The concept of covering a basic food product with some sort of coating before cooking has been around for a long period of time. Think of English fried fish in a batter coating (with chips, of course!) or Japanese Tonkatsu. A quality coating plays an important role in maximising product yields, while delivering flavour as well as textural and visual differences.
Today, the manufacture of these coatings is now a major industry which has challenged food processing machinery manufacturers to develop application equipment that keeps pace with increasingly complex coating formulations.
There are two main types of coated product: those with an outer layer of coating applied as a liquid (i.e. battered product) and those with the outer layer of coating applied in granular form (i.e. breaded product).
In general, the cost of the coating ingredients is lower per kilogram than the food to which they are applied, usually called the substrate. As a result, it can be cost-beneficial to add a high percentage of coating, or pick-up, so the product is accepted without loss of quality.
Once the pick-up for a particular product has been determined, it is important to maintain this percentage in order to achieve a consistent end-product. For some lower grade raw products, adding a coating may actually improve the quality of the finished product.
A main requirement of any application system is that the coating adheres to the substrate. A well-designed coating system will maximise coating adhesion, ensuring a high quality finished product.
The most prevalent of coated foods is a breaded product where a coating is usually applied on top of an adhesive batter on a pre-dusted substrate. The process is usually described as pre-dust, batter, or bread, and if a higher pick-up percentage is required, the batter/bread stage can be repeated.
Applying a pre-dust absorbs moisture on the surface of the substrate, allowing the better batter-adhesion to the product. Usually consisting of a flour base, pre-dusts can have a wide variety of additives with some being quite product-specific.
It is common for manufacturers to add gums or other starches to promote adhesion of the batter. Flavours can also be included in the pre-dust as this is the coating layer that is in direct contact with the food substrate.
In a flat breading applicator, the substrate is conveyed on a bed of pre-dust through a curtain of pre-dust falling from an upper hopper, so as to ensure that both upper and lower surfaces are evenly coated. Following this, a compression roller may be used to push the substrate into the bottom bed and help to coat the top and sides as well as improving adhesion to the substrate.
Drum breaders provide high capacity pre-dust or breading application. Flour dust or breading is thoroughly applied to all surfaces as product is gently tumbled inside a slowly revolving drum.
The pre-dust needs to be kept free of lumps—these occur as moisture is absorbed from the product and coagulates. This is usually done on a continuous basis at some convenient location on the applicator, by sifting to keep a light consistency.
Also, the inclusion of an air blow off to prevent excess coating material from being left on the substrate will prevent excess coating material from getting into the batter. This stops the batter from thickening, which would make it more difficult for manufacturers to maintain the right viscosity.
Flour is a non-free flowing material which bridges quite easily in hoppers and causes high belt tensions in applicator conveyors. Flour also ‘floats’ very freely in air, and dust control can consequently be a significant issue to be addressed by these machines, particularly those running highly seasoned flavoured pre-dusts!
There are a lot of specialty pre-dust units marketed by different equipment manufacturers which address the specific needs of the materials being applied.
Some machines have interchangeable parts which allow them to be converted for use as free flowing breaders. This can assist smaller processors, greatly increasing their flexibility to configure lines for products having different coating systems with a minimum level of equipment investment.
Once production increases and dedicated process lines are required, flexibility becomes less important, but a universal breading applicator can still be used as a backup when required.
After pre-dusting, the substrate is given a coat of adhesive batter on which the outer breading will be applied. Such batters start from a basic flour and water mix to which functional ingredients can be added to suit the product. Typically, these ingredients will include additional starches and gums such as alginates or carageenan for adhesion and guar or xanthan gum to increase viscosity.
Control of viscosity and temperature of the batter throughout production is essential for consistency of product texture, appearance and for coating system performance in the fryer. Viscosity is generally frequently checked during production using both manual and automated methods.
Batter can be applied in one of two systems. Low viscosity batters, which includes most adhesive batters, may be applied in a curtain (or weir type) applicator or in a submerger type applicator. Tempura batters are most commonly applied in a submerger type applicator.
In the curtain type applicator, the substrate passes through a shallow pool of batter whilst more batter is poured over the upper surface from a series of curtains. As the excess batter flows off the substrate, the sides of the product are also coated. Air knives and wipers may be used to remove excess batter which might otherwise carryover into the breader and cause wastage or unsightly coatings.
The applicator is also equipped with a pump to circulate the batter to the weirs and to prevent the solids from settling out when using lower viscosity batters. The pumps are specifically selected to reduce shear on the batter which can lower the viscosity and degrade the coating system.
Pumping generates heat which may need to be compensated for by inclusion of a chiller in the circulation loop to ensure the batter temperature is maintained within a certain range.
Outer Bread Coating
The final step in this process is to add the outer breading coating which comes in many forms and styles. The ‘crumb’ style breaders that are most frequently encountered are cracker meal, American crumb, and Japanese crumb.
Other categories include flour or powder breading. Based on wheat flour, a variety of other flours, starches, gums, seasonings and colouring agents may be added. This coating produces a very different bite to a crumb coating, but is capable of carrying much heavier flavours than crumb.
Flour coatings behave similarly to pre-dust and the same applicator can be used for both products. Dust control is very important here as the seasonings added to the flour can cause irritation if inhaled or rubbed into the eyes.
Some substrates can be quite difficult to cover fully with a flour breader due to their shape, flexibility or skin flaps. Special machines have been developed to flip the substrate two or three times during application so that the difficult-to-get- at places can be properly coated.
When the outer coating on the product is a batter, usually a tempura style will be applied. These batters have similar ingredients to the adhesive batter with the addition of leavening agents. These combine with the heat of the frying process to produce carbon dioxide, causing the batter to expand as it sets, forming the crisp open structure which characterises tempura coatings.
The basic process of pre-dust/batter with pre-dusting ensures good adhesion of the batter to the substrate. Some products will get a pre-treatment with a wash batter to ensure that the substrate surface has uniform moisture for even pre-dust coating and to increase the pick-up of coating in the final product. Wash batters are usually applied in a curtain type applicator.
In contrast, tempura or other puff style batters are most commonly applied in a submerger applicator because of the high viscosity of these coatings. Air knives are used to remove excess batter before the frying stage to avoid unnecessary fines building up in the cooking oil.
Control of temperature and viscosity of a tempura batter is even more critical than for adhesive batters; they are therefore usually mixed in small batches to enable them to be used within 5-10 minutes of mixing.
Alternatively, they may be mixed via an automated system which recirculates the batter between the applicator and a temperature controlled reservoir. Water and batter mix are added in accordance with the sensed viscosity of the circulating batter, to maintain the required values in the applicator.
Circulation pumps are usually of the positive displacement type to minimise batter shear damage. They are designed for easy removal of the impeller to facilitate cleaning and all piping is connected to the equipment with quick release couplings for the same reason.
There are also a number of special purpose batters that are used for specific products. Some of these are designed to act as binders, holding the breading onto the substrate during frying without changing the appearance of the product. Others are designed to reduce the amount of oil take-up in the fryer, which is attractive to the health conscious consumer.
Another clear coating can be applied to French fries which enables them to be presented crisp and palatable for up to 30 minutes after frying.
Frying And Oil Quality
Frying could be considered the most common method of cooking coated foods. It sets the coating, imparts flavour and produces colour with an appetising finish.
Choosing the right frying system is vital for processors to ensure they get optimum results. In order to maximise their return on the fryer systems, processors should take into consideration the following when selecting frying equipment:
- Type of product—Fully or par fried, physical size, shape
- Type of coating required
- Expected production throughput rates
- Amount of oil that is absorbed during frying
- Heat load on the fryer
- Optimum conveyor design
- Cleanability of fryer
- Oil filtration requirements
During the frying process, heat energy from the hot oil starts a chemical reaction leading to the formation of a crust on the coated product.
For example, water from the coating is heated to form steam and produces the bubbling normally associated with the initial frying of food. This skin then deepens to form a crust while simultaneously releasing steam from within the product. As the pressure from the escaping steam reduces, hot oil enters and fills the voids within the crust left after the water has been boiled away.
As the use of suitable frying oils is a major ongoing cost and concern, food processors must endeavour to maximise the oil used in their frying process. There are a considerable number of areas that can impact ongoing oil quality including: the use of the correct oil for the product, effective oil filtration, production rates, oil turnover time, fryer design, oil handling and storage and correct fryer operation practices.
Coating Application And Frying Equipment
A basic batter/bread system will do a good job in a coat and freeze situation, but when the product is to be further processed by frying or oven cooking prior to freezing, the coating system needs to be more sophisticated.
For the best coating on the finished product, a processor needs to work closely with both the ingredients supplier and the equipment supplier. Both the coating equipment and any subsequent cooking (fryers and/or ovens) should be considered as a whole with processing systems needing to work together to obtain an optimum result.
The selection of coating and cooking equipment involves decisions covering many facets including functionality, operational needs, marketing and consumer needs as well as hygiene integrity and legislative requirements.
Furthermore, market opportunities require the equipment to contend with current products and situations in addition to having to comply with possible future needs and requirements.