Food coating

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Coating is an industrial process which consists of applying a liquid or a powder onto the surface of a product of any possible shape to convey new (usually organoleptic) properties. Coating designates an operation as much as the result of it: the application of a layer and the layer itself. Coating takes much different meanings depending on the industry concerned.

Definitions[edit]

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 liquid and/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, or crystallisation). Food coating is not a “hard” science such as drying or cooling, which can be described by equations and be predictable. Food coating is rather a “soft” knowledge derived from the accumulation of know-how. One reason is the product and the ingredients considered have complex characteristics, variations and interactions.

Encapsulation is the application of a liquid layer on 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 to 1000 µ) and “food coating” (above this limit). Mere mechanical movement is not adequate and sufficient to fulfill the proper coating of minute particles.


Examples of coated products
Finished product Base Ingredient Post treatment Rate, % Objective
Ready-to-eat cereals Expanded cereals Sugar syrup Drying 20-50% Flavour and appearance enhancement
Dragees Nuts, chocolate, sweets Sugar Panning 10-100% Taste, flavour
Chocolate Hazelnuts, almonds Chocolate Cooling 30-50% Taste, flavour
Prepared vegetable Frozen vegetables Water, fats, flavourings Freezing 15-100% Taste, convenience
Processed cheese Grated/shredded cheese Anticaking None <2% Prevent agglomeration
Nuggets Meat Batter and crumb Frying 30-50% Palatability, cost, appearance
Snacks Expanded flour Oil and seasoning None 5-40% Palatability
Crunchy nuts Peanut Flour, binder, seasoning Frying, baking 30-100% Palatability

Pictures

Objectives of coating[edit]

  • Organoleptic

Coatings can be added for the enhancement of organoleptic properties of a food product. Appearance and palatability can be improved by adding colour (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).

  • Nutrition

Coatings also can be used to add vitamins and minerals (enriched white rice) or food energy.

  • Functional

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 lost of chewing gum]. Barrier effects are often difficult to achieve.

  • Cost

An ingredient may by cheaper than the product it coats and thus allows for a slight cost reduction.

The coating process[edit]

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.

Sequences of the coating process

  1. 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 stes 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), orthermal treatment (proteins set irreversibly when heated).

Coating as a system

The coating process seen as a system

.

A coating process can be broken into the following elements:

  • Inputs: base product, additives and ingredients
  • Additional flows: air as a carrier of product or ingredient, or for drying, energy in a mechanical (agitation, transfer, friction) or a thermal form (convection, conduction or radiation heating)
  • Outputs: end product, excess of coating ingredient, lost or to be recycled

Collaterals occur along the process:

  • Breakage of product
  • Generation of fines
  • Agglomeration of products
  • Clogging of system surfaces with product or ingredient
  • Airborne pollution, volatile organic component

These effects generally are to be avoided unless the end product is made more desirable.

The parameters of the system Parameters affecting the system are listed by origin:

Initial characteristics.
Base product End product Production
Shape, size, distribution, bulk density, nature, surface aspect, resistance, composition, flow behaviour, fines, hygroscopicity, temperature Capacity, end aspect, weight gain, storage behaviour, resistance Recipe changes, duration, cleaning

This first set of criteria governs the choice of the coating ingredient. The coating consists either in a single ingredient or a mix. This mix has different physical forms: solution, emulsion, suspension, powder, etc. It has its own characteristics. In addition, a fluid may be required such as spraying, cooling, heating or drying air.

Ingredient characteristics.
Additive Fluid.
Water or fat base, composition, concentration, viscosity, temperature, melting point, surface tension, setting behaviour Nature, temperature, relative humidity

The combination of the above characteristics drives the choice of the process principle. It has then to be precisely described.

Process characteristics.
Process Machine
Continuous, batch, residence time, ingredient temperature, fluid temperature, system temperature, flow volumes, tolerance to variations, number of functions to fulfill (feeding, dosing, recycling, drying) Form, internal surface, internal volume, size, mechanical movement, speed, temperature

The selection of the proper process and its control rely on the gathering of precise and reliable information.

Influence of temperature on adhesion.

The influence of some phenomena and their parameters is critical: crystallisation, water removal (drying), glass transition, viscosity, or surface tension.

Temperature Among the parameters, temperature has a choice place. It influences viscosity, surface tension, drying or crystallisation behaviour. Ultimately, it influences the coating rate (thickness, weight gain)and coating resistance. It therefore influences the degree of clogging of product and ingredient in the system. For example, fat will tend to set preferably on a cool product if the system wall is kept at a higher temperature.

Practice[edit]

Coating ingredients[edit]

Ingredients
Ingredient Form Usual coating rate Characteristics related to coating Coating aim Examples
Water Pure liquid 1-3% Wetting, adhesion, weight gain Dust prevention, freezer burn prevention
Water Saturated steam. 1-3% Wetting, adhesion Gluing of sugar on candies
Alcohol Solution 70% 0,1% Alcohol rate Antimicrobial, preservative, texture enhancement Preservation of packed pastries
Resin (shellac) Alcoholic solution 30% 1% Film forming Surface aspect, flow, moisture barrier Glazing of chocolate dragees
Wax (beeswax, carnauba, candelilla) Solid form, melted, micronized suspension in oil 1% Melting point Surface aspect, barrier Glazing of sugar dragees, fruit waxing
Sugars (saccharose, glucose, honey, polyols) Solution 70-90% 1-100% Crystallisation form, concentration, temperature Palatability, surface aspect (glazed or frosty) Sugar-coated ready-to-eat cereals
Natural hydrocolloid (gum arabic, xanthan, guar gelatine Solution 20-40% 3% Film forming, barrier Mechanical or chemical protection, carrier of additives Precoating of dragee with gum arabic and sugar prior to dragee pan coating
Starches (native or modified) Colloidal solution 20-40% 3% Characteristics depend on physical and chemical modifications, substitutes for more expensive ingredients (gum arabic, gelatine, titanium dioxide, etc.) Mechanical or physical protection Coating of fries before frying to reduce oil pick up
Flours Powder or thick suspensions 20-40% 10-20% Film forming, charge, viscosity, baking expansion Thickness, crispiness Coating of nuggets with batter prior to breading
Mineral or organic powders (talc, cellulose, potato flour, cellulose, starch) Pure powder 1% Moisture or fat absorption Anticaking, drying Coating of shredded cheese to prevent agglomeration
Oils and fats Pure 1 - 40% Melting point, viscosity Anticaking, adhesion, barrier against moisture migration Oiling of dry raisins or inclusions in ice cream
Seasonings, flavours, flavour enhancers, salt Powder, diluted or concentrated solutions 1-3% Concentration Palatability Flavouring of expanded snacks, salting of roasted nuts

Coating techniques[edit]

For the sake of classification, two categories can be split easily into batch or continuous processes. Then, the categories can be refined according to the way the product is set in motion and the ingredient applied. Then, techniques allow either for just coating or can combine coating and setting in the same equipment.

Techniques
Name Principle Example Alternative Batch/Continuous
Coextrusion Forming of an outside casing around an inner content Forming machine for sausages with forming of a collagen casing, further setting by coagulation and drying High-temperature, short-time cooking-extrusion of snacks with continuous filling of a flavoured paste. Continuous
Paddle mixer Mixing by agitation in a closed volume Snack coating Helicoidal, scraping paddles Batch
Vat mixer. IQF coating (and freezing) tumbler. Vacuum mixing Batch
Conveyor Application of the ingredient on the product spread across a conveyor. Topical coating of pastries. Spraying, screen, dipping coating. Continuous.
Drum. Application of the ingredient while the product is tumbled in drum. Snack seasoning. Simultaneous coating and drying of cereals with sugar. Continuous.
Screw. Application of the ingredient while the product is transferred and mixed in a trough fitted with screw(s)). Petfood. Twin-screw systems for a better mixing. Continuous.

Criteria for the selection of a technique.

  • Base product characteristics : shape, size, bulk density. Size is the first criterion.
  • Mechanical resistance of the base product.
  • Final thickness of the coating layer.
  • Complete/partial, top/side/bottom coating.
  • Number of sequences to repeat.
  • Processing time for each sequence.
  • Setting mode : drying, cooling, freezing…
  • Capacity.
  • Preferred batch or continuous system.

Comparison batch vs. Continuous. The demand for higher yields makes production managers want to shift from manual batch to continuous automated systems. One has to consider the pros and contras prior to go for a costly and risky decision.

Comparison
Batch. Continuous.
Advantages Flexible, easy to monitor and control, quick response, less time-dependent, easy recipe change, full traceability, tool for R&D. Efficient, justified if upstream and downstream processes are continuous.
Drawbacks. Limited capacity. Manpower. Expansive. Requires careful controls, feedback signals, consistent feeding and multiple peripherals.

Peripherals[edit]

Process in one glance.

Given the number of operations and steps, a coating process can be an extensive process considered as a whole. The process core machine requires peripherals to serve it. A few frequent ones are listed for information.

  • Storage.
  • Ingredient preparation.
  • Product feeding and metering.
  • Ingredient dosing.
  • Filtration or sieving.
  • Application system.
  • Recycling.

Measures[edit]

Test results can be immediately evaluated (visual aspect) but are preferably assessed by careful measures : to allow monitoring, to agree on commissioning, to certify conformity with customer requirements.

Typica measures :

  • Optical : colour, microscopy (homogenety, thickness), image analysis.
  • Weighing : weighing before and after treatment, weighing between batches or individual particles.
  • Specific measurements according to target : compaction, barrier property.

About coating.[edit]

Project management.[edit]

Information need to be gathered carefully along the above mentioned lines : base product, end product, ingredient, production constraints… Functions to fulfilly need to be clearly identified through Functional analysis, value analysis. Intuitive preference for a technique should not command premature decisions. A process is as much the result of the product target specifications as the consequence of production and technical environment conditions (product recipe changes, seasonal activity...).

Project steps The development of a product draws on limited time and money resources. A progressive approach spares both.

  1. Office work: preliminary analysis based on samples in order to define the technique.
  2. Laboratory : test on 100 g to 10 kg to demonstrate the physical feasibility and gather the parameters. Can the product be coated with this ingredient at all?
  3. Pilot. Test on 10 to 100 kg to demonstrate that what has been done manually can be mechanized and to prepare scale-up.
  4. Industrial test : test on 100 kg to tons to assess the sustainability of the process in industrial conditions.

Partners A coating process involves several partners, (not forgetting the consumer) :

  • Final marketer of the product.
  • Product manufacturers, sub-contractor, actual user of the process.
  • Ingredient producers.,
  • Machine manufacturers.
  • Research centers.

Some advices.
Coating is an easy operation in itself. Make the process last is the challenge.
Coating is nothing without the setting phase to which it is intrinsically tied.
The devil hides into the details. Minor factors can have huge detrimental effects on the quality of the product and the viability of the process.
Coating is more practice (trial and error) than science (equations).
Consequence : scale-up requires repeated evaluations as the volumes tested grow.
The choice of a coating technique is very dependent upon external conditions rather than the process itself.
Coating is a threesome play : product, ingredient, machine. Common sense tells that the ingredient and machine should adapt to the product arget, but a slight change in an ingredient can make a big difference in how easy the machine works.
Managing the peripherals is often the most difficult task in running the process.
Being at the end of the process line, the coating process receives little attention, is allocated little time until the end of the project where suddenly panic is risen over deadlines and budget limits.

References[edit]