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Edible Coatings for Deep-fat Frying of Starchy Products
Lebensm.-Wiss. u.-Technol., 30, 709–714 (1997)
Edible Coatings for Deep-fat Frying of Starchy Products P. Mallikarjunan, M. S. Chinnan*, V. M. Balasubramaniam and R. D. Phillips P. Mallikarjunan, M. S. Chinnan, R. D. Phillips: Center for Food Safety and Quality Enhancement, Department of Food Science and Technology, The University of Georgia, Griffin, GA 30223-1797 (U.S.A.) V. M. Balasubramaniam: Presently with National Center for Food Safety and Technology, Illinois Institute of Technology, Summit-Argo, IL 60501-1933 (U.S.A.) (Received October 1, 1996; accepted January 27, 1997)
The effectiveness in moisture retention and reduction of fat uptake of different edible coatings on a fat-free starchy product was determined. Mashed potato balls of 47 mm diameter were used as model food system. Samples were coated with corn zein (CZ), hydroxypropyl methyl cellulose (HPMC) or methyl cellulose (MC) film-forming solutions. Uncoated samples were used as control. Compared to the control, a reduction of 14.9%, 21.9% and 31.1% in moisture loss from the product was observed for samples coated with CZ, HPMC and MC films, respectively. Similarly, a reduction of 59.0%, 61.4% and 83.6% in fat uptake by the product was observed for samples coated with CZ, HPMC and MC films, respectively. Based on the overall performance in reducing moisture loss and fat uptake, MC exhibited the most significant barrier properties.
©1997 Academic Press Limited Keywords: edible film; deep-fat frying; potato; water content; fat reduction
Introduction Fat is one of the primary sources of energy in food and the per capita consumption of fat in the U.S. during 1987 was 28.4 kg (62.7 lb.) (1). The U.S. Surgeon General’s Report on Nutrition and Health included a dietary recommendation to reduce fat consumption by 25% and especially, to limit the energy consumed from saturated fat to less than 30% (2). Industry is responding to these challenges by introducing low-fat products. Moreover, interest in reducing oil uptake in fried foods has recently increased. Deep-fat frying is a widely used method for preparing foods with an attractive and tasty surface. The soft and moist interior along with the porous crispy crust provides increased palatability to foods (3). In the U.S., the fried-food business is flourishing. Reportedly, more than 500,000 institutional and commercial restaurants are involved in deep-fat frying operations (4). To meet the recent trends and consumer demand for low-fat products, there exists a need for reducing oil uptake during deep-fat frying. Most new low-fat fried products include fat substitutes in the formulation. Another approach is use of edible ingredients in the batter and breading mix to improve coating performance. Various ingredients such as alginate (5), powdered cellulose (6, 7), methyl cellulose (8) and soy protein isolates (9) have been used. Whey *To whom correspondence should be addressed.
protein, egg albumen and carboxymethyl cellulose (CMC) improved the breading adhesion significantly (10). Meyers (11) has discussed the functionality of hydroxypropyl methyl cellulose (HPMC) and methyl cellulose (MC) as barriers to fat absorption in breading and batters. The use of proteins (such as milk powder and egg albumen) as ingredients in fried products is also known, yet their functionality as edible coatings to reduce oil uptake by deep-fat fried foods have not been established (12). Limited research work has demonstrated the use of edible ingredients as a surface treatment. Fan and Arce (13) reported the use of amylose as a surface coating to reduce fat uptake during frying. Feeney et al. (14) used corn zein as an edible coating in French fried potatoes and reported a 28% reduction in oil uptake. Most of these new development approaches still rely on empirical tests to develop batter and breading formulations from protein and other hydrocolloids and most of the information is proprietary (15). We previously demonstrated the potential of edible films for moisture retention and reduction in fat absorption during frying of poultry products (16, 17). Compared to uncoated control samples, chicken nuggets coated with an edible HPMC coating had reduced oil absorption by up to 17.9% and 33.7% in the surface layer and the core, respectively. Coated samples also showed improved moisture retention (up to 8.6% in the surface layer and 16.4% in the core) compared to a
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©1997 Academic Press Limited
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Table 1 Formulations of edible film-forming solutions Film type
Base ingredient (g)
Water (mL)
Ethanol (950 g/L; mL)
Hydroxypropyl methyl cellulose
9
200
200
54 9
0 200
325 200
Corn zein Methyl cellulose
control sample (16). Our objective was to evaluate the moisture retention and fat reduction capabilities of different edible film coatings during deep-fat frying. In order to evaluate the effectiveness, a fat-free starchy product was selected.
Materials and Methods Mashed potato balls were used as the model food system. Baking potatoes were obtained from a local grocery store and were boiled for 45 min. After draining the water, the potatoes were peeled and mashed. The potato balls were formed using a commercial meat ball shaper (47 mm diam.).
Edible film coating To study the effectiveness of edible films/coatings, three different types were tested on mashed potato balls: CZ (corn zein), MC and HPMC. The film-forming solutions were formulated (Table 1) using the procedure described by Park and Chinnan (18). For HPMC (viscosity of 80–120 mPa.s in 2% aqueous solution, Aldrich Chemical Company, Inc., Milwaukee, WI) and MC (viscosity of 4000 mPa.s in 2% aqueous solution, Aldrich Chemical Company, Inc., Milwaukee, WI), the base ingredient was added incrementally to water maintained at 45 °C and allowed to dissolve with mixing. Then, 95% food grade ethanol (190 proof, Florida Distillers Co., Lake Alfred, FL) was added and thoroughly mixed. Polyethylene glycol (PEG) (Avg. Mol. Wt. 400, Aldrich Chemical Company Inc., Milwaukee, WI) was added as a plasticizer. The contents were covered with aluminum foil and left over night at ambient temperature to allow rising, coalescing and subsequent disappearance of air bubbles. For CZ (Regular grade, Freeman Industries, Inc., Tuckahoe, NY), the base ingredient was dissolved in 95% food grade ethanol. Anhydrous crystalline citric acid (C-0759, Sigma Chemical Company, St. Louis, MO) was added to enhance the dissolution of CZ by lowering the pH. For CZ, glycerin (G31-1, Fisher Scientific, Fair Lawn, NJ) was added as a plasticizer. The rheological characteristics of the solutions were determined using a rotational viscometer (Model LVTDV-II, Brookfield Engineering Laboratories, Stoughton, MA) equipped with a Thermosel system at ambient temperature. The apparent viscosity of the film-forming solutions was estimated using power law model: τ = Kγ˙ n, where, τ is the shear stress and γ˙ is the shear rate. The K
Plasticizer
Other
Polyethylene glycol (1 mL) Glycerin (11 mL) Polyethylene glycol (1 mL)
– Citric acid (1 g) –
(consistency coefficient) and n (the flow behavior index) of the solutions were estimated from experimental data as described by Rao and Webb (19). These coefficients would provide an insight to the applicability of spraying as a method for applying edible film solutions. Potato balls were coated by placing them individually on a rotating (15 rpm) platform and spraying with the film-forming solution for 1 min using a commercial spray gun (Model PM-50595, Popular Mechanics, Beckley, WV). After air-drying for 2 min, the procedure was repeated to ensure uniform coating of the film on the ball. To estimate film thickness on the product the filmforming solution was sprayed on a flat surface. The solution was sprayed first for 1 min and was sprayed again for another 1 min after air-drying for 2 min. This procedure simulated the coating process on the samples. The deposition rate was determined by spraying the solution for 3 min continuously on a flat surface. Data from difference in weight of the collecting surface, surface area and density of film-forming solution was used to calculate the film thickness and deposition rates.
Frying experiments Peanut oil at 175 °C was used as the frying medium. Uncoated and coated potato balls were fried for 240 s such that the center temperature reached 70 °C or above. All the experiments were carried out in triplicate. The temperatures of samples at different locations (2 mm below surface, mid point between center and surface, and geometric center) and oil in the vicinity of the samples were monitored using a T type thermocouple (TT-T-30, Omega Engineering, Stamford, CT) attached to a data logger (Hotmux, DCC Corporation, Pennsauken, NJ). The fried potato balls were air-cooled and excessive surface oil was gently wiped off using tissue paper. The samples were then weighed, placed in plastic bags and frozen at –20 °C until further analysis. Mass loss during frying was determined from the weight difference between precooked samples and deep-fat fried samples.
Determination of barrier properties of edible film To evaluate the effectiveness of edible film coatings, the moisture and fat contents of the surface layer and the interior core were determined. The samples were cut open into four quarters and the surface layer (0.1 mm)
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was scraped off and collected. The remaining portion of the sample was considered to be the core. The masses of surface layer and core portion were recorded. The moisture content of the surface layer and core samples was determined using a freeze-dryer (Research Freeze Dryer, Virtis Company Inc., Gardiner, NY). Freezedrying technique completely removed moisture and yielded porous samples for subsequent extraction of fat. The fat content of the freeze-dried samples were determined using AOAC method 960.39 (20). The overall moisture and fat contents of the fried potato balls were determined using: ¯ = M
F¯ =
Ws 3 Ms + Wc 3 Mc Ws + W c
Eqn [1]
Ws 3 Fs + Wc 3 Fc Ws + W c
Eqn [2]
where W = the mass (kg), M = moisture content (g/100 g), and F = fat content (g/100 g dry basis) of the respective layers. The subscripts s and c refers to the surface layer and core regions, respectively.
Statistical analysis Data were subjected to analysis of variance using SAS procedure GLM (21). Differences among packages were determined using the Duncan Multiple Range Test at 5% confidence level (or α = 0.05).
Results and Discussion Properties of edible film-forming solutions and coatings The density and viscosity values of edible film-forming solutions are shown in Table 2. The estimated film thickness ranged from 0.05 to 0.10 mm. CZ coatings were thicker than HPMC and MC coatings. This could be due to its viscosity and deposition rate, as CZ film had the lowest consistency coefficient (4.14 Pa.sn) and highest deposition rate (0.74 g/(m2.s)). The higher viscosity of CZ film-forming solution is due to its higher solids content than the HPMC and MC film-forming solutions. The values reported for the water vapor permeability of these edible films in the literature (18, 22–24) ranged from 89–200 3 10–12 g.m/(m2.s.Pa). The water vapor permeability of hydrophilic films depends
on film thicknesses and measurement conditions (temperature and relative humidity).
Moisture content The following visual observations were made during simultaneous frying of coated and uncoated potato balls. Initially, coated samples produced more bubbles in comparison to uncoated samples. As frying progressed, a reversal in the trend was observed with uncoated sample forming a larger envelope of bubbles compared to coated samples. This suggested that uncoated potato samples were losing more moisture to the frying medium than coated samples. The moisture content of the surface layer and core region is shown in Fig. 1a. The surface layer lost moisture rapidly and the final moisture content of surface layer ranged from 26 g/100 g for the control to 55 g/100 g for HPMC-coated samples. The edible filmcoated samples retained more moisture in the surface layer. The effectiveness in retaining more moisture was not significantly different among MC and HPMC. CZ retained more moisture than the control but significantly lower moisture than MC and HPMC. The difference in performance is mainly due to the type of edible film. CZ is a protein-based film whereas MC and HPMC are cellulose-based hydrocolloids. MC and HPMC form a protective layer due to thermal gelation above 60 °C. The methyl groups in MC and HPMC molecules undergo intermolecular association with adjacent molecules at temperatures above the incipient gelation temperature (IGT). The IGT is the temperature at which intermolecular association begins to occur very rapidly. Above the IGT, viscosity increases dramatically with increasing temperature, to the point where the solution gels (11). For MC and HPMC, IGT is in the range of 50 to 90 °C (depending on degree of substitution, concentration, etc.). This gel layer might have controlled the transfer of moisture and fat between the product and the frying medium. On the other hand, CZ forms a film layer upon contact with the surface of the product after spraying. The zein coagulates in moisture (moisture on the product surface) forming a thin film of precipitated corn zein on the surface. This insolubility of corn zein is mainly due to a high content of nonpolar amino acids and protein association through hydrogen bonding by glutamine (25). In addition, hydrophobic and hydrogen bonds
Table 2 Physical properties of edible film-forming solutions and coatings Viscosity
Film type
Density (kg/m3)a
Film thickness (mm)a
Deposition rate g/(m2 s)a
Consistency coefficient (Pa.sn)
Flow behavior index
Residual sum of squares
Hydroxypropyl methyl cellulose Corn zein Methyl cellulose
836.7±20.9 cb 877.9±6.3 b 967.5±4.9 a
0.05±0.01 b 0.10±0.02 a 0.06±0.01 b
0.37±0.11 b 0.74±0.11 a 0.31±0.06 b
46.47 4.14 387.54
1.02 0.98 0.98
168.23 9.28 971.00
a b
Means and standard deviations. Values with same letters in a column are not significantly different from each other at α =0.05.
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100
80 Moisture content (g/100 g)
develop in the film matrix (26) upon evaporation of the solvent (alcohol). Another possible contributing factor to CZ coatings resulting in lower moisture retention than HPMC and MC coatings was that CZ coatings were plasticized with glycerol while HPMC and MC coatings were plasticized with PEG. Glycerol is more hydrophilic than PEG resulting in poorer moisture barriers than PEG-plasticized films (23). In addition, CZ coatings contained higher amounts of plasticizer than HPMC and MC coatings. Moisture barrier ability of films decreases as the amount of added plasticizer increased (23). The core moisture content was not significantly different among the edible film-coated samples. The control lost more moisture compared to the coated samples.
(a)
a
b
a
a
a 60
a ab
40
b
20
0 30
Surface
Core
80 (b)
(a)
Moisture content (g/100 g)
Fat content (g/100 g)
a 20
15
b
10
c
c 5
MC
HPMC
c
60
40
20
a b
0
Surface
b
b
Core
0
Fig. 1 (a) Moisture and (b) fat content of surface layer and core of deep-fat fried potato balls coated with edible films. Columns with the same letter in a group are not significantly different from each other at (P < 0.05). Bars indicate standard deviation. (j) = control; ( ) = CZ; (C) = MC; (A) = HPMC
12
Fat content (g/100 g)
40
30 b
6 b
b
10
0
MC
b
4
2
CZ
(b)
8
20
Control
CZ
10
a
b
Control
a
50
Mass loss (%)
ab
b
25
0
a
HPMC
Fig. 2 Overall mass loss of deep-fat fried potato balls coated with edible films. Bars indicate standard deviation
c
Control
CZ
MC
HPMC
Fig. 3 Overall performance of edible films in (a) moisture retention and (b) reduction in fat uptake during deep-fat frying of potato balls. Columns in a group with the same letter are not significantly different from each other (P < 0.05). Bars indicate standard deviation
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Uncoated
Oil
Coated
Moisture
Oil
Moisture Edible
Crust
film
Crust
Core Core
Fig. 4 Conceptual diagram illustrating the effect of edible films on moisture and fat transfer during deep-fat frying of starchy products
Fat content A reduction in fat uptake was observed for deep-fat fried coated potato samples (Fig. 1b). The surface layer of coated samples absorbed less oil compared to control samples. The fat content of surface layer in control samples increased from 0 (undetectable levels by gravimetric analysis) to 28 g/100 g whereas MC-coated samples had only 13 g/100 g. The substantial reduction in oil uptake was mainly attributed to the barrier properties of the coatings to the transfer of moisture and fat (27, 28). The effectiveness in reducing oil uptake was not significantly different between MC and HPMC. Similarly, the effectiveness in reducing oil uptake was not significantly different between HPMC and CZ. Among all films tested, MC provided the lowest oil uptake during frying.
Mass loss The percent mass loss was calculated from mass before and after frying. Figure 2 shows the effect of edible film coating on mass loss. Control samples had 36% mass loss while mass loss for coated samples ranged between 19.8 and 22.2%. Coated samples reduced mass loss significantly and had approximately 40% lower mass loss than control samples. No significant differences were found among the edible coatings in reducing mass loss. The reason for lower mass loss could be attributed to the moisture retention properties of the edible films. In addition, the edible coatings acted as protective layers reducing material loss from the surface to the frying medium.
Overall performance of edible films The overall performance of different edible films during deep-fat frying of potato balls was calculated as described by Eqn [1] and Eqn [2] (Fig. 3). The overall moisture content was 54.5, 62.4, 71.4, and 66.4 g/100 g for control samples, and samples coated with CZ, MC
and HPMC, respectively. The overall fat content was 7.6, 3.1, 1.2 and 2.9 g/100 g for control samples and samples coated with CZ, MC and HPMC, respectively. In comparison to the control, percent moisture loss reductions were 31.1, 21.9 and 14.5 for MC, HPMC and CZ, respectively, and the percent reductions in fat uptake were 83.6, 61.4 and 59.0 for MC, HPMC and CZ, respectively. Among the films tested, MC exhibited the best barrier properties to provide moisture retention and reduction in fat uptake during deep fat frying. The better moisture barrier performance of MC coatings compared to HPMC coatings can be attributed to MC being less hydrophilic than HPMC (29).
Effect of edible films on moisture and fat transfer Figure 4 shows the role of edible films in reducing moisture loss and fat uptake in a conceptual manner. Based on the observations in the present study, the edible films form a protective layer on the surface of the samples. This protective layer is formed upon application of the coating as in the case of CZ and/or during the initial stages of frying as in the case of MC and HPMC. This protective layer inhibits the transfer of moisture and fat between the sample and the frying medium. Moisture removal and consequent fat uptake occur mainly in the crust (surface layers of the product) and, therefore, the role of the edible film coatings in retaining moisture and reducing fat uptake was limited only to the surface. Thus, the type of edible film was not significant with regard to moisture and fat contents of the core region, as evident by no significant difference in core moisture and fat content among coated samples. On the other hand, when considered with the surface effects, by selecting a suitable edible film, the moisture and fat transfer between the frying medium and the food can be controlled to produce an end product with desirable characteristics.
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Summary By suitable selection of edible films it is possible to control moisture and fat transfer between the frying medium and the food. Irrespective of the type of edible films used in our study, applying a coating reduced moisture loss and fat uptake during deep-fat frying. In comparison to the control, percent moisture loss reductions were 31.1, 21.9 and 14.5 for MC, HPMC and CZ, respectively, and percent reductions in fat uptake were 83.6, 61.4 and 59.0 for MC, HPMC and CZ, respectively. Among the films tested, MC showed most effective moisture and fat barrier properties.
Acknowledgements The study was supported in part by USDA Special Research Grant Program, USDA National Research Initiative-Competitive Grant Program (#95-37500-238) and with support from the University of Georgia. We thank Mr Glenn D. Farrell for his technical assistance.
References 1 USDA Per capita consumption of major food commodities.. Washington, DC: U.S. Department of Agriculture. US 1973–87. Table 680. (1988) 2 O’DONNELL, C. D. Presure keeps up to keep fat levels down. Prepared Foods, 163 (8), 70–71 (1994) 3 VARELA, G. Current facts about the frying of food. In: VARELA, G., BENDER, A. E. AND MORTON, I. D. (Eds), Frying of Food. Principles, changes, new approaches. Chichester, U.K.: Ellis Horwood Ltd, pp. 9–25 (1988) 4 BROOKS, D. D. Some perspectives on deep-fat frying. INFORM, 2, 1091–1095 (1991) 5 DUXBURY, D. D. Oil water barrier properties enhanced in fried foods. Food Processing, 50 (2), 66–67 (1989) 6 HENDERSON, A. Cellulose ethers — The role of thermal gelation. In: PHILLIPS, G. O., WILLIAMS, P. A. AND WEDLOCK, D. J. (Eds), Gums and Stabilisers for the Food Industry, Vol. 4. Oxford , U.K.: I.R.L. Press, pp. 265–275 (1988) 7 ANG, J. F. Reduction of fat in fried batter coatings with powdered cellulose. Journal of American Oil and Chemists Society, 70, 619–622 (1993) 8 PINTHUS, E. J., WEINBERG, P. AND SAGUY, I. S. Criterion for oil uptake during deep-fat frying. Journal of Food Science, 58, 204–205, 222 (1993) 9 MARTIN, M. L. AND DAVIS, A. B. Effect of soy flour on fat absorption by cake donuts. Cereal Chemistry, 63, 252–255 (1986) 10 SUDERMAN, D. R., WIKER, J. AND CUNNINGHAM, F. E. Factors affecting adhesion of coating to poultry skin: Effects of various protein and gum sources in the coating composition. Journal of Food Science, 46, 1010–1011 (1981) 11 MEYERS, M. A. Functionality of hydrocolloids in batter coating system. In: KULP, K. AND LOEWE, R. (Eds), Batters and Breadings in Food Processing. St Paul, MN: American Association of Cereal Chemists, pp. 117–142 (1990)
12 CUNNINGHAM, F. E. Developments in enrobed products. In: MEAD, G. C. (Ed), Processing of Poultry. London, U.K.: Elsevier Applied Science, pp. 325–359 (1989) 13 FAN, L. L. AND ARCE, J. A. Pretreatment of fried food products with oil containing emulsifiers. U.S. Patent, 4, 608, 264 (1986) 14 FEENEY, R. D., HARALAMPU, S. G. AND GROSS, A. T. Potato and other food products coated with edible oil barrier films. U.S. Patent, 5, 217, 736 (1993) 15 SAGUY, I. S. AND PINTHUS, E. J. Oil uptake during deep-fat frying: factors and mechanism. Food Technology, 49 (4), 142–145, 152 (1995) 16 BALASUBRAMANIAM, V. M., CHINNAN, M. S., MALLIKARJUNAN, P. AND PHILLIPS, R. D. The effect of edible film on oil uptake and moisture retention of a deep-fat fried poultry product. Journal of Food Process Engineering, 20 (1), 17–29 (1997) 17 BALASUBRAMANIAM, V. M., CHINNAN, M. S. AND MALLIKARJUNAN, P. Deep-fat frying of edible film coated products: Experimentation and modeling. Food Processing Automation IV, Proceedings of the FPAC IV Conference, Chicago, IL, November 3–5, St. Joseph, MI: American Society of Agricultural Engineers, pp. 486–493 (1995) 18 PARK, H. J. AND CHINNAN, M. S. Gas and water barrier properties of edible films from protein and cellulosic materials. Journal of Food Engineering, 25, 497–507 (1995) 19 RAO, V. N. M. AND WEBB, K. S. Flow parameters for nonnewtonian food using a co-axial cylinder viscometer. Journal of Food Processing and Preservation, 12, 243–249 (1988) 20 AOAC. Official Methods of Analysis. Washington, DC: Association of Official Analytical Chemists (1990) 21 SAS. SAS User’s Guide, Release 6.03. Cary, NC: SAS Institute (1988) 22 GREENER-DONHOWE, I. AND FENNEMA, O. The effects of plasticizers on crystallinity, permeability, and mechanical properties of methylcellulose films. Journal of Food Processing and Preservation, 17, 247–257 (1993) 23 PARK, H. J., BUNN, J. M., WELLER, C. L., VERGANO, P. J. AND TESTIN, R. F. Water vapor permeability and mechanical properties of grain protein-based films as affected by mixtures of polyethylene glycol and glycerin plasticizers. Transactions of ASAE, 37, 1281–1285 (1994) 24 GENNADIOS, A., WELLER, C. L. AND GOODING, C. H. Measurement errors in water vapor permeability of highly permeable, hydrophilic edible films. Journal of Food Engineering, 21, 395–409 (1994) 25 WALL, J. S. AND PAULIS, J. W. Corn and sorghum grain proteins. In: POMERANZ, Y. (Ed), Advances in Cereal Science and Technology, Vol. 2. St Paul, MN: American Association of Cereal Chemists, pp. 135–219 (1978) 26 POMES, A. F. Zein. In: MARK, H. F., GAYLORD, N. G. AND BIKALES, N. M. (Eds), Encyclopedia of Polymer Science and Technology: plastics, resins, rubbers, fibers, Vol. 15. New York, NY: Interscience Publishers, pp. 125–132 (1971) 27 NELSON, K. L. AND FENNEMA, O. R. Methylcellulose films to prevent lipid migration in confectionery products. Journal of Food Science, 56, 504–509 (1991) 28 TREZZA, T. P. AND VERGANO, P. J. Grease resistance of corn zein coated paper. Journal of Food Science, 59, 912–915 (1994) 29 KRUMEL, K. L. AND LINDSAY, T. A. Nonionic cellulose ethers. Food Technology, 30 (4), 36–38, 40, 43 (1976)
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Edible Coatings for Deep-fat Frying of Starchy Products P. Mallikarjunan, M. S. Chinnan*, V. M. Balasubramaniam and R. D. Phillips P. Mallikarjunan, M. S. Chinnan, R. D. Phillips: Center for Food Safety and Quality Enhancement, Department of Food Science and Technology, The University of Georgia, Griffin, GA 30223-1797 (U.S.A.) V. M. Balasubramaniam: Presently with National Center for Food Safety and Technology, Illinois Institute of Technology, Summit-Argo, IL 60501-1933 (U.S.A.) (Received October 1, 1996; accepted January 27, 1997)
The effectiveness in moisture retention and reduction of fat uptake of different edible coatings on a fat-free starchy product was determined. Mashed potato balls of 47 mm diameter were used as model food system. Samples were coated with corn zein (CZ), hydroxypropyl methyl cellulose (HPMC) or methyl cellulose (MC) film-forming solutions. Uncoated samples were used as control. Compared to the control, a reduction of 14.9%, 21.9% and 31.1% in moisture loss from the product was observed for samples coated with CZ, HPMC and MC films, respectively. Similarly, a reduction of 59.0%, 61.4% and 83.6% in fat uptake by the product was observed for samples coated with CZ, HPMC and MC films, respectively. Based on the overall performance in reducing moisture loss and fat uptake, MC exhibited the most significant barrier properties.
©1997 Academic Press Limited Keywords: edible film; deep-fat frying; potato; water content; fat reduction
Introduction Fat is one of the primary sources of energy in food and the per capita consumption of fat in the U.S. during 1987 was 28.4 kg (62.7 lb.) (1). The U.S. Surgeon General’s Report on Nutrition and Health included a dietary recommendation to reduce fat consumption by 25% and especially, to limit the energy consumed from saturated fat to less than 30% (2). Industry is responding to these challenges by introducing low-fat products. Moreover, interest in reducing oil uptake in fried foods has recently increased. Deep-fat frying is a widely used method for preparing foods with an attractive and tasty surface. The soft and moist interior along with the porous crispy crust provides increased palatability to foods (3). In the U.S., the fried-food business is flourishing. Reportedly, more than 500,000 institutional and commercial restaurants are involved in deep-fat frying operations (4). To meet the recent trends and consumer demand for low-fat products, there exists a need for reducing oil uptake during deep-fat frying. Most new low-fat fried products include fat substitutes in the formulation. Another approach is use of edible ingredients in the batter and breading mix to improve coating performance. Various ingredients such as alginate (5), powdered cellulose (6, 7), methyl cellulose (8) and soy protein isolates (9) have been used. Whey *To whom correspondence should be addressed.
protein, egg albumen and carboxymethyl cellulose (CMC) improved the breading adhesion significantly (10). Meyers (11) has discussed the functionality of hydroxypropyl methyl cellulose (HPMC) and methyl cellulose (MC) as barriers to fat absorption in breading and batters. The use of proteins (such as milk powder and egg albumen) as ingredients in fried products is also known, yet their functionality as edible coatings to reduce oil uptake by deep-fat fried foods have not been established (12). Limited research work has demonstrated the use of edible ingredients as a surface treatment. Fan and Arce (13) reported the use of amylose as a surface coating to reduce fat uptake during frying. Feeney et al. (14) used corn zein as an edible coating in French fried potatoes and reported a 28% reduction in oil uptake. Most of these new development approaches still rely on empirical tests to develop batter and breading formulations from protein and other hydrocolloids and most of the information is proprietary (15). We previously demonstrated the potential of edible films for moisture retention and reduction in fat absorption during frying of poultry products (16, 17). Compared to uncoated control samples, chicken nuggets coated with an edible HPMC coating had reduced oil absorption by up to 17.9% and 33.7% in the surface layer and the core, respectively. Coated samples also showed improved moisture retention (up to 8.6% in the surface layer and 16.4% in the core) compared to a
0023-6438/97/070709 + 06 $25.00/0/fs970263
709
©1997 Academic Press Limited
lwt/vol. 30 (1997) No. 7
Table 1 Formulations of edible film-forming solutions Film type
Base ingredient (g)
Water (mL)
Ethanol (950 g/L; mL)
Hydroxypropyl methyl cellulose
9
200
200
54 9
0 200
325 200
Corn zein Methyl cellulose
control sample (16). Our objective was to evaluate the moisture retention and fat reduction capabilities of different edible film coatings during deep-fat frying. In order to evaluate the effectiveness, a fat-free starchy product was selected.
Materials and Methods Mashed potato balls were used as the model food system. Baking potatoes were obtained from a local grocery store and were boiled for 45 min. After draining the water, the potatoes were peeled and mashed. The potato balls were formed using a commercial meat ball shaper (47 mm diam.).
Edible film coating To study the effectiveness of edible films/coatings, three different types were tested on mashed potato balls: CZ (corn zein), MC and HPMC. The film-forming solutions were formulated (Table 1) using the procedure described by Park and Chinnan (18). For HPMC (viscosity of 80–120 mPa.s in 2% aqueous solution, Aldrich Chemical Company, Inc., Milwaukee, WI) and MC (viscosity of 4000 mPa.s in 2% aqueous solution, Aldrich Chemical Company, Inc., Milwaukee, WI), the base ingredient was added incrementally to water maintained at 45 °C and allowed to dissolve with mixing. Then, 95% food grade ethanol (190 proof, Florida Distillers Co., Lake Alfred, FL) was added and thoroughly mixed. Polyethylene glycol (PEG) (Avg. Mol. Wt. 400, Aldrich Chemical Company Inc., Milwaukee, WI) was added as a plasticizer. The contents were covered with aluminum foil and left over night at ambient temperature to allow rising, coalescing and subsequent disappearance of air bubbles. For CZ (Regular grade, Freeman Industries, Inc., Tuckahoe, NY), the base ingredient was dissolved in 95% food grade ethanol. Anhydrous crystalline citric acid (C-0759, Sigma Chemical Company, St. Louis, MO) was added to enhance the dissolution of CZ by lowering the pH. For CZ, glycerin (G31-1, Fisher Scientific, Fair Lawn, NJ) was added as a plasticizer. The rheological characteristics of the solutions were determined using a rotational viscometer (Model LVTDV-II, Brookfield Engineering Laboratories, Stoughton, MA) equipped with a Thermosel system at ambient temperature. The apparent viscosity of the film-forming solutions was estimated using power law model: τ = Kγ˙ n, where, τ is the shear stress and γ˙ is the shear rate. The K
Plasticizer
Other
Polyethylene glycol (1 mL) Glycerin (11 mL) Polyethylene glycol (1 mL)
– Citric acid (1 g) –
(consistency coefficient) and n (the flow behavior index) of the solutions were estimated from experimental data as described by Rao and Webb (19). These coefficients would provide an insight to the applicability of spraying as a method for applying edible film solutions. Potato balls were coated by placing them individually on a rotating (15 rpm) platform and spraying with the film-forming solution for 1 min using a commercial spray gun (Model PM-50595, Popular Mechanics, Beckley, WV). After air-drying for 2 min, the procedure was repeated to ensure uniform coating of the film on the ball. To estimate film thickness on the product the filmforming solution was sprayed on a flat surface. The solution was sprayed first for 1 min and was sprayed again for another 1 min after air-drying for 2 min. This procedure simulated the coating process on the samples. The deposition rate was determined by spraying the solution for 3 min continuously on a flat surface. Data from difference in weight of the collecting surface, surface area and density of film-forming solution was used to calculate the film thickness and deposition rates.
Frying experiments Peanut oil at 175 °C was used as the frying medium. Uncoated and coated potato balls were fried for 240 s such that the center temperature reached 70 °C or above. All the experiments were carried out in triplicate. The temperatures of samples at different locations (2 mm below surface, mid point between center and surface, and geometric center) and oil in the vicinity of the samples were monitored using a T type thermocouple (TT-T-30, Omega Engineering, Stamford, CT) attached to a data logger (Hotmux, DCC Corporation, Pennsauken, NJ). The fried potato balls were air-cooled and excessive surface oil was gently wiped off using tissue paper. The samples were then weighed, placed in plastic bags and frozen at –20 °C until further analysis. Mass loss during frying was determined from the weight difference between precooked samples and deep-fat fried samples.
Determination of barrier properties of edible film To evaluate the effectiveness of edible film coatings, the moisture and fat contents of the surface layer and the interior core were determined. The samples were cut open into four quarters and the surface layer (0.1 mm)
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was scraped off and collected. The remaining portion of the sample was considered to be the core. The masses of surface layer and core portion were recorded. The moisture content of the surface layer and core samples was determined using a freeze-dryer (Research Freeze Dryer, Virtis Company Inc., Gardiner, NY). Freezedrying technique completely removed moisture and yielded porous samples for subsequent extraction of fat. The fat content of the freeze-dried samples were determined using AOAC method 960.39 (20). The overall moisture and fat contents of the fried potato balls were determined using: ¯ = M
F¯ =
Ws 3 Ms + Wc 3 Mc Ws + W c
Eqn [1]
Ws 3 Fs + Wc 3 Fc Ws + W c
Eqn [2]
where W = the mass (kg), M = moisture content (g/100 g), and F = fat content (g/100 g dry basis) of the respective layers. The subscripts s and c refers to the surface layer and core regions, respectively.
Statistical analysis Data were subjected to analysis of variance using SAS procedure GLM (21). Differences among packages were determined using the Duncan Multiple Range Test at 5% confidence level (or α = 0.05).
Results and Discussion Properties of edible film-forming solutions and coatings The density and viscosity values of edible film-forming solutions are shown in Table 2. The estimated film thickness ranged from 0.05 to 0.10 mm. CZ coatings were thicker than HPMC and MC coatings. This could be due to its viscosity and deposition rate, as CZ film had the lowest consistency coefficient (4.14 Pa.sn) and highest deposition rate (0.74 g/(m2.s)). The higher viscosity of CZ film-forming solution is due to its higher solids content than the HPMC and MC film-forming solutions. The values reported for the water vapor permeability of these edible films in the literature (18, 22–24) ranged from 89–200 3 10–12 g.m/(m2.s.Pa). The water vapor permeability of hydrophilic films depends
on film thicknesses and measurement conditions (temperature and relative humidity).
Moisture content The following visual observations were made during simultaneous frying of coated and uncoated potato balls. Initially, coated samples produced more bubbles in comparison to uncoated samples. As frying progressed, a reversal in the trend was observed with uncoated sample forming a larger envelope of bubbles compared to coated samples. This suggested that uncoated potato samples were losing more moisture to the frying medium than coated samples. The moisture content of the surface layer and core region is shown in Fig. 1a. The surface layer lost moisture rapidly and the final moisture content of surface layer ranged from 26 g/100 g for the control to 55 g/100 g for HPMC-coated samples. The edible filmcoated samples retained more moisture in the surface layer. The effectiveness in retaining more moisture was not significantly different among MC and HPMC. CZ retained more moisture than the control but significantly lower moisture than MC and HPMC. The difference in performance is mainly due to the type of edible film. CZ is a protein-based film whereas MC and HPMC are cellulose-based hydrocolloids. MC and HPMC form a protective layer due to thermal gelation above 60 °C. The methyl groups in MC and HPMC molecules undergo intermolecular association with adjacent molecules at temperatures above the incipient gelation temperature (IGT). The IGT is the temperature at which intermolecular association begins to occur very rapidly. Above the IGT, viscosity increases dramatically with increasing temperature, to the point where the solution gels (11). For MC and HPMC, IGT is in the range of 50 to 90 °C (depending on degree of substitution, concentration, etc.). This gel layer might have controlled the transfer of moisture and fat between the product and the frying medium. On the other hand, CZ forms a film layer upon contact with the surface of the product after spraying. The zein coagulates in moisture (moisture on the product surface) forming a thin film of precipitated corn zein on the surface. This insolubility of corn zein is mainly due to a high content of nonpolar amino acids and protein association through hydrogen bonding by glutamine (25). In addition, hydrophobic and hydrogen bonds
Table 2 Physical properties of edible film-forming solutions and coatings Viscosity
Film type
Density (kg/m3)a
Film thickness (mm)a
Deposition rate g/(m2 s)a
Consistency coefficient (Pa.sn)
Flow behavior index
Residual sum of squares
Hydroxypropyl methyl cellulose Corn zein Methyl cellulose
836.7±20.9 cb 877.9±6.3 b 967.5±4.9 a
0.05±0.01 b 0.10±0.02 a 0.06±0.01 b
0.37±0.11 b 0.74±0.11 a 0.31±0.06 b
46.47 4.14 387.54
1.02 0.98 0.98
168.23 9.28 971.00
a b
Means and standard deviations. Values with same letters in a column are not significantly different from each other at α =0.05.
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100
80 Moisture content (g/100 g)
develop in the film matrix (26) upon evaporation of the solvent (alcohol). Another possible contributing factor to CZ coatings resulting in lower moisture retention than HPMC and MC coatings was that CZ coatings were plasticized with glycerol while HPMC and MC coatings were plasticized with PEG. Glycerol is more hydrophilic than PEG resulting in poorer moisture barriers than PEG-plasticized films (23). In addition, CZ coatings contained higher amounts of plasticizer than HPMC and MC coatings. Moisture barrier ability of films decreases as the amount of added plasticizer increased (23). The core moisture content was not significantly different among the edible film-coated samples. The control lost more moisture compared to the coated samples.
(a)
a
b
a
a
a 60
a ab
40
b
20
0 30
Surface
Core
80 (b)
(a)
Moisture content (g/100 g)
Fat content (g/100 g)
a 20
15
b
10
c
c 5
MC
HPMC
c
60
40
20
a b
0
Surface
b
b
Core
0
Fig. 1 (a) Moisture and (b) fat content of surface layer and core of deep-fat fried potato balls coated with edible films. Columns with the same letter in a group are not significantly different from each other at (P < 0.05). Bars indicate standard deviation. (j) = control; ( ) = CZ; (C) = MC; (A) = HPMC
12
Fat content (g/100 g)
40
30 b
6 b
b
10
0
MC
b
4
2
CZ
(b)
8
20
Control
CZ
10
a
b
Control
a
50
Mass loss (%)
ab
b
25
0
a
HPMC
Fig. 2 Overall mass loss of deep-fat fried potato balls coated with edible films. Bars indicate standard deviation
c
Control
CZ
MC
HPMC
Fig. 3 Overall performance of edible films in (a) moisture retention and (b) reduction in fat uptake during deep-fat frying of potato balls. Columns in a group with the same letter are not significantly different from each other (P < 0.05). Bars indicate standard deviation
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Uncoated
Oil
Coated
Moisture
Oil
Moisture Edible
Crust
film
Crust
Core Core
Fig. 4 Conceptual diagram illustrating the effect of edible films on moisture and fat transfer during deep-fat frying of starchy products
Fat content A reduction in fat uptake was observed for deep-fat fried coated potato samples (Fig. 1b). The surface layer of coated samples absorbed less oil compared to control samples. The fat content of surface layer in control samples increased from 0 (undetectable levels by gravimetric analysis) to 28 g/100 g whereas MC-coated samples had only 13 g/100 g. The substantial reduction in oil uptake was mainly attributed to the barrier properties of the coatings to the transfer of moisture and fat (27, 28). The effectiveness in reducing oil uptake was not significantly different between MC and HPMC. Similarly, the effectiveness in reducing oil uptake was not significantly different between HPMC and CZ. Among all films tested, MC provided the lowest oil uptake during frying.
Mass loss The percent mass loss was calculated from mass before and after frying. Figure 2 shows the effect of edible film coating on mass loss. Control samples had 36% mass loss while mass loss for coated samples ranged between 19.8 and 22.2%. Coated samples reduced mass loss significantly and had approximately 40% lower mass loss than control samples. No significant differences were found among the edible coatings in reducing mass loss. The reason for lower mass loss could be attributed to the moisture retention properties of the edible films. In addition, the edible coatings acted as protective layers reducing material loss from the surface to the frying medium.
Overall performance of edible films The overall performance of different edible films during deep-fat frying of potato balls was calculated as described by Eqn [1] and Eqn [2] (Fig. 3). The overall moisture content was 54.5, 62.4, 71.4, and 66.4 g/100 g for control samples, and samples coated with CZ, MC
and HPMC, respectively. The overall fat content was 7.6, 3.1, 1.2 and 2.9 g/100 g for control samples and samples coated with CZ, MC and HPMC, respectively. In comparison to the control, percent moisture loss reductions were 31.1, 21.9 and 14.5 for MC, HPMC and CZ, respectively, and the percent reductions in fat uptake were 83.6, 61.4 and 59.0 for MC, HPMC and CZ, respectively. Among the films tested, MC exhibited the best barrier properties to provide moisture retention and reduction in fat uptake during deep fat frying. The better moisture barrier performance of MC coatings compared to HPMC coatings can be attributed to MC being less hydrophilic than HPMC (29).
Effect of edible films on moisture and fat transfer Figure 4 shows the role of edible films in reducing moisture loss and fat uptake in a conceptual manner. Based on the observations in the present study, the edible films form a protective layer on the surface of the samples. This protective layer is formed upon application of the coating as in the case of CZ and/or during the initial stages of frying as in the case of MC and HPMC. This protective layer inhibits the transfer of moisture and fat between the sample and the frying medium. Moisture removal and consequent fat uptake occur mainly in the crust (surface layers of the product) and, therefore, the role of the edible film coatings in retaining moisture and reducing fat uptake was limited only to the surface. Thus, the type of edible film was not significant with regard to moisture and fat contents of the core region, as evident by no significant difference in core moisture and fat content among coated samples. On the other hand, when considered with the surface effects, by selecting a suitable edible film, the moisture and fat transfer between the frying medium and the food can be controlled to produce an end product with desirable characteristics.
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Summary By suitable selection of edible films it is possible to control moisture and fat transfer between the frying medium and the food. Irrespective of the type of edible films used in our study, applying a coating reduced moisture loss and fat uptake during deep-fat frying. In comparison to the control, percent moisture loss reductions were 31.1, 21.9 and 14.5 for MC, HPMC and CZ, respectively, and percent reductions in fat uptake were 83.6, 61.4 and 59.0 for MC, HPMC and CZ, respectively. Among the films tested, MC showed most effective moisture and fat barrier properties.
Acknowledgements The study was supported in part by USDA Special Research Grant Program, USDA National Research Initiative-Competitive Grant Program (#95-37500-238) and with support from the University of Georgia. We thank Mr Glenn D. Farrell for his technical assistance.
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