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American Journal of Agricultural and Biological Sciences
Year: 2009  |  Volume: 4  |  Issue: 2  |  Page No.: 156 - 166

Effect of Methyl Cellulose Coating and Pre-Treatment on Oil Uptake, Moisture Retention and Physical Properties of Deep-Fat Fried Starchy Dough System

Jihad M. Quasem, Ayman Suliman Mazahreh, Khaled Abu-Alruz, Ibrahim A. Afaneh, Ala`a H. Al-Muhtaseb and T.R.A. Magee    

Abstract: Problem statement: The influence of edible methyl cellulose coating and blanching pre-treatment in reducing oil uptake and moisture loss during frying of starchy dough system was investigated.
Approach:
Potato dough cylinder of 60 mm length and 22 mm diameter was used as a model food system. Samples were coated with 0.5% methyl cellulose film-forming solution and uncoated samples were used as control. Compared to the control samples, a reduction of 80% in oil uptake was achieved, with an increase in frying temperature decreasing the oil uptake due to the gel formation of methyl cellulose which was enhanced by higher temperatures.
Results:
No effect of methyl cellulose coating on the final moisture content was observed. The effect of methyl cellulose coating on structural properties (diameter, length, volume and bulk density) was also examined. The results showed that methyl cellulose coating had a significant effect on dimensional and density changes, as it enhanced the formation of crust, leading to a buildup in pressure within the cylinder, which in turn caused considerable puffing of the sample.
Conclusion:
Blanching pre-treatment (100°C, 5 min) was found to be effective in reducing the oil content of the potato dough samples.

Fig. 1 and 2, reveal an added advantage of emulsifier addition to the dough.

A more in-depth series of frying experiments were then undertaken using potato dough (with emulsifier) as the raw material. As shown in Fig. 3, the experimental results for the potato dough exhibited the expected trend with respect to moisture content, i.e., the moisture loss was found to increase with increasing frying time and temperature. The experimental moisture content data were most accurately fitted to an exponential function with respect to time. In contrast, a linear relationship was reported by Gamble et al.[13,14] and Reddy and Dias[31].

The effect of frying temperature on moisture content is in agreement with numerous researchers including[6,7,18,19,22-24,30].

Fig. 1: Effect of emulsifier (MC) on the oil uptake during frying at 165°C (cylinder-60 mm length, 22 mm diameter)

Fig. 2: Effect of emulsifier (MC) on moisture content during frying at 165°C (cylinder-60 mm length, 22 mm diameter)

Fig. 3: Variation in moisture content with frying time and temperature (potato dough cylinder-60 mm length, 22 mm diameter)

Fig. 4: Variation in moisture loss rate with frying time and temperature (potato dough cylinder-60 mm length, 22 mm diameter)

Fig. 5: Variation in oil content with frying time and temperature (potato dough cylinder-60 mm length, 22 mm diameter)

The moisture loss data was further represented in terms of the moisture loss rate, as shown in Fig. 4. Three stages of moisture loss could be identified in the moisture loss rate versus time graph.

The corresponding oil content results for the potato dough (with emulsifier) samples are shown in Fig. 5. The maximum oil content was observed within the first 60 sec, followed by a continual decrease as the frying progressed.

Influence of blanching: Blanching is a pre-treatment method adopted during vegetable frying processes to enhance the frying characteristics and hence, product quality[15].

Figure 6 shows that the oil content of the pre-fry blanched sample has less oil compared to the standard sample (no pre-fry treatment).

As shown in Fig. 7, blanching also has a slight effect on the final moisture content of the samples. This is in agreement with the results of[18], who stated that blanching has an effect on the rates of moisture reduction of potato chips samples, due to starch gelatinization.

Fig. 6: Effect of blanching treatment on sample oil content during frying at 165°C (potato dough cylinder-60 mm length, 22 mm diameter)

Fig. 7: Effect of blanching treatment on sample moisture content during frying at 165°C (potato dough cylinder-60 mm length, 22 mm diameter)

Debnath et al.[7] found that pre-fry drying had a significant effect on the moisture kinetic coefficient during deep fat frying of chickpea flour-based snack food.

Potato dough with varying initial moisture content: In order to simulate products with differing initial moisture content, but the same structural characteristics, potato dough samples with initial contents of 1.0, 1.5 and 2.0 kg kg-1 (dry basis) were prepared and fried at 165°C. The oil uptake and the moisture loss rate were expected to be higher for the sample of higher initial moisture content. The experimental oil content data are shown in Fig. 8.

Physical properties:

Dimensional changes: Potato dough samples were fried at 135, 150, 165 and 180°C for different frying times and the dimensional changes (diameter and length) monitored. The diameter decreased as the water exited the potato dough. The cylinder was then pushed gently outwards by internal pressure. The results are shown in Fig. 11 and 12, respectively.

Fig. 8: Effect of initial moisture content on sample oil content during frying at 165°C (potato dough cylinder-60 mm length, 22 mm diameter)

Density: The variation in the density of potato dough samples during frying at different temperatures was also examined, as shown in Fig. 13, where it can be observed that it falls significantly during frying. Frying temperature was also found to affect the density, which decreases with increasing temperature, at a given time.

DISCUSSION

As shown in Fig. 1, the presence of emulsifier resulted in a significant decrease in the oil content of the sample. This may be attributed to the fact that following gelatinization, which takes place after approximately 30 sec, the emulsifier acts as oil barrier. The insoluble, oil gel formed causes the oil to diffuse in a counter direction (from the inside to the outside of the food). Although the emulsifier accelerated the formation of the crust, as revealed visually, this crust offers a barrier for oil penetration in comparison to the crust formed on sample without emulsifier (this has properties favorable to oil uptake). The final oil content of the dough (with emulsifier) was found to be comparable to that of the potato sample. This is in agreement with[3,8,13,17,23,28,32,34], who also reported the barrier effect which a crust has on oil uptake.

As shown in Fig. 2, the presence of emulsifier had a negative effect on the moisture loss from the dough sample. No effect on the final moisture content was observed. On comparison of potato and potato dough moisture loss curves, it can be seen that the relative degree of moisture loss is much lower for the potato dough. This could be attributed to the sample structure, with the dough samples having a decreased number of pores through which moisture can escape, compared with potato and also due to the properties of the coating film. Methyl cellulose coating film acted as protective layer reducing material loss from the surface[23].

During the early stage of frying, there is an initial heating period when no water is lost. Heat is transferred from the oil to the food via convection at the surface and conduction through the uncooked solid and the temperature of the food approaches the temperature of vaporization of the liquid present, i.e., the food temperature approaches 100°C[11]. This initial, short-heating phase was observed to proceed for approximately 10 sec. During this period all the heat entering the potato was used for sensible heating, with only a small amount of expanded air leaving the potato. Such observations are in accordance with those of Farkas[11,37] who reported that during the first stage of frying no evaporation of moisture occurred and the heat was transferred in convection mode between the oil and the sample. This period was followed by a sudden loss of moisture, characterized by formation of a larger number of small bubbles, which form over the entire surface of the sample and leave rapidly. The maximum water loss rate, in the period of ‘surface boiling’ occurs at between 30 and 100 sec. This surface boiling increases the heat transfer coefficient, which in turn allows more heat to be transferred to the food for the vaporization of water[24]. The rate of moisture loss was then observed to fall as the sample crust thickened; the moisture driving force and moisture content decreases. The formation of water vapor bubbles becomes limited to a relatively small number of formation sites. It is probable that these sites represent weak or damaged areas of the potato dough. It is worthy of note that qualitative evaluation of the operating oil temperature could be carried out by observing the volume of bubbles being formed during the early stages of frying.

The water loss rate decreases gradually until the bubble end point is reached and the potato dough was at its equilibrium moisture content (after approximately 120 sec of frying time at 165°C). This is in agreement with[7,20,22] who reported that the initial moisture loss was rapid and this was followed by a constant rate drying period.

During the first 60 sec of frying the oil replaces the lost moisture, however, as the frying progresses the methylcellulose begins to gelatinize forming a barrier (protective layer) on the cylinder surface, thus restricting further oil uptake. The dependence of oil uptake on temperature is opposite to that observed during the frying of potato, with an increase in frying temperature decreasing the oil uptake.

For low frying temperatures (below 135°C), although the oil temperature is sufficient to enhance the effect of methyl cellulose (gel formation), a certain amount of oil is still allowed to diffuse into the cylinder. However, at high frying temperatures (150°C and above) gel formation of methyl cellulose will be enhanced, resulting in less oil entering the sample. From frying temperatures greater than 150°C, the barrier formed causes a buildup of internal pressure, leading to the sample ‘exploding’ after a period of approximately 420 sec. Lower oil content during frying at a higher temperatures is in accordance with the results of[25-27,30].

This is due to the blanching effect on potato dough structure. Gelatinization of the potato dough prior to frying enhanced the resistance to oil uptake. This may be due to gel formation decreasing the pores on the cylinder surface. Selman and Hopkins[36], Lamberg et al.[21], Debnath et al.[7], Rimac et al.[32] and Pedreschi and Moyano[26] also found that pre-drying increases the crispness dramatically and significantly reduces the oil absorption of the blanched potato slices after frying.

The oil content at a given time was higher for samples with lower initial moisture content. This is due to crust formation and its favorable oil uptake characteristics. The 1.0 kg kg-1 sample has the lowest moisture content, thus crust formation was faster and therefore, oil uptake was higher. Therefore, it would appear that the effect of crust, which was enhanced by the addition of methylcellulose, had the greater effect than sample moisture content on the oil uptake characteristics. The corresponding moisture loss data and equations are shown in Fig. 9 and Table 1. Given that the moisture content plots display similar trends as confirmed by ‘b’ values of equal magnitude (Table 1), this indicates that the moisture loss rate is not affected significantly by initial moisture content.

At a given frying time, the final moisture content is dependent on the initial moisture content and frying conditions. Therefore, to eliminate the effect of the initial moisture content, it is more appropriate to compare the experimental data by adopting the parameter, oil content criterion (UR). This parameter is defined as the amount of oil uptake (g) divided by the amount of water lost (g). The experimental data were recalculated on this basis and the results are shown in Fig. 10.

The initial moisture content of the potato dough significantly affected the oil content criterion. The lower initial moisture content resulted in a higher oil uptake to water loss ratio. The higher initial moisture content results in pores of smaller radius being formed during frying due to the higher water diffusion rate[24]. Materials with a lower initial moisture content usually have higher porosity and have a greater tendency to build up pressure within the pores during frying, thus causing an enlargement of the pore size which results in a lower pressure drop for oil absorption, particularly during cooling[24].

Table 1: Moisture loss equation and R2 for data plotted in Fig. 9
*: Equation: y: ±a × exp (-bt) where, y: Sample moisture content (kg kg-1, dry basis); t: Frying time (sec)

Fig. 9: Effect of initial moisture content on sample moisture content during frying at 165°C (potato dough cylinder-60 mm length, 22 mm diameter)

Fig. 10: Oil content criterion against frying time during frying at 165°C (potato dough cylinder-60 mm length, 22 mm diameter)

Visual observations were carried out on each sample after frying, by cutting through sample cross-section to observe the internal structure. The crust area (very rigid texture) was clearly distinguished from the inner core of the sample which was completely gelatinized and rubbery in texture. This observation was used to explain the similarities in the moisture loss trend during frying at different moisture content and also different frying temperatures, due to the effect of methyl cellulose. The moisture loss was evenly distributed over the cylinder (visual evidenced for the bubbles) and the oil was forced to locate just within the crust, since the gelatinized internal structure as a barrier for oil penetration, along with the resistance caused by the space between the crust and the gelatinized layers. However, the higher initial moisture content delayed the formation of the crust layer. Mallikarjunan et al.[23] stated that moisture removal and consequent fat uptake occur mainly in the crust and, therefore, the role of the edible film coatings in retaining moisture and reducing fat uptake was limited only to the surface.

This is in agreement with[10,21,23,27] who reported that crust is the domain of interest regarding oil penetration since oil deposition during the frying process is mostly limited to the crust region.

As shown, the diameter of the potato dough sample was found to decrease at the onset of frying, irrespective of the frying temperature, due to the moisture evaporation. It was expected that the diameter would continue to decrease as the frying progressed, however, the diameter was found, in fact, to increase. This may be attributed to the presence of methyl cellulose, which gelatinizes during frying to form a solid structure. Although the potato dough losses a significant portion of its water content, the solid matrix remains fixed and it is this structure that ensures that dimensional changes are small. Methylcellulose has the further function of enhancing the formation of crust which leads to a buildup in pressure within the cylinder which in turn causes considerable puffing of the sample. This is revealed through the excluded samples when they had irregular puffing in certain places over the cylinder surfaces or in certain cases of sample explosion. Figure 11 also shows that the effect of time on the diameter changes was more significant than the effect of temperature; however, the effect of temperature was significant on sample length (Fig. 12).

The increase in length is attributed to the proportionately large degree of water movement within the sample in the axial direction toward the cylinder ends (visual evidence for moisture evaporation) which caused them to swell. Therefore, the increase in cylinder length is due to swelling at each end. As shown in Fig. 12, sample length decreased during the first 30 sec of frying, irrespective of frying temperature (135°C continued to decrease during the first 120 sec). As previously mentioned, the rate of moisture loss per unit area is greater in axial ends than the radial direction. The water movement within the sample towards the two ends created a certain pressure on the internal side of the two plat ends which changed the flat end to a significant concave shape. Therefore, the sample length increased. The increase in length may be partially attributed to crust development. In some cases, surface cracking occurred which led to no internal buildup and hence no visible puffing effect (e.g., 150°C diameter result).

Fig. 11: Variation in sample diameter during frying (potato dough cylinder-60 mm length, 22 mm diameter)

Fig. 12: Variation in sample length during frying (potato dough cylinder-60 mm length, 22 mm diameter)

Fig. 13: Variation in bulk density during frying (potato dough cylinder-60 mm length, 22 mm diameter)

Fig. 14: Variation in bulk density during frying of potato dough with different initial moisture content (cylinder-60 mm length, 22 mm diameter)

Table 2: Equation and R2 for data plotted in Fig. 14
*Equation: y: ±a × exp (-bx), where, y: Bulk density (g cm-3), x: Sample moisture content (kg kg-1, dry basis)

The results are in contrast to those of[9,29] who reported no changes in restructured potato sample volume during frying. However, Krokida et al.[20] reported that as the drying time increases, the specific volume of French fries samples decreases, which means that the shrinkage phenomenon gets more intense. Kawas and Moreira[16] and Taiwo and Baik[38] reported an increase in the diameter, thickness and volume with time during the frying of tortilla chips and sweet potatoes, respectively.

The relationship between moisture content and density was also determined; as moisture content increased the bulk density increased exponentially, as revealed in Fig. 14 and Table 2. However, it has been mentioned that, while there were slight changes in the dimensions of the potato dough cylinder during frying, the mass of the cylinder decreases considerably. Therefore, the changes in density can be considered to be controlled by the amount of moisture loss rather than dimensional variations. This finding is in agreement with Krokida et al.[20], Kawas and Moreira[16] and Taiwo and Baik[38] who reported a decrease in bulk density as moisture content decreased during frying of French fries, potato, tortilla chips and sweet potatoes, respectively.

Figure 15 shows the variation in volume during the frying process. During the first 30 sec the volume decreased slightly due to the evaporation of water, irrespective of the frying temperature and the absence of a crust. After 90 sec frying (for a temperature of 165°C) the crusts start forming (the equivalent at 135°C is 180 sec) and thus an internal gradient pressure was formed, hence, the volume exhibits an increase.

Fig. 15: Variation in sample volume during frying (potato dough cylinder-60mm length, 22 mm diameter)

CONCLUSION

The following conclusions can be stated from the present research in which frying of coated starchy food systems has been studied:

Methyl cellulose emulsifier has been shown to be capable of significantly reducing the amount of oil uptake during frying of potato dough with a negative effect on the moisture loss from the dough sample
The dependence of oil uptake on temperature is opposite to expectations, with an increase in frying temperature decreasing the oil uptake. Increasing temperature increased the gel formation of methyl cellulose, resulting in less oil entering the sample
Blanching pre-treatment process affected significantly oil uptake process. Gelatinization of the potato dough sample, prior to frying, decreased the pores on the surface and hence, enhanced the resistance to oil pick up
The oil content at a given time was higher for samples with lower initial moisture content. This is due, to crust formation and its favorable oil uptake characteristics
The effect of crust, which was enhanced by the addition of methylcellulose, had the greater effect than sample moisture content on the oil uptake characteristics
The structural properties were also affected by methyl cellulose coating, which increased the diameter and the length of the sample, while the density decreased with time and temperature

ACKNOWLEDGMENT

Researchers would like to thank Mr., Atalah Al-Khalayfeh, at the Department of Chemistry, for his technical support.

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