(Editor’s Note: This is an online-only article attributed to the August/September 2018 issue.)
Deep-fat frying is a common cooking method that uses fat or oil as the heat transfer medium in direct contact with the food at a temperature above the boiling point of water. Among all types of fried foods, fried fish and fried chicken products make up a substantial percentage of the items sold at fast-food restaurants. According to the U.S. Census data and Simmons National Consumer Survey, 93.8 million Americans consumed fried chicken in 2011. This figure was projected to increase to 101.04 million in 2020. From the consumers’ standpoint, fried food palatability is related to unique organoleptic and sensory characteristics, including flavor, texture, and appearance.
During the deep fat frying method, oil serves as a heating medium and absorbs into food, increasing the total fat content. For example, lipid content of French fries increases from 0.2 to 14 percent, lipid content may reach 40 percent in potato chips, and raw fish with 1.4 percent reaches 18 percent fat after frying. Fried foods have become a health concern and high consumption of fried foods has been associated with conditions including hypertension, low serum HDL cholesterol, obesity, and type 2 diabetes. Furthermore, the frying process generates various lipid oxidation products, some of which have been linked to premature aging and cancer. Therefore, there is much interest in reducing fat uptake during deep-fat frying.
On the other hand, reducing the fat content in fried foods is not that simple since it affects organoleptic properties such as taste and mouthfeel. In a November 1999 issue of LWT-Food Science and Technology, authors indicated that selection of an appropriate food coating before battering is one possible means of reducing fat-uptake. An edible coating is a thin layer of edible material formed as a coating on a food product. These coatings can act as barriers to moisture loss, which is important commercially, and reduces fat uptake during frying.
The effectiveness of a coating material is determined by its mechanical and barrier properties, which depend on its composition and microstructure, and by the characteristics of the food product to which it binds. Hydrophilic biopolymers can be used as water binders in a coating to reduce water loss from the coat. If water loss can be reduced, oil uptake would also be reduced. Most commercial biopolymer coatings that are claimed to act like this to reduce fat uptake are polysaccharide coatings.
Polysaccharide-based coatings are low-cost, biodegradable, and water-soluble; they do not require organic solvents before they are applied. Polysaccharides are hydrophilic, thus have poor water barrier property.
Starch. Starch is an abundant inexpensive biopolymer that can be obtained from corn, cassava, etc.
It consists of linear chain, amylose, and highly branched amylopectin. The predominant linear nature of amylose readily makes it easily form film. The amylose portion forms coherent, relatively strong, and free-standing films which are non-continuous and brittle. Starch films are odorless, tasteless, colorless, nontoxic, and do not interfere with the taste. However, the three hydroxyl groups per D-glycosylic unit impart a high degree of hydrophilicity to starch which does not favor its ability to lessons water escape.
Cellulose. Cellulose and its derivatives have been used as lipid barrier in deep-fat frying. Cellulose derivatives including hydroxypropyl methyl cellulose (HPMC), and hydroxypropyl cellulose (HPC), methyl cellulose (MC), carboxymethyl cellulose (CMC) have good film-forming characteristics. Cellulose and derivatives such as HPMC are able to form thermal gel to protect their content during frying.
MC, HPC, and gellan gum has been proven to fat absorption into pastry mix by 50 to 91 percent, but MC was found to reduce fat uptake more than other films. Also, MC and HPMC, corn zein and amylose were used to coat Akara, a West African food made from whipped cowpea paste. The effectiveness of the two methods (spraying and dipping) of application were evaluated as well; all three materials significantly reduced fat uptake in deep fried Akara in both application methods. MC coating applied by dipping was the most effective (49 percent) among all three materials. However, deep fried Akara coated with MC and HPMC had a shiny, smooth, and glazy appearance, contrary to the uncoated and products coated with corn zein and amylose.
Alginate. Alginate is another type of polysaccharide that has been used as lipid barrier in deep-fat frying. It can form coatings with or without gelation through solvent, electrolyte cross-linking (calcium) or injection of a water-miscible non-solvent for alginate.
Proteins are broad range of macromolecules that are found in both animals and plant sources. Protein-based coatings have been explored as potential coating materials in fat-uptake reduction. Plant sources of protein-based coatings include corn, wheat, and soy whereas animal sources include milk, whey, and muscle protein. Proteins are made up of 20 smaller units called amino acids which are attached to one another in long chains. However, polysaccharides such as starch and cellulose are consisted of glucose units that joined together. This imposes increased functional properties on the protein, mostly making it potent for intermolecular bonding. The amino acid consists of an amino group (-+NH3), a carboxyl (-COOH) group and a residue, R group, that is attached to a central carbon atom. The R group, also known as side chain, determines properties such as polarity, solubility, acidity, etc., of the amino acid. The arrangement of the amino acid determines the structure and name of the resulting protein. Peptide bond join several amino acids to form protein. Protein gelling is influenced by both intrinsic and extrinsic factors. Gelling occurs primarily due to an increased limited protein-protein interactions as well as a decrease in the protein-water interactions and these are influenced by external factors such as heat, pH, or ionic strength. Gelling is intrinsically influenced by protein concentration, composition, and extent of denaturation of the protein. Proteins have a three-dimensional conformational structure, and this tends to form networks during unfolding (denaturation). The loss of this native 3-D structure of protein by heat, acid, base, and/or solvent is necessary in order to form the more extended structures that are required to from film. Comparatively, proteins are able to form films with better mechanical and barrier properties than polysaccharides. However, films made from proteins are prone to cracking because of strong cohesive energy of the polymer.
The selection and use of protein materials in edible coating faces a major challenge as to its general acceptability. A number of people are allergic to certain proteins, as their Generally Recognized As Safe (GRAS) status must be a characteristic feature as well. Generally, unlike animal-based proteins, plant-based proteins are associated with the allergenic reactions, placing a limit on their broad application.
A 2009 issue of Journal of Food Engineering investigated the influence of the use of edible coatings from whey protein and soy protein isolate during deep frying of a pre-fried, frozen product performed from cassava. Authors showed that whey protein had the best results with fat absorption, presenting a reduction of 27 percent for the cassava puree product.
A 1997 issue of LWT – Food Science and Technology indicated almost 60 percent reduction in fat uptake on potato cuts coated with corn zein. They form films that are brittle, and therefore require plasticizers to increase their flexibility.
Soy protein is a protein obtained from a soybean after it has been dehulled and defatted. The use of soy protein in edible coating is characterized by many restrictions due to the allergic reactions associated with it. Potato pellet chips coated with soy protein isolate reduced more fat than all different concentrations of Carboxymethyl Cellulose (2 percent, 6 percent, 10 percent, and 14 percent w/v). Application of soy protein at 10 percent edible coating in doughnut mix resulted in 55.12 percent fat reduction.
Wheat gluten is a general term for water-insoluble proteins of wheat flour that is composed of a mixture of polypeptide molecules, considered to be globular proteins. A 2009 issue of Food Chemistry studied the impact of wheat gluten-based edible coating on dough sheets during deep-fat frying. High gluten content resulted in lower oil uptake in products with low moisture content.
Casein forms 80 percent of milk protein and has three principal components: α, β, and k-casein. A 2002 issue of Food Research International reported the failure of milk casein in reducing fat uptake, in which cereals coated with it rather had higher fat content than uncoated cereals. On the contrary, a 2005 issue of Food Science and Technology International found that potato chips coated with casein had 14 percent less oil than those that were not coated. Therefore, product type and characteristics could furnish a reason for differences in oil uptake observed by both research works. Authors also reported that potato slices coated with whey protein resulted in 5 percent reduction in fat uptake. The binding property of whey protein improved in the presence of sodium bisulphite in blanching a solution. Wheat flour battered chicken strips coated with 10 percent denatured whey protein isolate resulted in staggering 30.68 percent reduction in fat uptake as compared to coating without whey protein.
Lipids, unlike hydrocolloids exhibit good moisture barrier properties because of their hydrophobic nature. Generally, films made from lipid lack the structural integrity that films from protein or polysaccharide have. Also, they are not able to form cohesive films and have been used as coatings or incorporated into biopolymers to form composite films. This gives a better water vapor barrier due to their low polarity. Although incorporation of lipids into biopolymers may decrease the water vapor property of the film, it will create a film with non-uniform structure, according to an April 2012 issue of Journal of Food Engineering. Wax, acetoglycerides, etc. are the different types of lipids used in edible coating. Wax coatings have been reported to be substantially more resistant to moisture transport than most other lipids or non-lipid coatings. Nevertheless, coatings made from wax and other fat and oil materials are characterized by challenges like cracking, greasy surface, homogeneity, and undesirable organoleptic problems.
In summary, the superimposing characteristic feature of edible coating used in deep-fat frying is its ability to deter water escape from the product and oil imbibition by the product, which can be explained by their thermogelling and crosslinking characteristics. This feature helps to improve nutritional value of food products by reducing oil uptake during frying. They improve the sensory qualities of the food and can be used as a vehicle for incorporating other useful ingredients, such as antimicrobial and antioxidant agents. Oil absorption in deep-fat frying has been linked to water escape through the pores and consequent oil occupation of the voids created. When coating is applied to food, it forms a film on the surface of the product serving as a barrier to prevent water and fat movement during frying. As frying progresses, water loss is imminent, but the film formed reduces the size and number of pores. The amount of water loss from the surface consequently restricts fat inflow because coating makes the surface of the food stronger and more brittle during frying, improving the water-holding capacity of the product. Edible coating also works by changing the surface hydrophobicity of the food products.
Dr. Tahergorabi is an assistant professor of food science at NC A&T State University. Reach him at email@example.com.