Understanding the modern cold chain

Frozen food consumption is a necessity for millions of families and institutional consumers in both developed and developing countries. Our food distribution systems are largely dependent on refrigeration, so what technologies are available? Lars Eek finds out.

The ease of application of chilling and freezing techniques and the wide product range of foods entering the marketplace have made chilling or freezing an important and preferred preservation method in many countries. Urbanisation and the huge distances between the factory and the points of sale have made refrigeration an invaluable element to the bottom line.

The frozen food chain

Frozen foods are very close to, and sometimes better for the end consumer than, its fresh counterparts available. From a sensory as well as a nutritional point of view, the quality is generally higher than for any other preserved product. The need for a longer shelf life and improved taste and quality has given rise to the modern freezing industry.

Preserving surplus raw material and bridging seasonal gaps in the food supply, freezing today offers a number of advantages demanded by the consumer. Examples are convenience, availability, consistent quality and safety. Also, demographic changes have boosted consumption of frozen foods. Over the last few years, more women have joined the workforce, the number of single households and single parents have increased, the average disposable income has increased - all factors favoring frozen foods as well as fresh chilled foods.

The chilled food chain

It is claimed that chilled food is the fastest growing food sector in Western Europe. This does not, however, include traditional chilled products such as meat, meat products, fish, milk and dairy products. The consumption of these are stable and in some markets declining. The increase is to be found in 'new' products like entrees, salads, pizzas, etc.

Percentage-wise, this product group may be the fastest growing, but not necessarily with regard to tonnage consumed. The chilled food trend is not likely to compete with frozen foods. In the USA, some chilled foods are first frozen and then thawed in the retail outlets to achieve rational distribution and safety as well as increase their shelf life. Fish is a typical product that is sometimes handled this way.

The shelf life of chilled products is and can be enhanced by preservatives, modified atmosphere packing, irradiation and other methods. Consumer demands for fewer additives have given rise to a new problem at the marketplace, namely, microbiological hazard leading to increased risks of food poisoning. This problem is also relevant to some traditional chilled products that rely on preservation systems based on acids, salt, nitrate and phosphate. So far, the consumers have met irradiation with resistance. Furthermore, foods will, today and in the future, be produced far away from the place of consumption, which call for better preservation than by just cooling and chilled storage. While the infrastructure for frozen foods largely meets current requirements, improvements are necessary in chilled food distribution.

Green leafy vegetables like broccoli, spinach and beans lose half of their vitamin C content in a few days at ambient temperature. Fresh vegetables, transported across long distances domestically or, in many cases, between different global continents and then kept several days in the supermarket and at home in a refrigerator can be inferior to frozen food.

The freezing process

The freezing process may be seen as a lowering of the product temperature from its original value to the storage temperature. From each point within the product, the heat must be removed by conduction to the surface. From the surface the heat can be removed to the refrigeration medium. The freezing time depends on a number of factors, of which the most important are the dimension and shape of the product, the thermal properties, initial and final temperatures and, the temperature of the refrigeration medium.

Ice crystallisation

Most food items consist of or contain animal and/or vegetable cells forming biological tissues. The water solution of the tissue is contained between the cells - intercellular fluid -and within the cells - extra-cellular fluid. The concentration of salts and other solubles is higher within the cells than outside. The cell membrane acts as an osmotic barrier and maintains the difference in concentration.

When the product is frozen, the first ice crystals are formed outside the cells since the freezing point is higher for the more diluted fluid here than inside. If the freezing rate is low, the cell will lose water by diffusion through the membrane and the water will crystallise into ice on the surface of the crystals already formed in the intercellular space.

As the cells lose their water, the remaining solution within the cells becomes more and more concentrated and their volumes shrink causing the cell walls to collapse. The large ice crystals formed outside the cell wall occupy a larger volume than the corresponding amount of water and therefore will execute a physical pressure on the cell wall, contributing to an increased drip loss at the thawing. If the freezing rate is high, a large number of ice crystallisation nucleuses are formed, resulting in a much smaller size of the final crystals. Only at very high freezing speeds are small crystals formed uniformly throughout the tissue, both externally and internally with regard to the cell. Such higher freezing rates can only be achieved in comparatively small products.

The freezing time must, in good commercial practice, be determined for each product with regard to:

• Controlled freezing speed to secure an even textural quality level
• Microbiological considerations
• Mechanical losses -from product sticking to a conveyor belt, or product packages dropping to the floor - damage or down grading
• Dehydration
• The freezing process must fit into the production line.

Freezing equipment

Freezers can be classified under two main categories - integrated production and batch processing, the former being the most predominant. There are basically three main types of equipment based on the method of heat transfer:

• Air blast freezers that use air for heat transfer. Since air is the most common freezing media, this method of heat transfer has probably the largest range of designs.
• Contact freezers where heat transfer occurs through conduction. A refrigerated surface is placed in direct contact with the product or package to carry away the heat.
• Cryogenic freezers use liquid gases, nitrogen or carbon dioxide to produce vapours that pre-cool and freeze the products.

The design of the freezing equipment should optimise the total freezing process. Among the critical design parameters are product quality, minimum product losses, reliable operation, simple operation and maintenance, and refrigeration economy.

Freezing equipment can also be classified according to product form, either IQF products or packed products. Each product has unique freezing requirements dictated by factors ranging from handling sensitivity to portion size and moisture content, and each producer has clearly defined goals for quality, productivity and the cost of freezing.

The two most commonly used air blast freezing systems in the industry are fluidised bed freezers and spiral systems. Fluidised IQF freezer gives a superior individually quick frozen food product, answering round-the-clock volume freezing needs, and delivering top IQF quality on products ranging from sticky-cooked rice or delicate shrimp to bulky corn-on-the-cob. A self-stacking spiral freezer, on the other hand, delivers high throughput in a compact footprint. Its self-contained freezing zone, driven by the frictionless drive system, gives greater food safety, yield and quality, and gentle food handling. Its food product range covers the entire spectrum, from ready meal, bakery and poultry to ice cream hardening.

Selecting freezing equipment

The freezer is often the single most expensive part of the processing line. It is important to make sure that the equipment is properly integrated into the process and is working to optimum performance. Careful planning and knowledge is essential.

With reference to the equipment, the operating costs are of obvious importance. High operating costs can offset the advantage of a low initial investment cost. The value of the food products that pass through a freezer in a few weeks is often many times higher than the investment cost. Energy consumption, labour and maintenance, downtime and product losses from dehydration are crucial factors to take into consideration. The design and construction of equipment will, to an increasing extent, be focused on hygiene in order to facilitate as safe a product as possible from a microbiological point of view. Also, the rapid changes taking place in the food supply systems of today call for flexibility. A system that is easy to modify will make it possible to respond quickly to the requirements of the market place. Proper life cycle analyses will enhance the development when judging the final results achieved from the resources employed in the total flow from raw material to a final tasty 'added value' product.

Physical storage and distribution of frozen foods

The main problems affecting the frozen food industry for many years have been related to the storage time and temperature of the products. Not so much on the basic knowledge of maintaining shelf life, but on handling time and storage and transport temperatures. The quality and acceptance of frozen foods can be further improved if more emphasis is laid on avoiding temperature abuse.

More stringent legislation has and will improve the situation. Today a maximum product temperature of -18°C is becoming an accepted practice throughout distribution. Realising the importance of creating quality reserve for the consumers, most large food industries in Europe are demanding much lower temperatures in the first links of the chain. The last links of the chain, the retail cabinets, are often the culprits, as very often, it is difficult to create maximum attention with a large display area without overloading the cabinet. Frozen products delivered to the retail outlet are also not placed in the cabinet immediately. Problems such as temperature increase during defrosting can be minimised by using night covers and programming the defrosting to the night hours. The storage before the exposure in the cabinet should be carried out in specially constructed stores -backstores. This system is used in the most modern and largest retail outlets today as well as in the quick service restaurants. The need for a low and steady temperature is fully accepted by everyone involved in the food chain. Two main challenges for the years to come will be to improve on the education and understanding of proper handling of frozen goods throughout the frozen chain, and to find simple and inexpensive ways for control and checking. It is likely that temperature indicators will be used not only to check the physical distribution, but also throughout the entire processing as a safety 'instrument' in HACCP-work or other similar systems.

Courtesy: Minett Media