Coating is a process that consists of applying a liquid or a powder into the surface of an edible product to convey new (usually sensory) properties. Coating designates an operation as much as the result of it: the application of a layer and the layer itself. Coating takes different meanings depending on the industry concerned.
Definitions
This article concerns coating applications in the food industry. There are many similarities between coating processes and numerous examples of technology transfer to and from the food industry.
Coating in the food industry is the application of a layer of liquids or solids onto a product. The operation essentially relies on mechanical energy. It consists mostly in setting the product particles in motion and simultaneously applying the coating ingredient in a certain pattern to expose one to the other. It involves such phenomena as adhesion, friction, viscosity, surface tension and crystallisation. Food coating is not a “hard” science such as drying or cooling, which can be described by equations and are predictable. Food coating is rather a “soft” knowledge derived from the accumulation of know-how. One reason is that the product and the ingredients considered have complex characteristics, variations and interactions.
Encapsulation is the application of a liquid layer to very small particles. It relies on an array of principles: entrapping a molecule inside a matrix, chemical bonding, and polymerisation. Encapsulation aims at the protection and controlled release of active molecules when immersed in an environment. As a rule of thumb, particle size can discriminate between “encapsulation” (below 300 µm to 1000 µm) and “food coating” (above this limit). Mere mechanical movement is not adequate and sufficient to fulfill the proper coating of minute particles.
Objectives of coating
Coatings can be added for the enhancement of organoleptic properties of a food product. Appearance and palatability can be improved by adding color (white dragee, brown chocolate), changing the surface aspect (glazed sweets or rough, crispy nuggets); changing or adding tastes (sweet dragee, salted snack) or flavours (fruit-glazed sweet goods), or texture (breaded crispy nuggets).
Coatings also can be used to add vitamins and minerals (enriched white rice) or food energy.
Coating conveys functional properties, such as particle separation (oiled dry fruit, shredded cheese), antioxidant effect (fruit cubes), or a barrier effect [water migration between a layer of ice cream and a biscuit (cookie) or against moisture loss of chewing gum]. Barrier effects are often difficult to achieve.
An ingredient may be cheaper than the product it coats and thus allows for a slight cost reduction.
The coating process
The coating process begins with the application of the coating on the food product, but the end product must be stable throughout its shelf life. Therefore, a coating process is completed by a stabilizing process, either by freezing, cooling, heating or drying. The sequences of this process are:
Application: To apply minute quantities of an ingredient, spraying is used to disperse it first, instead of just pouring it. This hastens the dispersion on the whole surface of the product. For larger ratios of coating to substrate, mixing or dipping can be used. Multiple stages also can be used; breaded meats, for example, may have a dry application (predust) followed by a wet batter dip and then another dry crumb application. Adhesion: the coating must adhere to the product, meaning there must be a degree of affinity between the ingredient and the product. Coalescence: in case of a liquid, the multiple droplets may merge to form a uniform continuous layer. Characteristics of the ingredient in relation to the product, such as viscosity and surface tension associated to a mechanical effect (friction) are critical. Stabilisation : depending on the nature of the coating ingredient(s) and substrate product, the ingredient is stabilised by elimination of the solvent (drying and evaporation of water, alcohol), crystallisation (sugar crystallises when water is evaporated, fat crystallises when cooled), or thermal treatment (proteins set irreversibly when heated).