Effect of Defatted Maize Germ Addition on the Functional and Textural Properties Of Wheat Flour

Abstract

The objective of this study was to assess functional properties of wheat flour blends with defatted maize germ flour (DMGF), a byproduct of the corn oil industry, at 5–25% levels. The bulk density, oil, and water absorption capacities, emulsion/foaming capacity and stability, objective color, least gelation concentration, and rheological properties (apparent viscosity and dough compression) were determined in control and flour blends. With DMGF addition, bulk density and foaming capacity decreased from 0.62 g/mL to 0.55 g/mL and 33.7% to 25.7%, respectively, both at 25% level. In general, when compared to control, oil and water absorption and emulsion capacities increased significantly in flour blends with >10% DMGF. Overall, regardless of the DMGF level, complete or partial gelling was observed at ≥ 8% gelation concentration. The apparent viscosity increased with increasing DMGF levels (0–25%) in all flour blends and also at all 4 concentrations from 5% to 20%. The control flour dough had a hardness value of 7.56 N, which increased significantly to 84.6N, when the DMGF level increased to 25% in the flour blend. These results indicate that most of the functional properties of wheat flour blends improved with DMGF addition, thus DMGF has a great potential to be used in a variety of food products.

Keywords:

• Defatted-maize-germ flour
• Functional properties
• Texture
• Dough

INTRODUCTION

Maize (Zea Mays L.), more commonly referred to as corn in the U.S., is processed commercially using two main processes, wet or dry milling; and recently, to a lesser extent, by dry grinding for ethanol production. The U.S. maize production in 2007 was 332 million metric tons[1]; with the above processes utilizing about one-fourth of the total crop. In both dry and wet milling processing of maize, the germ is separated from the kernel for oil production.[2] Defatted maize germ (DMG) cake, a byproduct of the corn oil industry, is almost entirely used for animal feed. The use of this relatively low-cost ingredient warrants further research for exploiting its potential use in human foods.

The maize germ, which accounts for 5–14% of the weight of a maize kernel, depending on variety and grain size, is high in protein content, dietary fiber and minerals.[3–5 ] The proteins in DMGF mostly consist of albumin and globulin,[6,7] and are balanced in most of the essential amino acids; [4] lysine, a major limiting amino acid in wheat, accounts for 5–6% of the total proteins in DMG, which is more than twice than that in wheat flour. According to Anjum et al.[8] protein content and the contribution that essential amino acids make to the total are the most important factors from a nutritional point of view. Besides nutritional considerations, the functional properties of flours, such as emulsifying properties, foaming capacity and stability, water and oil absorption, and solubility also contribute significantly to the final quality of a processed food product. The defatted maize germ meal or flour has been studied previously for quality characteristics or in product development.[4,5] The maize germ protein flour has been shown to have excellent water retention and fat binding capacity which helps stabilize emulsions by absorbing or binding excess water,[9,10] subsequently leading to higher yield of end products.[11]

Among various food quality attributes, texture is one of the major criteria, which consumers use to judge the quality and freshness of many foods. Texture is important in determining the eating quality of foods and can have a strong influence on food intake and nutrition. Perceived texture is closely related to the structure and composition of the food, and both microscopic and macroscopic levels of structure can influence texture.[12,13] Instrumental methods to measure mouthfeel are based on the science of rheology, which measures the deformation and flow of materials. Texture and rheology are key factors for foods to be palatable and for the movement of foods through the digestive tract.[14] The flow and deformation behavior of doughs, another important functional property, is recognized to be central to the successful manufacturing of bakery products. Thus, rheological measurements are used at numerous points in the development of new products and processes during the optimization stage or manufacturing to ensure consistent quality.[15]

The nutrient-dense composition of DMGF offers a good potential for its use as an ingredient or fortificant in a variety of foods, such as bread, cookies, muffins, cake, etc. In spite of the excellent nutritional quality, very limited information is available in the literature on the functional characteristics of DMGF that can greatly benefit product development endeavors. The objective of this study was to investigate the functional and rheological properties of DMGF-wheat flour blends, which may aid in enhanced use of DMGF in different food systems.

MATERIALS AND METHODS

Preparation and Proximate Analysis of DMGF

The maize germ was obtained from Sanghera Agri. Farm (Okara, Pakistan). The DMGF was processed from maize germ according to the method of Tate et al. with some modifications.[16] Defatting of the germ (100 g) was carried out by continuous fluxing with 15 × volume of n-hexane in Soxhlet apparatus for 8 hours. The resulting defatted germ was dried at 23ºC overnight and ground in a lab grinder. The flour was sieved manually using a 250-μm stainless steel sieve (W.S. Tyler Co., Mentor, Ohio, USA). The flour samples were stored in polyethylene bags at 23ºC until required for further analysis. The proximate composition and mineral content of DMGF were analyzed by AACC (2000) and AOAC (1990) methods, respectively.[17,18]

Preparation of DMGF-Wheat Flour Blends

DMGF was added to all-purpose wheat flour–control, with 12.28% moisture content (King Milling Co., Howell, Michigan, USA) at 5, 10, 15, 20, and 25% (w/w) levels to prepare the treatment flour blends. Each treatment containing both types of flours was mixed thoroughly by sieving to achieve equal distribution of particles in the flour blends. The moisture content of the wheat-DMGF blends was 12%, 11.71%, 11.43% 11.14, and 10.86% in 5%, 10%, 15%, 20%, and 25% DMGF blends, respectively. All flour blends were stored in woven polypropylene bags at 23ºC.

Physical and Functional Properties Flour Blends

Hunter color

Hunter color measurement of DMGF and flour blends was done with a Hunter Color Meter (Model D25 L; Hunter Associates Lab., Reston, Virginia, USA), using four replicates each,. A 100-g flour sample of flours was placed in a sample cup and color values were recorded as “L” (0, black; 100, white), “a” (–a, greenness; +a, redness), and “b” (–b, blueness; +b, yellowness). The standard white tile, supplied by the manufacturer, had “L”, “a”, and “b” values of 94.8, −0.7, and 2.7, respectively and used to calibrate the colorimeter. The data thus obtained was used to calculate the Chroma and Hue angle according to method of Little,[19] as follows:

(1)

(2)

Bulk density, water and oil absorption capacities

Bulk density (loose) was determined according to the method of Okaka and Potter. [20] A 50-g sample was filled into a 100-mL graduated measuring cylinder. The cylinder was tapped gently several times on a laboratory bench to a constant volume. The results for bulk density were reported as g/mL. Water and oil absorption capacities were determined by the method of Sosulski et al.[21] Two grams of flour were mixed with 20 mL distilled water or refined corn oil at room temperature (23 ± 1ºC) and centrifuged (Model: Sorvall RC-5B; DuPont Instrument, Newtown, Conn., USA) at 2000 × g for 30 min. The water or oil absorption capacity was expressed as a percentage (%) of flour weights.

Emulsion activity and stability

Emulsion activity and stability were determined by the method of Yasumatsu et al. (1972).[22] Two grams flour and 20 mL distilled water in a test tube were mixed, followed by the addition of 20 mL refined soybean oil. The contents were mixed for 5 min with vigorous shaking. The resulting emulsion was transferred into 50-mL centrifuge tubes and centrifuged at 2000 × g for 30 min. The ratio of the height of the emulsion layer to the height of the liquid layer was calculated, and the emulsion activity expressed as percentage. The emulsion stability was determined after heating the emulsion (in a 50-mL centrifuge tubes) in a water bath at 80ºC for 30 min, cooling to 23 ± 1ºC and centrifuging at 2000 × g for 30 min. The emulsion stability, expressed as a percentage, was calculated as the ratio of the height of the emulsified layer to the height of the liquid layer.

Foaming capacity and stability

Foaming capacity (FC) and stability (FS) were determined as described by Okaka and Potter.[20] A 2-g flour sample was added to 50 mL distilled water in a 100-mL graduated cylinder at 30ºC and the suspension was mixed and shaken till foaming occurred (approx. 5 min); the volume of foam thus formed was expressed as FC using the following formula:

(3)

The volume of foam was recorded one hour after whipping to determine foam stability as percent of the initial foam volume as follows:

(4)

Least gelation concentration

The least gelation concentration was determined by the method of Sathe and Salunkhe.[23] Flour dispersions of 2, 4, 6, 8, 10, 12, and 14% (w/v) were prepared in 5 mL of distilled water in test tubes and heated for one hour in a water bath at 95 ± 1ºC. The heated dispersions were cooled to 10 ± 1ºC. The least gelation concentration was determined based on visual observation whether any drops from the emulsion slipped out to the top or not in inverted tubes. The results were expressed as no (−), complete (+), or partial (±) gelation.

Apparent viscosity

Apparent Viscosity of flour dispersions was determined using Brookfield DV-II viscometer (Brookfield Engg., Middleboro, Mass., USA) by preparing dispersions of 5, 10, 15, and 20 g flour in 100 mL of distilled water in 250-mL beaker. The apparent viscosity was measured using spindle #2 at shear rate of 100 rpm at 23 ± 1ºC. The readings were recorded after 30 s shearing time. All viscosity measurements were done in triplicate and results are reported in mPa.s.

Texture Analysis of DMGF-Wheat Flour Doughs

The texture analysis of dough under compression mode was measured using a Stable Micro Systems texture analyzer (Model: TA-XT2i, Texture Technologies, Scarsdale, New York, USA), equipped with a 500-N load cell and a 35-mm cylindrical probe. The flour samples were kneaded using a Hobart mixer (Model N-50, Hobart Corp., Troy, Ohio, USA) for 3 min by adding water (60% on flour weight basis). Dough samples for texture analysis were obtained by sheeting with a rolling pin over a rectangular platform and frame of 10-mm height to form cylindrical doughs (10 mm × 35 mm). The doughs thus obtained were subjected to uniaxial compression.[24] A crosshead speed of 100 mm/min was used to compress the doughs to 50% of their original height. The parameters derived from the force-deformation curve included hardness, the maximum resistance for the compression peak (height of peak) and stickiness, the maximum value of the negative peak of the compression.

Statistical Analysis

All data were analyzed using JMP IN software, version 5.1 (SAS Institute, Inc., Cary, NC, USA). The separation of means or significant difference comparisons were done using Tukey's HSD test and the statistical significance was defined as P < 0.05.

RESULTS AND DISCUSSION

Proximate and Mineral Composition

The proximate composition of DMGF (germ-weight basis) consisted of 27.6% crude protein, 3.1% crude fiber, 7.5% ash, 5% fat, 6.6% moisture, and 51.2% carbohydrates (by difference). These findings are consistent with the previously reported results for DMGF; 24.8–31.0% protein and 3.3–10.3% crude fiber and 8.4% minerals.[5,25] Some compositional differences can be anticipated owing to crop variety, different climatic and soil conditions, agricultural practices, post-harvest handling, and processing techniques. Many other researchers have also reported the nutrient-dense properties of DMGF as a good source of balanced proteins, fiber, and minerals.[6,26,27]

The mineral composition of DMGF was 2.56% phosphorus, 2.35% potassium, 1.12% magnesium, 0.017% calcium, and 0.021% iron; all on dry weight basis. Our results are consistent with previously reported mineral content in corn germ flour.[28,29] The high concentration of potassium in DMGF has significance in that an average human diet is deficient in this mineral[30]; thus adding DMGF to food formulations can minimize potassium deficiency to a great extent.

Physical and Functional Properties Flour Blends

Hunter color

Hunter color values of DMG-wheat flour blends are given in Table 1. Wheat flour was “whiter” than DMGF as indicated by the higher “L” values. As compared to control (wheat) flour, the DMGF addition in the range of 5–25%, resulted in a decrease in the color lightness, as evidenced by lower “L” values, and an increase in yellow tint (higher “b” values in positive range), both significantly (p < 0.05), whereas “a” values (red-to-green) exhibited no clear pattern.

Table 1 Hunter color values of DMGF—wheat flour blends

DMGF conc. (%)	Hunter color values
	“L”	“a”	“b”	Hue angle (h°)	Chroma
0	62.08a ^1	−0.70a	5.33d	−82.51c	5.37d
5	61.74a	−0.80a	6.18c	−82.62c	6.23c
10	61.55a	−0.88a	6.68b	−82.53c	6.73b
15	59.20b	−0.73a	6.95b	−84.04b	6.99b
20	59.03b	−0.78a	7.48a	−84.08b	7.52a
25	58.53b	−0.75a	7.78a	−84.49a	7.81a
100% DMGF ^2	54.85	0.60	11.43	–88.96	11.44
^1Means sharing the same letter in columns are not significantly different from each other (Tukey's HSD test, p < 0.05). ^2Data given for reference only; not compared with flour blends.

For color analysis, the hue angle (h°), represented by the ratio of arctan (b/a), is a good indicator of changes in color by a single value. Hue angle is the attribute of color perception by means of which an object is judged to be red, yellow, green, blue or purple, while chroma is the attribute of color perception that expresses the degree of departure from the grey of the same lightness.[19] The DMGF had a hue angle of −86.99 indicating it being more yellowish in color compared to wheat flour (−82.51), which was also supported by the value of chroma (11.44). Chroma values increased from 6.23 to 7.81 with increasing levels of DGMF in the blends, while hue angle also decreased from −82.62 to −84.49 indicating more yellowish color in the flour blends.

Bulk density, water, and oil absorption capacities

Bulk density of DMGF (0.43 g/mL) was significantly less than that of wheat flour (0.62 g/mL). The DMGF addition (5–25%) resulted in lower bulk density in flour blends (p < 0.05, Table 2). In a study by Akpata and Akubor,[31] defatting process is shown to result in decreased bulk density. The lower density of DMGF can be potentially beneficial for addition to many foods, especially weaning food formulations.

Table 2 Bulk density and water and oil absorption capacities of DMGF—wheat flour blends

DMGF ^1 conc. (%)	Bulk density (g/mL)	Water absorption (%)	Oil absorption (%)
0	0.62a ^1	85.0c	75.0d
5	0.61a	96.7bc	90.0c
10	0.60b	105.0b	96.7bc
15	0.58b	113.3b	101.7b
20	0.56bc	133.3a	103.3b
25	0.55c	143.3a	111.7a
100% DMGF ^2	0.43	363.3	180.0
^1Means sharing the same letter in columns are not significantly different from each other (Tukey's HSD test, p < 0.05). ^2Data given for reference only; not compared with flour blends.

The higher degree of water absorption by DMGF (363.3%) versus 85% for wheat flour (Table 2), could be attributed to higher protein content,[26] and probably the germ defatting process itself. Owing to this characteristics, the addition of DMGF in flour blends resulted in gradual and significant increase in water absorption capacities. Water-retention capacity is an important factor for additives used in food systems. The amount of water present in processed products will depend on the extent to which the dry ingredients absorb or adsorb water under various environmental conditions.[32] Nielsen et al. [25] reported water-binding capacity of 420−690% for corn germ meal, which are significantly higher than those observed in the present study. Maize germ protein flour rich in starch has also been reported to stabilize emulsions by absorbing or binding excess water, enabling more water to be added.[9,10] The results regarding water absorption capacity have shown that DMGF-added blends might be useful in bakery products to improve handling properties.

Oil absorption capacity also increased significantly (P < 0.05) with increasing levels of DMGF in the flour blends (Table 2). Our results for oil absorption capacity of DMGF are in close agreement to those reported previously,[11,33] in sausage batters with added maize germ protein flours. These studies showed that maize germ protein flours increased water-holding capacity and decreased cooking losses of sausage batters, most likely by binding fat and water to increase product yields.

Emulsion capacity and stability

The DMGF was shown to have 63.6% emulsion capacity. With the addition of DMGF, the emulsion capacity of wheat flour (50.0%) increased gradually (Table 3), however, this increase was significant at ≥ 10% levels only. It may be noted that, as compared to control (wheat flour), the emulsion stability of flour blends increased significantly at all levels of DMGF. These results for emulsifying properties agree to those reported for DMGF previously.[11] Maize germ proteins have been reported to improve not only emulsifying capacity but also emulsion stability which in turn can improve texture[33] thus forming stable gels. The relatively higher emulsion properties of DMGF could be attributed to high soluble protein content[6] that form the protective barrier around fat droplets thus preventing their coalescence.[34] The DMGF, having high emulsion activity and stability, has a potential to be used as an ingredient in processed bakery and meat products, and as a stabilizing agent in the colloidal foods.

Table 3 Emulsion and foaming properties of DMGF—wheat flour blends

DMGF ^1 conc. (%)	Emulsion		Foaming
	Capacity (%)	Stability (%)	Capacity (%)	Stability (%)
0	50.0b ^1	47.4d	33.7a	21.2d
5	51.6b	49.3c	33.4a	23.6d
10	53.4a	50.0bc	29.9b	24.3d
15	53.8a	50.6ab	27.9bc	29.0c
20	54.4a	51.2a	26.4c	33.2b
25	54.7a	51.6a	25.7c	38.3a
100% DMGF ^2	63.6	58.5	19.7	90.0
^1Means sharing the same letter in columns are not significantly different from each other (Tukey's HSD test, p < 0.05). ^2Data given for reference only; not compared with flour blends.

Foaming capacity and stability

The foaming capacity of DMGF was 19.7%, and with increasing DMGF levels, foaming capacity of the flour blends decreased gradually from 33.7% in control to 25.7% in 25% DMG-Wheat flour blend (Table 3); however, this decrease was significant only at or above 10% DMGF level. The defatting process and higher mineral content (7.5%) of DMGF might possibility have contributed towards lower foaming capacity in DMGF. Vani and Zayas[35] reported that maize germ protein flour had lower foaming capacity than both wheat germ and soy flours at 1% concentration level. The foaming stability of DMGF was 90.0% versus 21.2% for wheat flour. As expected, DMGF addition resulted in a significant increase in the stability of the foam in flour blends, but only at 15% or higher levels.

Least gelation concentration

In general, the complete or partial gelling was observed at 8% or higher gelation concentration of flour blends in the dispersion (Table 4). At lower gelation concentrations (≤6%), only partial gelling was noticed at >15% DMGF in flour blends. Gelling power is reported to increase with defatting process.[31] Protein gels are aggregation of denatured molecules and the defatting process also might have resulted in higher concentration of denatured protein in DMGF resulting in more gelling power. Sathe et al.[36] reported the DMGF could be a valuable addition in foods such as puddings and sauces, which require thickening and gelling.

Table 4 Least gelation concentration of DMGF—wheat flour blends

DMGF conc. (%)	Concentration of flour blends in the dispersion (%)
	2	4	6	8	10	12	14
0	– ^1	–	–	–	±	±	+
5	–	–	–	±	±	±	+
10	–	–	–	±	±	+	+
15	–	–	±	±	±	+	+
20	–	–	±	±	+	+	+
25	–	±	±	±	+	+	+
^1Gelation levels: None (−), complete (+), partial (±).

Apparent viscosity

Apparent viscosity (η) of the different flour blends is shown in Fig. 1. As expected, the apparent viscosity increased with increasing concentration of DMGF in the flour blend. The apparent viscosity increased from 12.5 to 36 mPa.s when the concentration increased from 0 to 20%, respectively. The increase in viscosity can be attributed to the higher water absorption capacity of DMGF in the blend compared to wheat flour. Another possible reason for increase in viscosity of blends could be due to the presence of insoluble matter in the dispersion; this effect was reported by Marsh et al.[37] for tomato concentrate. The concentration of the insoluble suspended matter has shown to have a profound effect on the viscosity and the type of viscous flow.[14]

Figure 1 Apparent viscosity of DMG-wheat flour blend dispersions at various concentrations.

Texture Analysis of Doughs

The force-deformation curve recorded for doughs prepared from different flour blends is presented as Fig. 2. As mentioned in the methodology section, the water added to prepare the dough was kept constant at 60%, on flour wt. basis; appropriate amount of water is needed to provide the desired the dough consistency. The control dough had a hardness value of 7.56 N, which increased significantly (p < 0.05) to 84.6N, when the DMGF level increased to 25%. The steep increase in hardness values could be again attributed to the high water absorption capacity of DMGF. On the other hand, the stickiness also increased significantly (p < 0.05) from −0.372 N to −4.610 N when the DMGF level was 25% in the dough. The increase in stickiness is probably due to the low oil content of DMGF, and due to increased inter-particle friction.

Figure 2 Force-deformation curves of dough made from flour blends containing different levels of DMGF (control = 100% wheat flour) .

The viscoelastic properties that affect dough machinability depend strongly on water distribution into the dough.[38] Many researchers have reported that the mobility of water in food systems can be studied by measuring the protons spin–spin relaxation time.[38,39] In the wheat flour dough system, water interacts with gluten and starch to form a continuous network with dispersed particles, which are responsible for dough elasticity and extensibility.[40,41] The properties of the dispersed starch phase, the continuous gluten phase and interaction between the components contribute to the dough viscoelasticity. Rolée and Le Meste [42] who studied the change of the storage modulus (G′) in wheat starch preparations at different water content, observed an increase in the initial modulus when the moisture content was decreased. Studies on water distribution in dough are very complex because of the presence of numerous components such as starch, gluten, lipids as flour constituents, sucrose at different physical states and added fat.

CONCLUSION

Functional and rheological properties of flours play an important role in the quality of many processed foods, such as bakery products. Our results showed that the addition of DMGF to wheat flour, improved the functional properties of theses flour blends; most noticeably an increase in water and oil absorption and emulsion capacities, which all contribute the textural attributes of a processed product. The apparent viscosity and dough hardness also increased significantly with DMGF addition to wheat flour. In addition, the DMGF was shown to be rich in many nutrients, especially lysine—a major limiting amino acid in wheat. Thus, besides desirable effect on functional properties, another advantage of DMGF addition can be improved nutritional quality of food products to which it is added. The present study demonstrated that DMGF, a byproduct of the corn oil industry, has a great potential to be used for value-addition in a variety of food products.