Food coating
Encyclopedia
Coating is an industrial process
which consists in 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.
. 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 and physical principles. 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.
Pictures
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).
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 lost of chewing gum]. Barrier effects are often difficult to achieve.
An ingredient may by cheaper than the product it coats and thus allows for a slight cost reduction.
Sequences of the coating process
Coating as a system
.
A coating process can be broken into the following elements:
Collaterals occur along the process:
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:
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.
The combination of the above characteristics drives the choice of the process principle. It has then to be precisely described.
The selection of the proper process and its control rely on the gathering of precise and reliable information.
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.
Criteria for the selection of a technique.
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.
Typica measures :
, 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.
Partners
A coating process involves several partners, (not forgetting the consumer) :
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 affects 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.
Industrial process
Industrial processes are procedures involving chemical or mechanical steps to aid in the manufacture of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry....
which consists in applying a liquid or a powder onto the surface of a product of any possible shape to convey new (usually organoleptic) properties. Coating
Coating
Coating is a covering that is applied to the surface of an object, usually referred to as the substrate. In many cases coatings are applied to improve surface properties of the substrate, such as appearance, adhesion, wetability, corrosion resistance, wear resistance, and scratch resistance...
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
This article concerns coating applications in the food industryFood industry
The food production is a complex, global collective of diverse businesses that together supply much of the food energy consumed by the world population...
. There are many similarities between coating processes and numerous examples of technology transfer
Technology transfer
Technology Transfer, also called Transfer of Technology and Technology Commercialisation, is the process of skill transferring, knowledge, technologies, methods of manufacturing, samples of manufacturing and facilities among governments or universities and other institutions to ensure that...
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 and physical principles. 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
Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
, friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
, viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
, or crystallisation). Food coating is not a “hard” science such as drying
Drying
Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be considered "dried", the final product must be solid, in the...
or cooling
Cooling
Cooling is the transfer of thermal energy via thermal radiation, heat conduction or convection. It may also refer to:-Techniques:* Air conditioning* Air cooling* Computer cooling* Cryogenics* Conduction * Infrared solar cells* Laser 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.
| 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
- Organoleptic
Coatings can be added for the enhancement of organoleptic
Organoleptic
Organoleptic is an adjective used to refer to the aspects of food or other substances as experienced by the senses, including taste, sight, smell, and touch.-USDA uses:...
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
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
- 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
.
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:
| 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.
| 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 | 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.
The influence of some phenomena and their parameters is critical: crystallisation, water removal (drying), glass transition
Glass transition
The liquid-glass transition is the reversible transition in amorphous materials from a hard and relatively brittle state into a molten or rubber-like state. An amorphous solid that exhibits a glass transition is called a glass...
, 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.
Coating 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 |
| 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 Gum arabic 220px|thumb|right|Acacia gumGum arabic, also known as acacia gum, chaar gund, char goond, or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal... , xanthan, guar Guar The guar/guwar/guvar bean or cluster bean is an annual legume and the source of guar gum. It grows best under conditions with frequent rainfall, but tolerates arid conditions well. About 80% of world production occurs in India, but, due to strong demand, the plant is being introduced into new areas... 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
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.| Name | Principle | Example | Alternative | Batch/Continuous |
|---|---|---|---|---|
| Coextrusion | Forming of an outside casing around a 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.
| 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.
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
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.
Project management.
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 analysisFunctional analysis
Functional analysis is a branch of mathematical analysis, the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure and the linear operators acting upon these spaces and respecting these structures in a suitable sense...
, 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.
- Office work: preliminary analysis based on samples in order to define the technique.
- 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?
- Pilot. Test on 10 to 100 kg to demonstrate that what has been done manually can be mechanized and to prepare scale-up.
- 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 affects 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.