COMMERCIAL CANNING OPERATIONS
Shelf-stable foods go through various operations before making them ready for marketing, distribution, and consumption. Some typical operations in the commercial canning process are schematically represented in Figure 3.15.
RAW MATERIAL SELECTION
The quality of processed foods depends largely on the quality of the incoming raw product, which in turn depends on how the food is procured or harvested, handled, and stored. Harvesting at the proper maturity is an important step in thermal processing of fruits and vegetables.
Most fruits are harvested and processed in the “softripe” stage. Exceptions include bananas, pears, and some apples, which, when harvested at a mature stage, produce a higher quality processed product than those harvested at the “soft-ripe” stage.
Vegetables; for freezing preservation are generally harvested at a tender stage, while for canning they are harvested at a somewhat more mature stage so they will have sufficient strength to withstand the cooking required for achieving commercial sterilization.
With meat and fish, this refers to postmortem or post-catch quality, in which case the slaughtering and catching practices – and subsequent storage conditions – will affect the raw material quality. Raw material selection is the first step in the process for the implementation of quality control and quality assurance concepts, including the application of hazard analysis critical control point (HACCP) approaches.
The purpose of cleaning is to remove undesirable foreign material and should be designed to obtain:
– maximum separation efficiency consistent with minimum wastage of good material;
– complete removal of separated contaminants and avoidance of recontamination;
– a clean product surface in an acceptable condition;
– minimum quantity and concentrations of residues.
The foreign material found on fruits and vegetables can be grouped under the following headings:
Mineral: soil, sand, stones, metallic particles;
Plant: twigs; foliage, stalks, pits, skins, husks, rope, and string;
Animal: excreta, hair, insect eggs, body parts;
Chemical: spray residues, fertilizers;
Microbial: microorganisms and their by-products
There are two basic types – dry and wet – that are usually used in combination.
Dry Cleaning Methods
The main dry cleaning methods are based on screens, aspiration, or magnetic separations.
Dry methods are generally less expensive than wet methods and the effluent is cheaper to dispose of, but they tend to be less effective in terms of cleaning efficiency. A major problem is recontamination of the material with dust.
Precautions may be necessary to avoid the risk of dust explosions
Screens Screens are essentially size separators based on perforated beds or wire mesh by which larger contaminants are removed from smaller food items (e.g., straw from cereal grains, or pods and twigs from peas). This is termed ‘‘scalping’’ (Figure 1.1a). Alternatively ‘‘de-dusting’’ is the removal of smaller particles (e.g., sand or dust) from larger food units (Figure 1.1b).
The main geometries are rotary drums (also known as reels or trommels) and flatbed designs. Some examples are shown in Figure 1.2.
Cleaning by abrasion between food particles or between the food and moving parts of cleaning machinery is used to loosen and to remove adhering contaminants. Trammels, tumblers, vibrators, abrasive discs, and rotating brushes are used for this purpose
Aspiration This exploits the differences in aerodynamic properties of the food and the contaminants. It is widely used in the cleaning of cereals, but is also incorporated into equipment for cleaning peas and beans. The principle is to feed the raw material into a carefully controlled upward air stream. Denser material will fall, while lighter material will be blown away depending on the terminal velocity
Magnetic cleaning is designed to detect and remove metallic contaminants that can damage preparatory equipment from the raw products. Rotating or stationary magnetic drums, magnetized belts, magnets located over belts carrying the food or staggered magnetized grids, through which the food is passed, are used involving permanent and electromagnets
Wet Cleaning (Washing)
Wet cleaning is effective in removing firmly adhering soils. The addition of detergents and sanitizers can greatly improve cleaning efficacy. However, there are disadvantages. The large amounts of water used subsequently produce a considerable volume (15,000 L/metric ton of canned food) of highly polluting effluent that requires expensive waste treatment. The resulting wet surfaces can spoil more rapidly and may require dewatering to provide clean material suitable for processing or storage.
Passive soaking is the simplest wet cleaning method. It is often used as a preliminary stage in the cleaning of root vegetables heavily contaminated with adhering soil. The soil is softened and partly removed along with stones, sand, and other abrasive materials.
The cleaning efficiency is improved by agitation via caged propeller stirrers built into the tank, by slow-moving paddles, or by a horizontal perforated rotating drum partially submerged in the soak tank.
While warm water improves the soaking efficiency it can induce increased microbial growth and spoilage.
The use of detergents is increasing, especially in foods contaminated with spray residues and mineral oil. These must be selected with care as they may affect the appearance and texture of the food. For example, sodium hexametaphosphate has a softening effect on peas, and some metal ions, such as calcium, can increase toughness.