Evaluation of Fat Uptake of Polysaccharide Coatings on Deep-Fat Fried Potato Chips by Confocal Laser Scanning Microscopy

Abstract

In the present investigation, potato slices of 3 cm diameter and 1.5 mm thickness with edible coating (1% Okra and 1% Okra + Carrageen polysaccharide coating solutions) and without any coating treatment (control samples) were fried in sunflower oil at temperatures from 170–180°C for 5 min. Confocal laser scanning microscopy of fried chips was recorded using fluorescence mode of the microscope. We observed gas cells and fat globules in the confocal laser scanning microscopy micrographs of fried chips. The results indicated that both 1% Okra and 1% Okra + Carrageen polysaccharide were effective in reducing the moisture loss and decreasing oil uptake (p ≤ 0.05), but we found the highest effect in those samples treated with 1% Okra + Carrageen polysaccharide coating. These results substantiate the application of edible coating with 1% Okra and 1% Okra + Carrageen polysaccharide to the potato chips resulting in better moisture retention capacity, eventually leading to chips with lower fat content.

Keywords:

• Moisture content
• Confocal laser scanning microscopy
• Fat content
• Sunflower oil
• Potato chip
• Polysaccharide
• Gracilaria corticata
• Okra

INTRODUCTION

Snacks are an essential part of packaged foods segment. A snack is defined as a type of small quantity of food consumed between meals or in place of a meal at home while watching TV and pubs and bars (where they are served free).^[1] Snack food generally includes bakery products, ready-to-eat mixes, chips, and other light processed foods. According to the Ministry of Food Processing, the snack food industry is estimated at about Rs. 100 billion in value and over 4,00,000 tonnes in terms of volume.^[2,3] Branded snacks have a higher sales margin when compared to unbranded products because people in large numbers prefer it during teatime.^[4] Of the wide range of snacks available, potato chips have a global preference.^[5] Potato chips constitute a major segment of the Indian snack food industry, according to India Info line. The potato chip industry is continually growing around the world. Deep fat frying is widely used in the preparation of tasty snack foods like potato chips with an attractive appearance.^[6] Potato chips are one of the deep fat fried snack food worldwide with an oil content that ranges from 35.3 to 44.5% w.b. giving it the unique attribute that makes the chips palatable.^[7] The most important attributes of fried food are texture, appearance, and flavor.^[8] Deep fat frying is a dry cooking process where the oil/fat transfers heat and also migrates into the food giving it nutrients and flavor which makes it desirable. Fat uptake is one of the critical point of deep fat frying. The achievement of high and constant quality of fried products with appropriate oil content is of vital interest in the food industry and with consumers.^[9,10] The Saturated and trans-fat are main elements related to causing diseases, and many researchers have conducted studies to reduce or remove the oil content from fried foods because of the obesity and the negative effects of excess oil consumption on human health.^[11] This point has caught the attention of researchers. There has been strong encouragement in the recent years to reduce oil content of fried foods and many researches are working on the development of food products that have reduced fat and cholesterol levels. There is a need to develop a better understanding of how foods absorb oils when they are deep fried and how the processing conditions influence the quantities absorbed.^[12] The structure developed during deep fat frying defines some of the quality attributes of the coating system. A good understanding of the physical properties of foods, especially at microscopic scale, provides the basis for optimization of the processes and sets the stage for development of newer and higher quality products.^[13] To meet the consumer demand for low-fat products as a part of health concern, there exists a need for reducing oil uptake during deep fat frying. The application of edible coating with natural polysaccharides is one of the means devised to reduce fat uptake during the frying operation aside from the fact that they add more value such as improved texture, appearance, taste, and volume, to fried foods.^[14] Recent studies have shown that edible coatings using natural polysaccharide can act as potential barriers to the incorporation of oil during the deep fat frying process.^[15] The main challenge is, therefore, to improve the frying process by controlling and lowering the final oil content of the fried product. Carrageen, a hydrocolloid polysaccharide composed of α-d-1,3 and β-d-1,4 galactose residues that are sulphated at up to 40% of the total weight; strictly negative charge over normal pH and associated with ammonium, calcium, magnesium, potassium, or sodium salts. The objective of this study was to characterize microstructural properties and quantify fat distribution in deep-fat fried potato chips coated with okra (Abelmoscus esculentus) and Gracilaria corticata polysaccharide using confocal laser scanning microscopy (CLSM).

Sample Collection and Preparation

We collected seaweed species (Gracilaria corticata) from Mandapam coast in Gulf of Mannar, Tamilnadu southeast coast of India at a latitude 9°45_N and longitude 79°0_E on low tide during December 2014. We washed collected samples with tap water to remove epiphytes and other marine organisms. The samples then transported to the laboratory in sterile polythene bags. In the laboratory, we rinsed the samples with tap water and were shade dried and powdered in a mixer grinder. Similarly, we collected okra (Abelmoscus esculentus) bio waste (upper crown head) from canteen, Anna University, Chennai, and rinsed with tap water and shade dried. Dried okra wastes were broken to powder form in an electrical mixer. We then sieved the powder into particle sizes 0.5–4 mm and stored in glass bottles. Potatoes (Solanum tuberosum) used for frying experiments were purchased from local markets for research (Chennai, India) while refined sunflower oil used for frying, was purchased from local supermarket. All the potatoes were stored at +4°C temperatures and 95% relative humidity in a dark place. Oil was preserved at laboratory conditions with (15–25)°C and far from light and humidity for further use. All reagents used in the experiment had analytical purity.

Extraction of Mucilage Polysaccharide/Gum

We extracted the mucilage polysaccharides by the method followed by Distantina et al.^[2] Five grams of clean, dried seaweed and okra powder sample was soaked in distilled water for 15 min. After soaking, we separated the water from the seaweed by filtration. First, a known amount of solvent (methanol, acetone; [1/50 g/mL]) was heated in a beaker as an extractor which emerged in a water bath equipped by a stirrer. If the temperature of solvent reached 85°C, the samples were then added into solvent, and we started counting the time of extraction. The speed of stirrer was set constant at 275 rpm. We maintained the constant ratio of seaweed weight to solvent volume (1/50 g/mL) by adding hot water. We stopped the extraction after 45 min. We separated the filtrate from the residue using filter cloth and it was immediately poured into 3 volumes of cold (5°C) acetone (90% w) which caused the precipitation of polysaccharides. The precipitation was carried out for 30 min by stirring gently by hand, then we purified the separated polysaccharides with the addition of 96% ethanol in 1:3 ratios. The precipitated polysaccharides were collected and oven dried at 50–60°C until they reached a constant weight. We carried out the above experiments with different solvents, such as acetone and some parameters, namely the yield, chemical structure, swelling index, and emulsifying capacity of the extracted polysaccharides were determined.

Preparation of Gum Suspensions

Coating solutions were prepared from the extracted mucilage polysaccharide/gums. For each coating material, solutions of 1 % w/v were prepared. We heated each solution to 90ºC for 5 min and then cooled to room temperature. Then we added Glycerol at a level of 3% w/v to these solutions as plasticizer.

Sample Preparation

We washed the potato tubers washed, hand-peeled, and cut to slices with a thickness of 1.5 mm by operating a potato-slicing device, following which the prepared slices of potato cast by steel mould with a diameter of 3 cm. We blanched the prepared slices of potato in water at 85°C for 3.5 min. After blanching, the slices of potato were immersed in a 1% aqueous solution of okra polysaccharide and 1% combination of Okra and Carageenan polysaccharide solution from Gracilaria corticata at room temperature for 5 min. In addition, the potato slice samples without polysaccharide coatings kept as control in the same condition. We kept the samples at room temperature for 10 min for uniform coatings.^[16,17] Subsequently we dried the slices with special paper towels to remove their surface starches and they finally were deep fried in sunflower oil.

Frying Experiment

Frying was carried out in a thermostatically temperature-controlled fryer (HOME KING Germany) having a capacity of 1 L of oil. The slices were fried at 150 ± 10°C for 10 min in sunflower oil. In recent years, a significant portion of edible oils is covered by sunflower oil, with a sharp focus on its healthiest options with its high nutritional quality and desired industrial functionality—for example, high oleic sunflower oil. The phenolic components and tocopherols from sunflower seeds are the most important antioxidants, which provide good storage stability, as well as nutritional quality. Cells contain a complex system of antioxidant defenses to protect against the harmful consequences of activated oxygen species produced in deep fat fried foods. After cooling at room temperature, we carried out the analysis of the moisture content as well as oil content of all fried samples. In industries, the most common frying temperature for potato product is 180°C. In this study, we selected a medium (150°C) frying temperature since previous findings showed that acryl amide (a possible carcinogen in humans) formation in potato chips could be reduced significantly by decreasing the frying temperature.^[18,19] All experiments were run in triplicate and the present results are the average of the obtained results.

Samples Analysis

Moisture content was determined by drying in an oven at 105°C until constant weight reached. After 3 h, all the samples were weighed every 5 min until they attained a constant weight. The oil content was determined by a simple and rapid method that consists of weighting, crushing of sliced chips, and immersing in a 250 mL beaker containing 200 mL of Hexane per 100 g slices at 28 for 45 min.

CLSM

We evaluated the microstructure and starch distribution in the potato tissue by confocal microscopy. Two cylindrical samples (8 mm in diameter) taken from each potato disk. Two sections (200 mL) obtained from each cylindrical sample using an oscillating tissue slicer (EMS 5000, Electron Microscopy Sciences Inc., and Hatfield, PA). A Zeiss LSM 510 multiphoton microscope (Carl-Zeiss Inc., Thornwood, NY) used for high-resolution imaging of the potato tissue microstructure. Multiphoton fluorescence images were obtained using 880 nm laser excitation and a 500–530 nm band pass emission filter using a 40_ oil objective (NA = 1.4). The Z-projection image processed using a Zeiss LSM image analysis toolbox. We used Calcofluor white stain used for the visualization of cell walls.^[20] We used a 0.2% rhodamine-B solution for the visualization of starch.

Sensory Evaluation

We selected a panel of assessors with experience in sensory evaluation to evaluate the samples of potato chips. The intensities of sensory attributes considered were odor, taste, color, texture and flavor. Fried slices was taken immediately after production and presenting five samples of four slices to the panel in random order to be scored for intensity using an ordinal scale and a five-point hedonic scale from 1 (like extremely) to 5 (dislike extremely). The procedure and analysis of results carried out following standard methods.

RESULTS AND DISCUSSION

CLSM

CLSM is one of the most advanced techniques in microscopy, which involves, focusing at a specific point on the specimen sample and getting the images in x/y, y/z, and x/z planes. In this technique, the specific point of interest on the specimen and the pinhole spot of the microscope lie on the same focal plane. The oil locations in the potato chips obtained by using the florescence mode of the CLSM with the help of different fluorescent dyes and surface topography studied by using the reflective mode.

We obtained a three-dimensional image of the potato chips sample from the CLSM by varying the Z-direction dimension of the sample. The penetration depth and the step size varied from sample to sample. Potato crisps were fried in the sunflower oil heated to 170–180°C. Principally, the oil covering the cells in the intercellular spaces remained trapped within the microstructure and we observed the details of the cell structure, such as size heterogeneity, in the micrographs of high resolution CLSM.^[21] The oil globules of different sizes and irregular shapes noticed higher in the micrographs of controlled chips without any coating compared to that of polysaccharide-coated potato chips. The cell walls stained homogeneously with rhodamine B dye and starch granules oval in shape appeared as clusters in organized form inside the cells. The size of the starch granules appeared to increase in the control chips indicating the beginning of starch gelatinization (swelling of starch granules) as shown in Fig. 1. The presence of gas cells seen higher in the micrographs of control chips than the polysaccharide (1% Okra and 1% Okra + Carageenan) coated chips. These could have led to the development of voids/pores in the control chips that contained air and fat with little moisture. This higher amount of gas cells in the control chips attributed to the effects of water vapor released from the pores on the surface potato slices without edible coating and moisture loss during deep fat frying. Presence of higher amount of gas cells in the control chips without edible coating resulted in the higher fat uptake due to moisture loss than that of 1% Okra and 1% Okra + Carageenan polysaccharide coated chips, is shown in Figs. 2 and 3. This implies that fat absorption was lower in edible-coated chips than the control chips without any coating application with same frying time and temperature. This was in agreement with the results obtained from the Soxhelet method for fat content determination. The images showed that dye intensity increased with both frying temperature in control chips compared to the coated chips. The type of oil used might also lower fat uptake in fried foods as reported earlier.^[22] In the present study, we used sunflower oil for frying potato chips. Sunflower oil had lower viscosity compared other frying oils like corn oil, palm oil, etc. Sunflower oil with lower viscosity had relatively less adherence to the surface of the product and led to easy drainage of oil when the fried food removed from the fryer. Similar results were obtained by Mecit et al.^[16] for the potato chips fried at 170ºC for 4.5 min in hazelnut oil with higher viscosity and fat content than sunflower oil or corn oil.

FIGURE 1 Confocal laser scanning microscopy (CLSM) micrographs of control chips without polysaccharide.

FIGURE 2 Confocal laser scanning microscopy (CLSM) micrographs of 1% Okra polysaccharide coated.

FIGURE 3 Confocal laser scanning microscopy (CLSM) micrographs of 1% Okra + Carageenan polysaccharide coated chips.

We also observed the microstructural changes in the potato tissue caused by the frying process by CLSM micrographs. Polyhedral cells with small intercellular spaces and globular to ellipsoid starch granules were also observed in all three samples. We observed cell walls with thin lining of darker color and starch granules were in dark color due to staining with rhodamine-B dye. Signs of slight hydration and swelling of starch granules observed more in control chips compared to polysaccharide-coated samples. The size of the starch granules tend to increase in control chips, indicating the beginning of starch gelatinization. The starch granules in the control chips ruptured and amylose leached out and led to loss of integrity. This loss of cell turgor and integrity between adjacent cells were the main factors that could affect the firmness of fried snack products. This process of amylose release was comparatively lesser in edible-coated chips due the barrier properties of the coating materials. The CLSM analysis clearly showed that the gelatinization of potato starch took place in control chips without polysaccharide edible coating and indicated changes in the membranes and the cell walls, which attributed to the decrease in firmness and loss of texture in control chips. The gelation of polysaccharide coating had promoted the barrier-resistant effect to oil uptake and moisture loss in coated chips.

Effect of Coating on the Moisture Content and Fat/Oil Uptake of Fried Products

The results obtained have shown that use of polysaccharide coatings had a significant effect on the final moisture and fat content of fried potato chips (p ≤ 0.05). Results showed that using edible coatings from both Okra and Carageenan polysaccharide for frying of the potato slices increased the percentage moisture contents of the products after frying as shown in Fig. 4. The polysaccharides coated samples showed an increase in moisture than the non-coated control sample. The hydrocolloid polysaccharides coated on the potato samples acted as barrier material by not allowing more water to evaporate on the surface and thereby maintained adequate moisture within the sample. We noticed that both 1% Okra and 1% Okra + Carageenan polysaccharide were effective in increasing final moisture content of fried products. The control chips without any coating treatment had lower moisture content than the 1% Okra and 1% Okra + Carageenan polysaccharide edible-coated chips. The combination of 1% Okra + Carageenan polysaccharide showed best results with respect to the efficiency of the edible coatings in increasing percentage moisture content during the frying process of the potato chips. This may be because of the fact that the polysaccharides were hydrophilic biopolymers, which acted as water binders in a coating and increased the moisture content of the product. Similar results were observed previously where the increasing moisture content with application of different concentrations of methylcellulose films, soy protein isolate, and whey proteins concentrate coating on potato strip chips.^[23] The highest moisture content in the fried chips treated with 1% Okra + Carageenan polysaccharides combination might be due to the higher gel strength of the biopolymer which resulted in less evaporation damage and hence, to a lower water diffusivity. Similar results were observed with mixture of coating material resulting in higher moisture content of fried products.

FIGURE 4 Effect of coating on moisture content of fried chips.

Figure 5 shows the reduction in fat/oil uptake of coated and uncoated potato chips. A considerable reduction in fat uptake observed for deep-fat fried polysaccharide coated potato chips. The edible-coated potato chips absorbed lesser oil compared to non-coated control chips samples.^[24] Among the two polysaccharide concentrations (1% Okra and 1% Okra + Carageenan) tested, the combination of 1% Okra + Carageenan polysaccharide provided the lowest fat content and highest reduction in oil uptake during frying. Oil absorption was a surface phenomenon as proved in earlier studies by different authors. Therefore, the edible coatings as gel-forming compounds form a fine network structure, which help to cover the pores on the surface of the potato slice and thus, prevents oil absorption into the potato tissue during the frying process.^[25] The reductions in oil uptake of the coated chips were again due to the difference in the properties of the coating materials used such as viscosity, gel strength that governed the moisture retention in the test samples. Therefore, the Okra and Carageenan polysaccharide with high viscosity and gelation property^[26] might have formed a matrix on the chips during frying, thereby preventing the loss of moisture and not allowing the entry of the oil through capillary action during frying.^[27] The purpose of coating of potato slices was to reduce the fat content of the final product. Thus, the results obtained were satisfactory and followed the same trend with the results of Garmakhany et al.^[8] for French fries with different layers of hydrocolloids coatings using combination of carboxy methylcellulose and pectin, which showed reduction in the moisture loss and lowest oil uptake of French fries of products during frying because of their better barrier properties.

FIGURE 5 Effect of coating on fat uptake of fried chips.

Sensory Evaluation

Descriptive sensory evaluation showed five attributes identified in the taste, odor, flavor, texture, and color in potato chips samples. The consumers generally liked all potato chip samples and consumer sensory evaluation indicated that there was no clear preference of one sample over the others, despite the differences observed through descriptive sensory analysis and physio-chemical characteristics. Among two treatments (1% Okra polysaccharide and 1% Okra + Carageenan polysaccharide) and the control sample, all of the sensory parameters at level of p ≤ 0.05 no significant difference seen.

CONCLUSION

The present work demonstrated the suitability of using Okra (Abelmoscus esculentus) as new hydrocolloid on reducing the fat uptake in potato chips. From the results of the above experiments it could be envisaged that the use of Okra (Abelmoscus esculentus) and Carageenan polysaccharide from as Gracilaria corticata as coating material in preparation of potato chips decreased oil uptake and increased moisture content of potato chips unlike uncoated control samples. Therefore, the application of Okra and Carageenan polysaccharide as edible coating could be acceptable for decreasing the oil uptake of potato chips from both nutritional and economical aspects. The potato slices were marinated or coated with hydrocolloid for 5 min, which reduces fat uptake without increasing the costs of production too severely compared to uncoated chips. We used CLSM analysis to study the surface topography of potato chips and the oil location on the surface. Images showing fat distribution in control and polysaccharide coated chips were obtained. Sensory evaluation of quality of the products showed that application of edible films did not influence sensory characteristics of the fried coated potato chips and did not change the acceptance by the consumers. The edible coating of chips with polysaccharide hydrocolloid produced significant effect of on fat distribution and moisture retention of the final fried product. There was good correlation observed between fat distribution from CLSM images and fat content determined by the conventional method.