Study of oil uptake during deep-fat frying of Taro (Colocasia esculenta) chips

Abstract

The effects of a water blanching pretreatment (BP, 85°C for 3 min), sample thickness (1 and 2 mm), oil temperature (180 and 200°C), and frying time (1 and 3 min) on the oil uptake (OU) behavior during the deep-fat frying of pre-dried (oven dried at 70°C for 20 min) taro (Colocasia esculenta) chips were investigated. Results demonstrated that using short frying times and high oil temperatures causes OU to decrease in both blanched and non-blanched samples (p < 0.01). In addition, higher product thicknesses were found to increase OU in non-blanched taro chips, while the opposite trend was found for the blanched slices (p < 0.01). BP also affected the OU, yielding lower fat contents (up to 80% of OU reduction) (p < 0.05), thus allowing the development of a fried taro product with reduced fat content (less than 30% fat content in dry basis).

Se investigó el efecto del escaldado con agua (85°C, 3 min), espesor de la muestra (1 y 2 mm), temperatura del aceite (180 y 200°C) y tiempo de freído (1 y 3 min) sobre la absorción de aceite (AA) durante el freído por inmersión de chips de malanga (Colocasia esculenta) pre-secados (70°C, 20 min). Los resultados demostraron que el uso de tiempos cortos de freído y temperaturas altas del aceite disminuyó la AA en las muestras escaldadas y no escaldadas (p < 0,01). Además, se encontró que la AA fue mayor en los chips de malanga no escaldados, mientras que la tendencia inversa se observó en los chips escaldados (p < 0,01). El escaldado disminuyó significativamente la AA (hasta un 80% de reducción) (p < 0,05), permitiendo el desarrollo de un producto frito de malanga con contenido reducido de grasa (menor al 30% base seca).

Keywords:

• blanching
• chips
• deep-fat frying
• oil uptake
• taro (Colocasia esculenta)

Palabras clave:

• chips
• escaldado
• freído por inmersión
• ganancia de aceite
• malanga (Colocasia esculenta)

1. Introduction

Taro (Colocasia esculenta) is one of the most widely cultivated edible roots in the tropical and subtropical countries. This tuber represents an important source of calories in developing regions of the world (4.2–4.4 kcal/g dry matter) and compares favorably with other similar products such as cassava (1.3–1.5 kcal/g dry matter) or sweet potato (3.9 kcal/g dry matter) (Emmanuel-Ikpeme, Eneji, & Essiet, 2007; Kaushal, Kumar, & Sharma, 2013). In addition, taro corms are rich in starch (21.1–26.2% wet basis) have reasonably high contents of potassium and magnesium (2251–4143 mg/100 g dry matter and 118–219 mg/100 g dry matter, respectively), and their essential amino acid profile is very similar to the Food and Agriculture Organization reference pattern, except for the sulfur-containing amino acids tryptophan and histidine (Huang, Chen, & Wang, 2007; Kaushal et al., 2013). Despite its nutritional relevance, taro has not received sufficient attention in the literature to enhance its potential, allowing a widespread use. Most of the existing studies on taro have focused on the characterization of physicochemical, thermal, and microstructural properties of flours, starches, and pastes (Aboubakar, Njintang, Scher, & Mbofung, 2008; Antonio-Estrada et al., 2009; Njintang et al., 2008; Nwokocha, Aviara, Senan, & Williams, 2009). However, the current technology for taro flour production is energy-intensive, as the tubers need to be boiled for as long as 30 min before they can be further processed into flour because of their high oxalate content (20.5–42.1 g/kg dry matter) (Aboubakar, Njintang, Scher, & Mbofung, 2009; Kaushal et al., 2013). The possibility of increasing its utilization lies in developing suitable processing technologies, securing consumer acceptance with marketable products, and achieving economic feasibility (Emmanuel-Ikpeme et al., 2007; Quiroz-Moreno, Fontes-Gagiola, Rouzaud-Sández, & Vidal-Quintanar, 2013). In this regard, Rodríguez-Miranda et al. explored the development of extruded snacks from taro and nixtamalized maize flour blends (2011); yet a cooking technology with shorter heating times would be economical from the industrial standpoint (Aboubakar et al., 2009). As it happens with other starchy foods such as potatoes, cassavas, plantains, yams, or sweet potatoes, a feasible way of increasing taro consumption is through fried product development.

Deep-fat frying is one of the oldest and most widely used cooking methods to prepare tasty and crispy foods (Pedreschi, 2012; Pedreschi & Moyano, 2005a). This operation consists of the immersion of the food materials in edible oils heated above the water boiling point and produces desired quality attributes such as color, flavor, texture, and mouthfeel (Pedreschi, 2012). However, this process is incompatible with recent consumption trends (e.g. healthier foods and low-fat products), which has motivated the investigation of strategies to reduce the oil uptake (OU), mainly through the use of various pretreatments such as blanching (Pedreschi, Cocio, Moyano, & Troncoso, 2008; Pedreschi & Moyano, 2005b; Reis, Masson, & Waszczynskyj, 2008), drying (Debnath, Bhat, & Rastogi, 2003; Pedreschi & Moyano, 2005a; Song, Zhang, & Mujumdar, 2007a, 2007b; Tajner-Czopek, Figiel, & Carbonell-Barrachina, 2008), osmotic dehydration (Ikoko & Kuri, 2007; Pedreschi, Moyano, Santis, & Pedreschi, 2007), coatings (Suárez, Campañone, García, & Zaritzky, 2008), or their combinations (Fan, Zhang, & Mujumdar, 2006; Moyano & Pedreschi, 2006; Rimac-Brnčić, Lelas, Rade, & Šimundić, 2004). Blanching in hot water is frequently used prior to frying for color and texture improvement. The use of a blanching pretreatment on its own has been demonstrated to enhance OU during frying because of increased moisture content and microstructural changes in the product (Fan et al., 2006; Moyano & Pedreschi, 2006; Pedreschi & Moyano, 2005b; Pedreschi et al., 2008). However, blanched samples subjected to further drying have been reported to absorb less oil than fresh and non-pretreated dried samples (Fan et al., 2006; Moyano & Pedreschi, 2006). In addition to the aforementioned impact of blanching on the OU behavior, a hot water pretreatment may help in extracting and reducing the naturally high oxalate content of taro corms, as demonstrated by Savage and Mårtensson (2010). Besides, frying conditions such as oil temperature, frying time, and sample size have an important effect on the OU behavior of food products. For example, oil absorption is enhanced for longer frying times and higher specific areas of foods (Krokida, Oreopoulou, & Maroulis, 2000). On the other hand, the use of increasing frying temperatures may contribute for reducing OU (Maneerote, Noomhorm, & Takhar, 2009, 2008; Pedreschi et al., 2007; Pedreschi & Moyano, 2005b). Nevertheless, the use of increasing frying temperatures has shown a reverse OU trend in some studies (Debnath et al., 2003; Krokida et al., 2000). Thus, several studies aimed at establishing process conditions for the development of low-fat products have been published (Sobukola, Awonorin, Sanni, & Bamiro, 2008; Song et al., 2007b).

To date frying of starchy foods has mainly been studied for potatoes with very scarce publications concerning the frying behavior of other tubers such as cassava (Vitrac, Dufour, Trystram, & Raoult-Wack, 2002), sweet potato (Da Silva & Moreira, 2008; Farinu & Baik, 2008; Taiwo & Baik, 2007), or yam (Sobukola et al., 2008). Selected studies on fried taro products include those of Emmanuel-Ikpeme et al. (2007), which explored both storage stability and sensory evaluation of pre-blanched (80°C, 5 min) taro chips fried in pure and blended edible oils (220°C); and of Ahromit and Nema (2010), which studied the heat and mass transfer of taro cylinders (7.5 cm long, 1 cm diameter), fried at a single temperature (180°C). Recently, Ukpabi, Chijioke, and Mbanaso (2013) investigated the feasibility of producing energy-rich taro crisps with acceptable color, taste, and crispness during frying with either crude red or refined palm oils. However, the authors did not report any of the process conditions used (temperature, time, product-to-oil ratio). No information about the combined effect of pretreatments and processing variables on the oil absorption during frying of this lesser-known tuber has been published, which is essential for the production of ready-to-eat foods with desirable properties such as reduced fat content. Therefore, the objective of this study was to investigate the effect of a blanching pretreatment and process conditions (frying temperature, frying time, sample thickness) on the OU behavior during the deep-fat frying of pre-dried taro chips.

2. Materials and methods

2.1. Raw material pretreatments

Taro (C. esculenta) corms were kindly supplied by local producers from Valle Nacional, Oaxaca (Mexico). Taro corms were washed thoroughly with tap water, peeled by hand with a knife, and then cut into slices using an electric slicing machine (Tecmal, model Argenta, Italy). The resulting slices were soaked in a 4% (w/v) aqueous NaCl solution at 25°C for 2 min (taro to brine ratio ≈ 0.75 w/v). The BP consisted of the immersion of taro slices in hot water at 85°C for 3 min (taro to water ratio ≈ 0.3 w/v). Blanched and non-blanched taro slices were blotted with a paper towel before drying to remove the excess water or brine. All samples were oven-dried at 70°C for 20 min prior to frying (Binder, model ED 53, Germany).

2.2. Frying process

A complete two-level factorial design was conducted to evaluate the effect of frying temperature (180 and 200°C), product thickness (1 and 2 mm), process time (1 and 3 min), and blanching pretreatment (with and without blanching) on the OU behavior of fried taro chips. Taro slices were deep-fried in hot general purpose soybean oil (Bakers & Chefs, Mexico) using a 3.5 l capacity electrical fryer (Turmix, model FT-2, Mexico) adapted with a PID temperature controller (Yuyao Gongyi meter, model XMTG-818, China) to maintain the set frying temperature within ±1°C. The fryer was filled with 2 l of oil and the taro-to-oil ratio was kept at 1:40 w/v. The oil was preheated for 30 min prior to frying and discarded after 6 h. Before each frying test, the oil level was checked and replenished as needed. After frying, the slices were allowed to drain and cool for 5 min at room temperature (25°C). From each experiment, OU (expressed as kg oil/kg dry solids) was obtained as the response variable. All treatments were carried out in triplicate. The complete design included 48 experiments. The experimental data obtained from frying experiments were fitted to the following linear regression model with main and interaction effects:

(1)

where x₁, x₂, and x₃ are the coded levels for the frying temperature, product thickness, and process time, respectively.

Relative OU reduction () between treatments with and without blanching pretreatment was calculated according to

(2)

2.3. Analytical methods

The moisture content of both fresh taro and taro slices after blanching was measured by drying the samples in a convection oven at 105°C until constant mass weight. Oil absorption of fried samples was determined gravimetrically. After frying, taro chips were ground and oven-dried until constant mass by heating at 105°C. Then, in order to determine their oil content, 2 g of dried samples were extracted with boiling petroleum ether (35–60°C boiling point range) for 4 h using a Soxhlet apparatus and later oven-dried to eliminate the solvent. The oil content in product was calculated as the weight difference between oven-dried samples before and after the extraction procedure. In addition, color, texture, and moisture content of the four frying treatments with the lowest OU were determined. The CIE (Commission Internationale de l’Éclairage) L*, a* and b* color parameters of fried chips were measured using a MiniScan 45/0 L colorimeter (Hunter Lab, Inc., Reston, VA, USA), which was calibrated according to the apparatus specifications. The texture of fried taro slices was determined using a TA-XT2i Texture Analyzer (Texture Technologies Corp., Scarsdale, NY) as the required force to break the chips using a three point cell at 0.5 mm/s of velocity. All analyses were carried out in triplicate.

2.4. Statistical analyses

Tukey’s pairwise comparisons were performed to determine significant differences in OU behavior between the corresponding treatments with and without blanching (p < 0.05). The dependency of OU on the process variables (frying temperature, product thickness, and process time) for both blanched and non-blanched taro chips was expressed using a linear regression model with main and interaction terms. Following data fitting, an analysis of variance (ANOVA) of the linear regression models was carried out to evaluate the effect of process conditions on OU behavior of taro chips. Statistical analyses were performed using the Matlab Statistics Toolbox (Mathworks Inc., Natick, MA). Additionally, the goodness of fit of the obtained models was evaluated using the coefficient of determination (R²).

3. Results and discussion

The complete experimental design along with the mean OU obtained for each treatment is shown in Table 1. The OU ranged from about 26.04 ± 0.92 to 71.45 ± 9.08 kg oil/100 kg dry solids in non-blanched taro chips, while in the blanched slices, the OU was found to fluctuate between 7.50 ± 0.34 and 30.49 ± 3.37 kg oil/100 kg dry solids. As shown in Table 1, the effect of BP application on the OU was significant with the resulting lower fat contents (up to an 80% OU reduction) (p < 0.05).

Table 1. Experimental design for the study of blanching pretreatment and process variables on the oil uptake behavior of pre-dried taro chips.

Tabla 1. Diseño experimental para el estudio del efecto del tratamiento de escaldado y variables de proceso sobre la absorción de aceite de chips de malanga pre-secados.

Run	Factors*			Oil uptake (kg oil/kg dry solids)**		Relative OU reduction (%)
	Temperature (°C)	Product thickness (mm)	Frying time (min)	Blanched slices	Non-blanched slices
1	180 (−1)	1 (−1)	1 (−1)	0.3019 ± 0.0190^a	0.4290 ± 0.0161^b	29.63
2	180 (−1)	1 (−1)	3 (1)	0.3049 ± 0.0337^a	0.7145 ± 0.0908^b	57.33
3	180 (−1)	2 (1)	1 (−1)	0.1690 ± 0.0052^a	0.6439 ± 0.0277^b	73.75
4	180 (−1)	2 (1)	3 (1)	0.2889 ± 0.0059^a	0.6142 ± 0.0007^b	52.96
5	200 (1)	1 (−1)	1 (−1)	0.0750 ± 0.0034^a	0.2951 ± 0.0288^b	74.58
6	200 (1)	1 (−1)	3 (1)	0.1661 ± 0.0038^a	0.4427 ± 0.0098^b	62.48
7	200 (1)	2 (1)	1 (−1)	0.0779 ± 0.0051^a	0.2604 ± 0.0092^b	70.08
8	200 (1)	2 (1)	3 (1)	0.1072 ± 0.0029^a	0.5909 ± 0.0675^b	81.86

Notes: *Values in parenthesis represent the coded factor levels.

**Data expressed as mean ± standard deviation of three independent experiments. Values in a row followed by the same lowercase letter indicate no significant difference between blanched and non-blanched samples processed in the same temperature–thickness–time combination according to Tukey’s test (p < 0.05).

Notas: *Los valores en paréntesis representan los niveles codificados de los factores.

**Datos expresados como la media ± desviación estándar de tres experimentos independientes. Los valores en una fila seguidos por la misma letra minúscula indican que no existe diferencia significativa entre las muestras escaldadas y no escaldadas procesadas a la misma combinación de temperatura-espesor-tiempo de acuerdo a una prueba de Tukey (p < 0.05).

The summarized statistics for the linear regression analysis of blanched and non-blanched fried taro chips are shown in Tables 2 and 3. The corresponding experimental and predicted results are plotted in Figures 1 and 2. A good agreement was found between the experimental and predicted data for the blanched (R² = 0.9846) and non-blanched (R² = 0.9569) taro chips.

Table 2. Analysis of variance (ANOVA) for determining the effect of process variables on the oil uptake of fried taro chips without blanching pretreatment.

Tabla 2. Análisis de varianza (ANDEVA) para determinar el efecto de las variables de proceso sobre la absorción de aceite en chips de malanga fritos sin pretratamiento de escaldado.

Parameter	Value × 10^2	DF	SS × 10^4	MS × 10^4	F	p-Level
	49.885	1	59,723.3	59,723.3	3221.4	<0.001
	−10.156	1	2475.6	2475.6	133.53	<0.001
	2.8514	1	195.13	195.13	10.525	0.005
	9.1727	1	2019.33	2019.33	108.92	<0.001
	−0.0125	1	0.0037	0.0037	0.0002	0.989
	2.7791	1	185.36	185.36	9.9979	0.006
	−1.6529	1	65.570	65.570	3.5367	0.078
	6.2280	1	930.92	930.92	50.2119	<0.001
Residual		16	296.64	18.540

Table 3. Analysis of variance (ANOVA) for determining the effect of process variables on the oil uptake of fried taro chips with blanching pretreatment.

Tabla 3. Análisis de varianza (ANDEVA) para determinar el efecto de las variables de proceso sobre la absorción de aceite en chips de malanga fritos con pretratamiento de escaldado.

Parameter	Value × 10^2	DF	SS × 10^4	MS × 10^4	F	p-Level
	18.637	1	8336.2	8336.2	4111.9	<0.001
	−7.9815	1	1528.9	1528.9	754.15	<0.001
	−2.5628	1	157.63	157.63	77.753	<0.001
	3.0419	1	222.08	222.08	109.54	<0.001
	1.1625	1	32.435	32.435	15.999	0.001
	−0.0318	1	0.0243	0.0243	0.0120	0.914
	0.6911	1	11.464	11.464	5.6547	0.030
	−2.2320	1	119.56	119.56	58.975	<0.001
Residual		16	32.437	2.0273

Figure 1. Experimental and predicted oil uptake behavior of fried taro chips (process time = 1 min).

Figura 1. Absorción de aceite experimental y predicha de chips de malanga fritos (tiempo de procesamiento = 1 min).

Figure 2. Experimental and predicted oil uptake behavior of fried taro chips (process time = 3 min).

Figura 2. Absorción de aceite experimental y predicha de chips de malanga fritos (tiempo de procesamiento = 3 min).

The effect of frying temperature, product thickness, and process time on OU may be deduced from the model parameters. As expected, OU of both blanched and non-blanched fried taro chips was higher with longer frying times (p < 0.01), which is in agreement with previous reported data in other products (Diaz, Totte, Giroux, Reynes, & Raoult-Wack, 1996; Fan et al., 2006; Krokida et al., 2000; Moyano & Pedreschi, 2006). In addition, the statistical analysis of results demonstrated that increasing oil temperature causes a decrease in OU in both the blanched and non-blanched samples processed in the temperature range of 180–200°C (p < 0.01). The reduction in OU for increasing frying temperatures has been widely documented during the deep-fat frying of other starchy foods such as potato chips, for frying temperatures of 120–180°C (Moyano & Pedreschi, 2006; Pedreschi & Moyano, 2005a, 2005b; Pedreschi et al., 2007) or rice crackers in the range of 200–240°C (Maneerote et al., 2009). However, in some studies, using higher frying temperatures increased OU. For example, Diaz et al. (1996) reported an increase in fat gain of deep-fat fried plantain chips by using higher temperatures from 110°C to 150°C, but the opposite trend was observed in the range of 150–190°C. Vitrac et al. (2002) observed that reducing the oil temperature from 160°C to 140°C resulted in a lower oil content of cassava chips for the same frying time. At high frying temperatures, the OU reduction may be explained by the enhanced crust formation, which acts as a physical barrier for oil absorption (Dana & Saguy, 2006).

The thickness of taro chips had a significant effect on the amount of oil absorbed during frying in both blanched and non-blanched samples (p < 0.01). For the non-blanched slices, a higher OU was obtained with thicker taro chips. However, a reversed trend in the OU was obtained in the blanched slices, which can be explained by an enhanced moisture gain of the thinner samples during BP because of their higher surface/volume ratio or structural changes due to tissue softening. In fact, the moisture content of taro chips increased from 64.47 ± 0.25 g water/100 g product in fresh taro to 68.54 ± 0.41 and 65.86 ± 0.45 g water/100 g product in the 1 and 2 mm slices, respectively. Krokida et al. (2000) found that the oil content during frying of only-blanched potato strips was higher for lower thickness values and for the same frying times in strips having a cross-section of 5 × 5, 10 × 10, and 15 × 15 mm² and a length of 40 mm. Recently, Tajner-Czopek et al. (2008) found that fat gain during frying of blanched and pre-dried potato strips of 10  mm × 10 mm × 70 mm was higher than that obtained in samples of smaller size (8 mm× 8 mm × 70 mm). Nevertheless, these authors used a longer frying time for the bigger potato strips to obtain an equivalent texture in the final products.

The present findings can be used to select process conditions allowing the development of a reduced-fat fried product from taro when a blanching pretreatment is used. However, it is important to notice that the combination of the shortest frying time (1 min) and lowest frying temperature (180°C) resulted in undercooked products. On the other hand, the combined use of longer times (3 min) and higher temperatures (200°C) produced chips partially burnt on the surface. Therefore, these products were discarded for further characterization and only color parameters, texture, and moisture content of the remaining treatments were investigated (Table 4). As expected, a significant difference in breaking force was obtained between the two sample thicknesses (p < 0.05). All these fried chips can be considered as acceptable in terms of sensory attributes and storage stability as they contain about 10% or less of moisture content and 30% or less of oil content (Sobukola et al., 2008). It is important to notice that the fat content in some selected fried taro chips was lower than those observed in two commercial fat-reduced potato snacks (27.32 ± 1.24 kg oil/100 kg dry solids in fried potato chips and 12.91 ± 0.48 kg oil/100 kg dry solids in baked potato strips), which is valuable from the nutritional standpoint.

Table 4. Color, texture, and moisture content analyses of the treatments with the lowest oil uptake in blanched samples.

Tabla 4. Análisis de color, textura y humedad de los tratamientos con menor absorción de aceite en muestras escaldadas.

Parameter	Treatments*
	180°C, 1 mm, 3 min	180°C, 2 mm, 3 min	200°C, 1 mm, 1 min	200°C, 2 mm, 1 min
L*	48.93 ± 8.02^a	53.65 ± 5.56^a	48.73 ± 3.06^a	54.52 ± 3.65^a
a*	11.36 ± 5.22^a	5.13 ± 1.63^b	7.77 ± 1.01^a	2.33 ± 1.30^c
b*	15.76 ± 3.40^a	22.74 ± 2.2^b	12.8 ± 1.60^a	15.14 ± 0.26^a
Hardness (N)	214.75 ± 94.64^a	337.8 ± 61.88^b	207.88 ± 59.48^a	355.45 ± 90.42^b
Moisture content (% w.b.)	4.51 ± 0.37^a	7.34 ± 0.39^b	4.16 ± 0.23^a	10.75 ± 4.33^b

Notes: *Data expressed as mean ± standard deviation of three independent experiments. Values in a row followed by the same lowercase letter indicate no significant difference between treatments according to Tukey’s test (p < 0.05).

Notas: *Datos expresados como la media ± desviación estándar de tres experimentos independientes. Los valores en una fila seguidos por la misma letra minúscula indican que no existe diferencia significativa entre tratamientos de acuerdo a una prueba de Tukey (p < 0.05).

4. Conclusions

The relationship between OU and processing parameters during production of fried taro chips was successfully determined. All studied variables had a significant effect on OU behavior of the product. The application of a blanching and drying pretreatment before frying was demonstrated to reduce OU of taro chips up to 80%. Overall, OU reduced with shorter frying times and higher oil temperatures in both blanched and control samples. However, some factor combinations lead to opposite OU behaviors in blanched and non-blanched samples, which cannot be simply extrapolated from studies on other products due to differences in structure and composition. Therefore, similar studies are needed in order to characterize the complex frying behavior in other lesser-known tubers such as taro. The present findings can be used for the development of a fried product from taro with less than 30% of fat content (dry basis).

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

I.I. Ruiz-López http://orcid.org/0000-0002-6592-6838