 ^c	82 ± 0.8 ^c	85.8 ± 0.7 ^c	13.2 ± 0.6 ^c
Blanched	74.4 ± 0.1^a	80.5 ± 0.1^b	82.2 ± 0.3^a	9.5 ± 0.4^a
Native	68.0	71.8	78.5	10.6	Avula^[Citation48]
Hot air dried	-	-	-	-
Drum dried	-	-	-	-
Acetylated	55.9	63.2	67.3	1.7
Enzyme modified	73.5	76.9	86.9	11.4
Distilled water	51.26 ± 2.15	67.58 ± 1.97	74.73 ± 2.64	8.43 ± 3.38	Ngoma et al.^[Citation21]
Citric acid
5g/L	56.28 ± 0.67	70.83 ± 0.25	84.35 ± 0.46	12.78 ± 0.23
10 g/L	60.92 ± 1.71	72.65 ± 1.26	5.38 ± 1.05	9.89 ± 1.63
15 g/L	57.81 ± 1.91	69.30 ± 1.56	77.58 ± 3.56	10.05 ± 2.14
Sodium metabisulphite
5 g/L	63.40 ± 1.06	75.72 ± 1.56	85.28 ± 5.60	9.15 ± 3.65
10 g/L	58.12 ± 2.02	70.88 ± 2.46	83.17 ± 3.14	9.63 ± 2.94
15 g/L	63.65 ± 6.35	73.26 ± 5.25	81.38 ± 2.86	8.76 ± 1.92

Varietal differences, environmental conditions and the experimental protocols like the level of moisture, sample preparation, rate of heating and instrument are among the factors that affect the gelatinization temperatures values.^[37] With regards to pre-treatment methods, such as drying or blanching, different methods affect differently the gelatinization temperatures of the flours. Therefore the flours produced from CaCl₂ pretreatments and enzyme-modified can be incorporated in food formulation that requires such gelation temperatures such as in bread making. Besides, it can be incorporated in soups, sauces, gravies, bakery and dairy products as viscosifiers and texturizers.

The endotherm peaks of native sweet potato flour appeared between 68 and 78.5°C (Table 4). Such flour would be suitable for products manufactured at low processing temperatures and requiring low gelatinization temperatures and high swelling strength, such as soup, sauce, pudding, and older foods.^[34] Other studies have concluded that low gelatinization temperatures provide greater availability of starch to amylolytic enzymes during the baking cycle that is desirable in bread making.^[2] The native flours also can be used in refrigerated and frozen foods, as emulsion stabilizers and for encapsulation.

Considerable variability in the values of the gelatinization enthalpy is observed (Table 4). Gelatinization enthalpy is found to depend on modification and pretreatments as revealed by Avula^[48] and Ndangui et al.^[3] from studies using different modifications and pretreatments. Significant differences appeared in all treatments. The lowest values were observed in the flours obtained after blanching treatments, compared to SSP, SSPCaCl₂, and SSPB. (Table 4). This is because of some reduction of reducing sugar content as a result of blanching^.[3] The DSC data of flour from two driers (hot air and drum driers) did not indicate the gelatinization peak (Table 4). This indicates the loss of crystallinity in that flours.^[48] This suggests that different treatments may have a differential effect on the gelatinization enthalpy of sweet potato flours.

The lower values of gelatinization temperatures and enthalpy are noticed for the acetylated flours, compared to the native and enzyme modified (Table 4). This can be attributed to the presence of changes in their structural integrity as a result of substituent groups that in turn, varies the degree of crystallinity and degree of chain branching.^[48] This indicates that lesser energy is needed to break the intermolecular bonds in starch granules of such flour to achieve gelatinization.^[25] Significant differences were observed with respect to onset, peak, endset and the ∆H values of sweet potato flours after enzyme modification (Table 4). Gelatinization enthalpy depends on some factors such as crystallinity, intermolecular bonding.^[37,48]

Morphological properties

Flour modification involves physical, chemical and biochemical spectacles on the surface of contacting phases.^[53] The granule size is reported to affect some functional properties and physiology function.^[37,54] Wide variability in the size and shape of starch granules has been observed (Figures 3 and 4). Scanning electron microscopy has played an important role in increasing understanding of the granular structure of modified flour starches and used to detect structural changes caused by chemical modifications and the most substituted regions in starch granules.^[53]

Figure 3. A) PSP Flour Preheated at 90°C for 45 minutes, 35.57% Degree of Gelatinization. b) PSP Flour Preheated at 90°C for 60 minutes, 39.02% Degree of Gelatinization. c) PSP Flour Preheated at 90°C for 75 minutes, 49.08% Degree of Gelatinization. d) Native (Non Preheated) PSP Flour, 14.94% Degree of Gelatinization e) PSP Flour Preheated at 90°C for 30 minutes, 28.11% Degree of Gelatinization. f) PSP Flour Preheated at 90°C for 15 minutes, 25.77% Degree of Gelatinization. Scanning electron micrographs of native PSP flour and preheated flour (a- f). Reprinted from S. Nurdjanah, N. Yuliana, S. Astuti, J. Hernanto, and Z. Zukryandry, ‘Physico Chemical, Antioxidant and Pasting Properties of Pre-heated Purple Sweet Potato Flour’, J. Food Nutr. Sci., vol. 5, no. 4, pp. 140–146, 2017, with kind permssion of Authors, ©2017.

Figure 4. Microstructure of sweet potato flour manufactured under different pretreatment and drying conditions (SEM). (a–c) Freeze-dried after treatment with distilled water, NaHSO3 and CaCl2. (d–f) Air-dried at 55, 60 and 65 C after treatment with distilled water. (g – i) Air-dried at 55, 60 and 65 C after treatment with NaHSO3. (j–l) Air-dried at 55, 60 and 65 C after treatment with CaCl2. Reprinted from M. Ahmed, A. M. Sorifa, and J. B. Eun, ‘Effect of pretreatments and drying temperatures on sweet potato flour’, Int. J. Food Sci. Technol., pp. 726–732, 2010, with kind permission of Authors, © 2010.

Native sweet potato flour starch granular are mostly round, hexagonal, and spherical with 4 to 26 µm size whereas, the modified flour are partially disappeared.^[40] This may be due to the differences in the native granule architecture and fragility.^[53] Their results were in line with Nurdjanah et al.^[54] who found that the native sweet potato flour contains starch granule which is round, spherical and surrounded by cell wall material (Figure 3). Ndangui et al.^[3] studied the impact of thermal and chemical pretreatments on physicochemical, rheological, and functional properties of sweet flour and they observed composed of smooth, round, and spherical granules with quite irregular that exhibits dents .

Yadav et al.^[34] noticed indentations for acetylated sweet potato flour and exo corrosion for enzyme-modified flour. Similarly, preheating treatment up to 75 min at 90°C of sweet potato before flouring could change the granular characteristic partly and developed a more compact flour (Figure 3).^[54]The authors found that preheating have a noticeable effects and that is due to partial gelatinization as a result of preheating. Modification process and heat treatment are among the factors that differ the morphological features of the flours.^[48] An inverse result was reported by Ndangui et al.^[3] who found that pretreatments (calcium chloride and blanching) did not have noticeable effect on starch granular change among the flours under the SEM images.

Ahmed et al.^[55] found significant processing methods effect on the scanning electron micrographs of sweet potato flour prepared with different pretreatments and drying methods. It was observed that the structures of untreated flour granules were larger than those of CaCl₂, NaHSO₃ and distilled water pretreated flours (Figure 4). They also observed that at higher temperatures, pretreated flours progressively lost granular morphology, probable due to gelatinization (Figure 4(i,l)). The authors concluded that the variations of the structure observed among the flours were due to the internal modification of the starch granules through the action of NaHSO₃ and CaCl₂ during processing. Using laser diffraction granulometry studies on the sweet potato flour revealed mixed populations of large, medium, and small granules particle size distribution obtained.^[3]

Yadav et al.^[40] reported that the treatment of sweet potato flour with glucoamylase alters the granule morphology. The modified flour showed serrated surfaces and breakage of outer layers in some granules due to more reducing sugars is released as a result of amylolysis.^[40] Hence, acetylation, enzyme modification and preheating treatments could be used as a method for modifying and preserving functional properties to suit various applications of sweet potato flour in the food industry.

Value added products from sweet potato flours

Among the tuber crops sweet potato contributes in terms of economic values and used as food where its grown. Sweet potato flours are stable ingredients that are used in many food formulations in the production region. Along with the traditional usages, it’s now using for the production of the different value-added products, as shown in Table 5.

Table 5. Some value added products made from sweet potato reported by different authors.

S. no.	Product name	Major components	References
1.	Complementary food	Sweet potato and soybean flours and other ingredients are used.	Amagloh et al.^[Citation56]
2.	Cookies	Cookies are prepared from sweet potato and wheat flours with different proportions and sugars, fat, shortening and eggs are incorporated.	Saeed et al.^[Citation57]
		Flour containing various proportions of sweet potato flour, and whey protein concentrate (80%) are used for cookies preparation.	Seelam et al.^[Citation58]
		Cookies were formulated which includes 55% sweet potato flour with corn, sorghum and rice flour at the ratio of 1:3:5 respectively and other ingredients.	Giri and Sakhale^[Citation29]
3.	Amala	Cassava and sweet potato flours are used to produce amala.	Idowu et al.^[Citation59]
		Prepared from elubo sweet potato flours.	Fetuga et al.^[Citation10]
4.	Biscuit	Different combination of sweet potato and wheat flours with other ingredients used in the preparation of Biscuits	Srivastava et al.^[Citation60]
		Wheat- Sweet potato flour composites were prepared at different ratios and other ingredients are used.	Toan et al.^[Citation61]
		Biscuits were prepared from doughs containing starch, sweet potato flour and fiber at different levels and also from dough containing wheat flour as control.	Fetuga et al.^[Citation10]
5.	Bread	The bread is prepared from wheat flour substituted with different parts of yellow sweet potato flour.	Nogueira et al.^[Citation1]
		Sweet potato and wheat flours and other ingredients are used.	Kamal et al.^[Citation62]
		Sweet potato and wheat flours are used for the bread making.	Mu et al.^[Citation14]
6.	Extruded pasta products	Carbon methyl cellulose, a pinch of salt and an egg are added to the sweet potato flour, mixed thoroughly to produce pasta.	Olubunmi et al.^[Citation11]
7.	Instant noodles	Wheat flour, sweet potato and other major ingredients are used for the noodles preparation.	Taneya et al.^[Citation63]
8.	Ketchup	Made from sweet potato, water, vinegar, sugar and flavorings.	Oke and Workneh^[Citation64]
9.	Sweet potato leather	Steamed sweet potato chunks are blended with water, sugar, salt, citric acid, and optionally with artificial fruit flavors.	Oke and Workneh^[Citation64]

Conclusion

Sweet potato flour is becoming increasingly important and food producers, marketers and consumers are drawing attention to it. In fact, sweet potato can generate high income for the farmers because of high market value and profitability. The information presented here shows that sweet potato flours have great potential for the production of numerous food products. Functional properties are the set of data, which provide information about the fields of application in food formulations. It can be used for product development as a guideline. However, most information on the sweet potato’s functional characteristics is based on starch rather than flour. Not only the starch but also other flour components provide for the functional properties of flour as additional components such as protein, fat, etc. are available in the flour. Different kinds of modifications are used to modify the structural, rheological and functional properties sweet potato flours. The studies on the different varieties and processing methods reveal the vast variability available among the functional properties of sweet potato flour. The range of characters observed makes the sweet potato flour amenable to different applications based on their functional properties. The functional properties of sweet potato flours obtained from different varieties and processing methods varied both within and between species and processing methods. The high stability in the heating and cooling processes of drum dried and hot air-dried flours make these flours very useful in food industry especially where sterilization is required such as baby food. The high bulk density of modified flours can make them useful in food preparations such as confectioneries, as well as in pharmaceuticals industry as a drug binder and disintegrant. The flour prepared using blanching as pretreatments which have high swelling power can be partly incorporated with wheat flour for bread making. The high paste viscosity of enzyme-modified flour makes these flours to act as a good thickening agent. The less digestibility of acetylated flours makes them suitable for specific and targeted food products such as for diabetics and obese people who may have a lower digestibility ability. In deduction, acetylation, enzyme modification and preheating treatments could be used as a method for modifying and preserving functional properties to suit various applications of sweet potato flour in the food industry. After going through whole discussion and the available data, it can be concluded that sweet potato flours are valuable foodstuff and have multifunctions. Hence, in view of huge amounts in which sweet potatoes are used up all over the world, sweet potatoes could be a very good vehicle for addressing some health related problems and also serve as food security.

Acknowledgments

This review article contains information gathered from numerous published resources, and thus, we would like to acknowledge all authors of the references used in this review.

Disclosure statement

All authors declare no conflict of interest