Optimization of Blanching Conditions Prior to Deep Fat Frying of Yam Slices

Abstract

The effect of low-temperature blanching and frying time at a frying temperature of 170°C on moisture and oil contents, breaking force and colour of yam chips was investigated using response surface methodology to establish the optimum blanching conditions and frying time. A central composite rotatable design was used to study the effects of variation in levels of blanching temperature (60–80°C), blanching time (1–5 min) and frying time (2–6 min) on quality attributes of yam chips. The effect of blanching temperature and frying time was more significant than the time of blanching on the quality attributes. The response variables were fitted to predictive models applying multiple linear regressions. Statistical analysis with response surface regression showed that moisture content, oil content, breaking force and L∗ (lightness) parameter were significantly (P < 0.05) correlated with blanching temperature and time and frying time. However, the regression equation showed a poor fit for a∗ and b∗ respectively. The optimum conditions were a blanching temperature of 70–75°C, blanching time of 4–5 min while frying for about 5 min.

Keywords:

• Blanching
• Yam slices
• Deep fat frying
• Optimization
• Yam chips

INTRODUCTION

Yams (Dioscorea spp) are annual or perennial climbing plant with edible underground tubers. More than 95% of the world's yams are currently grown in Sub-Saharan African with the remainder grown in the West Indices and parts of Asia and South and Central America.[1] Its production is confined to the “yam zone” comprising Cameroon, Nigeria, Benin, Togo, Ghana and Cote d'Ivore where 90% of the 37.5 million tones originated from. Yam is the second most important root or tuber crop in Africa with production reaching just under one third level of cassava.[1] Yam tubers are consumed in many different forms including roasted, baked, pounded, mashed and fried.[2]

Deep fat frying is a simultaneous heat and mass transfer process, which leads to a succession of physical and chemical changes in a product.[3] The fried products have an attractive colour (golden brown), distinctive mouth feel, pleasant taste as well as fried flavours and unique textural properties (crispy crust formation). Thus in spite of significant fat transfer to the products, the frying has been favoured due to its ability to create unique organoleptic properties. In deep-fat fried products, both health and sensory aspects should be addressed to meet consumers demand. High heat transfer rates are largely responsible for the development of the desired sensory properties in fried products.[4] Due to public health concerns, there is a strong demand to reduce the oil content of fried foods.[5,6] Pretreatments can have a great effect, particularly on the sensory properties and oil contents of fried products. In recent years, much research has been concentrated on the development of food products that have reduced fat and cholesterols levels.

There are alternative methods to the manufacture of fried products with reduced fat and cholesterols levels. Instead of frying in fat substitutes, reformulation of products can result in less fat pick-up during the frying process. Uses of such ingredients technology have been reported previously.[7,8] Another pretreatment method involves conventional frying with premature removal from the fryer at a high moisture content (about 10%) and finish processing using conventional air drying.[8] The use of superheated steam instead of hot air has also been proposed.[9,10] However, this method have the disadvantages of higher product temperature (>100°C) which can cause damage to heat sensitive products and the need for some additional devices for loading and unloading of the material.

The effects of pre-frying drying on frying kinetics and quality of French fries have also been examined.[11,12] Their results showed decreased fat content of French fries and significantly affected colour and structural properties of French fries. Another technique, a low-temperature blanching (LTB) process, has been reported by numerous authors, offering a promising approach to improving sensory properties of fried products. The literature gives different optimum blanching conditions for different processing operations.[13,14,15,16,17] Blanching in hot water or chemical solutions before frying have been reported to improve colour and texture of potato and could reduce oil uptake in some cases by gelatinization of the surface starch.[18,19,20] Blanching pretreatment has been performed on potatoes and other vegetables to denature the enzymes responsible for unacceptable darkening and off-flavour. It also results in loss of soluble solids, air removal from tissue, hydrolysis, and solubilization of structural polymers, such as protopectin and gelatinization of starch granules.

One of the most commonly used optimization technique is response surface methodology (RSM). RSM has been widely and effectively used in industrial investigations and other processes such as the development and/or improvement of the nutritional products due to its practical utility in their optimization.[21] This methodology presupposes the use of experimental design techniques to investigate and learn about the functional form of the process or system that involves one or more response variables that are influenced by various factors, or independent variables. An appropriate experimental design is fundamental for enabling the researchers to explore the process under study and leading him successfully to its optimization obtaining the maximum or the minimum, if they exist, or to determine a region in the total space of the factors in which certain desirable operating conditions are satisfied.[22]

Most research reports available in the literature relates to products like Tofu[23]; French fries[24]; potato strips[11]; potato[16,25,26,27,28]; Donut[29]; Meat ball[30] and cassava chips.[31] No information is available on the deep fat frying of yams slices with respect to optimizing the conventional blanching conditions as it affects quality parameters. Hence, the purpose of the present research was to investigate how yam chips quality is affected by LTB applied prior to frying. A second objective based on the result is using response surface methodology (RSM), to find the optimum blanching temperature, and time and frying time to improve the quality of yam chips with respect to moisture and oil contents, breaking force, and colour parameters.

MATERIALS AND METHODS

Matured yam tubers (Dioscorea rotundata) were purchased from a local market in Abeokuta, Nigeria and were of specific gravity 1.368kg/m³. Refined bleached deodorized palm olein supplied by Ngo Chew Hong Ltd was used as the frying medium. Yam tubers were stored at 25°C and 85% RH before use. Slices of 30 × 45 × 1.2mm were obtain from the tubers using stainless steel knife and vegetable slicer (Model ART No: SF-923-1, Texas USA). Slices were then fixed on stainless steel rods to ensure defined and constant position of slices in water thus preventing clogging and floatation. Surfaces starch was removed by lowering the rods with slices vertically into bath maintained at 30 ± 1°C for 2–3 min. Slices were then placed between moistened towels to remove excess surface water.

Blanching Operation

Conventional blanching in hot water was carried out by immersing rods containing yam slices in a water bath (Clifton NE 1–22/15136, England) at 60, 70, and 80°C for 1–5 min with a constant product weight/water volume of 1:20. The method had earlier been used.[16,32] Blanched samples were then air dried at 27–30°C (±1°C) for about 1 min to remove surface moisture before frying.

Frying Operation

To actually study the effect of blanching condition and thus optimizing, a fixed frying temperature (170 ± 1°C) was used throughout the experiments. Pre-treated and control slices were fried in a deep fat fryer (Model S 576, Hong Kong, China) with temperature control of ± 1°C. The vat holds 1.5l of oil and equipped with a 2kW electric heater. Constant temperature of frying was maintained by keeping the yam-to-oil weight ratio as low as possible (∼0.0035).[11,33] For each selected frying time, the slices were drained after frying over a stainless steel basket and allowed to cool to room temperature before analyses were carried out. The oil was preheated for 1hr before frying and discarded after 6hrs of use.[34] Slices were fried for 10–240 s.

Optimization Procedure

RSM technique was used to optimize the blanching pretreatment condition prior to frying of yam slices. Three independent variables in the process were chosen for the study thus 15 combinations including three replicates of the center point were performed in random order, based on a central composite rotatable experimental design for three factors. Range finding experiments were performed at the outset of this work in order to ascertain what blanching temperature and time and frying time could be applied to the yam slices so that the product would be acceptable to consumers on the basis of sensory attributes. Six independent variables namely moisture content (Y₁), oil content (Y₂), breaking force (Y₃), lightness (Y₄), redness (Y₅), and yellowness (Y₆) were considered to evaluate the effects of the independent variables.

Analyses

Moisture content determination was done by the oven dry method at 105 ± 1°C for 5 h.[35] Analysis was done in triplicates. The oil content was determined using the soxhlet extraction method with diethyl ether.[36] The breaking force interpreted as chips crispness was carried out using a universal testing machine (Model –M500, Testometric AX, Rochdale, England) equipped with a 50 kN load cell. Samples of uniform sizes were selected and placed on a metal support with jaws at a distance of about 25 min. A cylindrical flat end plunger (70 mm diameter) moving at a speed of 2.5 mm/min presses the samples in the middle. The measurement was recorded by a computer connected directly to the equipment. The breaking force was obtained as the peak force from the force-deformation curve.[37,38,39]

Colour parameters (L∗, a∗ and b∗) were measured using a colourimeter (Colour Tec –PCM TM, USA). The equipment was standardized each time with a white and black ceramic plate. Samples were scanned at five different locations to determine lightness (L∗), redness (a∗), and yellowness (b∗) values as the average of five determinations.

Statistical Analysis

Data obtained were analysed using SPSS 10.0 version and significance expressed at the P < 0.05 level. For optimization procedure, differences in dependent variables as a result of different blanching conditions were studied by ANOVA using Tukey's test with a 95% confidence interval for the comparison test means. Data were analysed by multiple linear regression using the method of least squares to fit the second other equation to all the responses for coefficient of determination (R²) value, probability value, F-ratio and the regression coefficients.

(1)

where a_o, a₁ – a₃, a₁₁ – a₃₃, and a₁₂ – a₁₃ are the equation regression coefficients for intercept, linear, quadratic and interaction coefficients respectively, X₁ – X₃ are coded independent variables. The significance of the equation parameters for each dependent variable was assessed by F-test. The analysis was carried out by Statistical Analysis System (SAS, Version 8.0, SAS Institute Inc. Cary, NC, USA).

RESULTS AND DISCUSSION

Moisture and Oil Contents

The moisture and oil contents for both control and blanched samples are as shown in Figure 1. The moisture content curves showed a classical drying profile with no constant drying rate for a frying temperature of 170°C. The drying profile takes place majorly in the falling rate period which is similar to behaviour of many agricultural products during processing. Conventional hot water blanching has been reported to cause starch gelatinization resulting in different microstructure, however, no significant difference in the drying profile was observed (P > 0.05). Blanching is known to increase the permeability of cytoplasmic membranes, allowing the blanch water to penetrate cells and intercellular spaces, driving out gases and other volatile compounds,[40] causing losses of soluble substances such as vitamins, salts and sugars, and increasing the moisture content of the samples.[17] In this work, blanching before frying had no significant effect (p > 0.05) on the moisture content which increased slightly when the slices were blanched before frying. After blanching, the slices appeared swollen but moisture was rapidly released when placed in hot oil in response to loss of osmotic integrity and cellular damage as a result of blanching.[16] Similar trend were observed for samples blanched at different temperatures (60–80°C) for land 3 mins.

Figure 1 Effect of blanching temperature for 5 min on Moisture and oil contents of fried yam chips fried at 170°C.

With respect to oil content, blanching temperature and time seems to affect oil transfer during deep fat frying. As shown in Figure 1, blanching temperature of 70°C at 5 min for short frying time (about 50 s) resulted in lower oil content as compared to other blanching temperatures. Blanching has been reported as a pre-treatment operation that could improve the colour and texture of chips and reduce their oil uptake by gelatinization of surface starch.[18] Besides, some authors have reported that low-temperature blanching (e.g., 55 – 70°C) before frying activates pectin esterase enzyme (PME) and the resulting reactions decrease porosity and reduced oil uptake.[41,42] What the enzyme does is to demethylate the carboxymethyl groups of pectic polysaccharide chains. The decrease in the degree of methylation may in turn trigger different processes related to texture and firmness such as cross-linking by Ca²⁺ ions, increased hydration at the demethylated sites, reduced susceptibility for heat induced β-degradation of pectin's, and enhanced shielding and repulsion forces by the electric charges within the biopolymer matrix of the cell walls.[17] Since oil adsorption may be regard as a surface phenomenon, the activities of these enzymes may favour or otherwise the oil content of yam chips. In this work, our results is contrary to those reported,[16] of whom stated that blanching at high temperature and short time (e.g., 97°C for 2mins) before frying resulted in higher oil content of potato chips that in control samples. Differences can be attributed to observable differences in the structure of our test material (yam) and potato.

Figure 2 shows the effect of blanching time on the oil content of yam chips. Slices blanched at 70°C for 5min resulted in observable lower oil content which is significantly (P < 0.05) different from slices blanched for shorter time at the same temperature. The longer the period of blanching, the more gelatinized the surface starch which decreased porosity and subsequently lower oil content. When the curves (moisture and oil) in Figures 1 and 2 are closely examined, it appears the oil content is related to moisture content. These seems to agree with the report of[11,33] for potato chips. When yam slices are placed in hot oil, the free water is rapidly lost in the form of bubbles. As frying continues, the outer surface dries out; improving hydrophobicity and oil many adhere to the chips. When the chips are removed from the fryer, the vapour inside the pores condenses and the difference in pressure between the surrounding tissue and the pores causes the adhering oil on the surface to be absorbed into the pore spaces. However, if yam chips contain more moisture, this higher moisture content will prevent the oil from entering the pore spaces.

Figure 2 Effect of blanching time at 70°C on Moisture and oil contents of fired yam crisps fried at 170°C.

Optimization of Blanching Conditions

Table 1 shows the coded and uncoded values for the independent variables considered in this optimization procedure. According to the response surface regression analysis of each independent variable in the model, the main effect of temperature and time of blanching and time of frying have obvious effects on the response variables. Regression coefficients of each response model fitted were tested for significance and are presented in Table 2, together with coefficient of determination, R², F-Ratios, and P-values. Response models with R² <0.75 were omitted as they showed very low percentages of explained variability, indicating a significant lack of fit [16,43] but will be discussed based on trends.

Table 1 Coded values of independent variables

	Codes
Variable	−1	0	+1
X1 (°C)	60	70	80
X2 (min)	1	3	5
X3(min)	2	4	6

Table 2 Regression coefficients, coefficient of determination (R²) and analysis of variance of regression models for moisture and oil contents, breaking force and colour parameter (L∗) for yam chips at the design response surface

Coefficients	Y1	Y2	Y3	Y4
ao	224.266	100.464	8256.822	179.729
a1	−3.942∗∗	−2.735∗	−90.479∗∗∗	−5.232∗
a2	−6.033∗	−0.037∗	−379.419∗∗∗	9.589
a 3	−24.205∗∗∗	−1.079∗	−1227.381∗∗∗	15.675
a 11	0.021	0.020	0.172	0.044
a 22	0.649	−0.023	18.734	−0.998
a 33	1.534∗∗∗	−0.197∗	53.766∗∗∗	−0.507
a 12	0.069	−0.001∗	5.215	−0.073
a 13	0.123	0.046	8.181	−0.184∗∗∗
a 23	−0.544∗∗	0.049	−15.188∗∗	0.606
R^2	0.9857	0.9532	0.9954	0.7811
F-Ratio	38.294	11.312	119.623	1.983
P-value	0.00044	0.00784	3E −005	0.2334
Subscript: 1 = blanching temperature; 2 = blanching time; 3 = frying time Significant level: ∗0.001; ∗∗0.01; ∗∗∗0.05.

The model for moisture content of blanched samples fitted with R² > 0.85 had significant linear (temperature and time of blanching and time of frying), quadratic (time of frying) and interaction (time of blanching and frying) terms. To visualize the combined effects of these factors on the response, the response surface (Figure 3) and contour plots (Figures 4 and 5) were generated for each of the fitted model as a function of the factors concerned. Figure 4a–c, shows the moisture content of blanched yam chips as time of frying (X₃ = 0), blanching time (X₂ = 0), and blanching temperature (X₁) are held constant respectively. The contour plots clearly showed decreasing moisture content as blanching temperature and frying time increases while blanching time decreases. Since chips with lower moisture content corresponds to better crispness which is desirable by consumers, obtaining such chips might involve blanching temperature of >70°C for 4–5 min and frying time of 5–6 min. Fig. 4d – f presents the contour plots of the oil content as affected by the independent variables. From the plots, an increase in temperature of blanching and decrease in blanching time seems to produce yam chips of increasing oil content. Obviously, an increase in time of frying increases the oil content of chips. The fitted model had a significant linear (a₁, a₂, and a₃), quadratic (a₃₃), and interaction (a₁₂) terms.

Figure 3 The surface plots of moisture content (Y₁, %), oil content (Y₂, %), breaking force (Y₃, N), lightness (Y₄), redness (Y₅) and yellowness (Y₆) of fried yam chips as affected by blanching temperature (X₁), blanching time (X₂), and fixed level of frying time (X₃ = 0).

Figure 4 The contour plots for Moisture content (Y₁, %), Oil content (Y₂, %) and Breaking force (Y₃) of fried yam chips as affected by Blanching temperature (X₁), blanching time (X₂), and frying time (X₃) for chips fried at 170°C.

Figure 5 The contour plots for colour parameters: Lightness (Y₄) of fried yam chips as affected by Blanching temperature (X₁), blanching time (X₂), and frying time (X₃) for chips fried at 170°C.

Most studies on potato and tortilla chips have claimed a higher initial moisture in samples results in higher oil content.[11,38,39,42] Similar trend was observed with yam chips and it appears that having a higher moisture content in the final product resulted in a low final oil content since the internal volume of the slice to be occupied by oil during frying is less. Hence, increasing the water content in undamaged cells should therefore be one of the main purposes of the blanching step. Conventional blanching in hot water seems to make the frying process mainly a function of time as decreasing the frying time decreases oil content throughout the process. Considerable lower oil content in yam chips can be obtained using blanching temperatures of 70–75°C for 3–5 min while frying for about 5 min.

Conventional blanching of yam slices before frying significantly (P < 0.05) affected the breaking force which is interpreted as chips crispness. Deep fat frying is a dehydration process and tends to reduce the moisture content of the product. A lower breaking force corresponds to a crispier product which is usually preferred by consumers. The model had significant linear and quadratic terms for time of frying (a₃ and a₃₃, respectively), a significant linear term for temperature and time of blanching (a₁ and a₂, respectively) and the interaction was significant in both time of blanching and frying (a₂₃). A close examination of the contour plots in Figure 4g–i showed that at blanching temperature of 70–80°C for >4 min, the breaking force tends to decrease to less than 1000N. The plot also showed a clear dependence of breaking force on frying time. As frying proceeds, more moisture is being removed from the product thus tending towards crispness. Similar observations have been reported for carrot and potato chips respectively.[38,39] From Table 2, the quadratic effect of time of frying was greater than its main effects on breaking force. The contour plot showed that as blanching temperature and frying time increased and approached the upper limit of the experimental range, there was a decrease in breaking force hence increase in crispness that is desired in yam chips.

Colour of yam chips is an important parameter to be controlled during processing together with crispness, oil and moisture contents. Yam chips colour is the result of the Maillard reaction that depends on the content of reducing sugars and amino acids or proteins at the surface, the temperature and time of frying. The aspect of colour of the food surface is the first quality parameter evaluated by consumers and is critical in the acceptance of the product, even before it enters the mouth.[33] For an acceptable yam chips, the colour must be light, golden yellow and not dark. The proposed model will only be used for L∗ (lightness parameter) since the R² > 0.75 but trends during processing will only be discussed for a∗ (redness) and b∗ (yellowness) parameters. Common factors that cause poor modelling are a skewed response distribution, curvature in the relationship between a response variable and a design factor, large difference between-replicate variation and outlying experiments.[44] The model fitted for L∗ parameter had a significant linear term for temperature (a₁), a quadratic term for time of frying and an interaction term for temperature of blanching and time of frying (a₁₃). Blanching before frying significantly changes the shape of plots. The L∗ values was observed to be decreasing as blanching temperature and time was increasing with the minimum and maximum values being 43.582 and 46.478, respectively. However, the quadratic effect of time of frying was more significant for L∗, as it increases with time of frying to a maximum level of 49.622 (Figure 5b) when blanching time was fixed (X₂ = 0).

When blanching temperature was fixed (X₁ = 0), the maximum value of L∗ was in the region of 47.7846 while the lowest value of 36.3102 was observed as shown in Figure 5c. For the a∗ parameter (redness), the values increases as temperature of blanching and frying time increases (Figures not shown) while it decreases as blanching time increases. Yam slices tends to get darker (more red) as frying proceeds (as a result of surface non-enzymatic browning reactions) as indicated by the progressive increasing of a∗ values with frying time. Blanching produces chips of lighter colour thus improving colour properties since it could leach out reducing sugars from the yam tissues and avoiding undesirable dark colour of slices after frying.[45] Low reducing sugar contents are required to minimize colour development during frying.[46] Fried chips colour is as a result of maillard, non-enzymatic browning reactions that depends on the superficial reducing sugar content, temperature and time of frying.[26] For the b∗ (yellowness) parameter, the trend is such that it decreases with increase in blanching temperature but increases as blanching time and frying time increases (Figures not shown). Since a higher values of b∗ means more yellow colour which is desirable for a fried product, frying time seems to have a more significant effect. This is so because irrespective of the condition of blanching, as frying time increases, the b∗ continues to increase even when other independent variables are kept constant. An overall consideration of the effect of blanching on colour parameter shows an acceptable, light coloured and yellow yam chips.

CONCLUSION

In this work conventional hot water blanching of yam slices was carried out at a temperature range of 60–80°C for 1–5 min before frying for 2–4 min. A classical drying profile depicts water removal while blanching conditions affected oil content of yam chips. Response surface methodology techniques was used to optimize the blanching conditions with frying time. Dependent variables investigated include moisture and oil content, breaking force and colour parameters as a result of blanching conditions and frying time. For the production of yam chips of acceptable properties, the optimum blanching conditions and frying time suggested is a blanching temperature of 70–75°C for about 4–5 mins while frying for about 5mins. This condition could produce yam chips of lower moisture and oil contents, lower breaking force (higher crispness), higher L∗ (lighter), lower a∗ (less dark), and higher b∗ (more yellow) parameters.

ACKNOWLEDGMENT

The authors wish to thank the management of the International Institute of Tropical Agriculture (IITA) Ibadan and National Centre for Agricultural Mechanization (NCAM) Ilorin, Nigeria for the use of their equipment.

Notes

1. FAO. World Crop Production Statistics. Food and Agriculture Organization of the United Nation Statistical database 2002. Online service, 00100 Rome Italy.