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Cooling, chilling, cold stabilisation
Cooling can be defined as a processing technique that is used to reduce the temperature of the food from one processing temperature to another or to a required storage temperature. Chilling is a processing technique in which the temperature of a food is reduced and kept at a temperature between –1°C and 8°C. The objective of cooling and chilling is to reduce the rate of biochemical and microbiological changes in foods, in order to extend the shelf-life of fresh and processed foods, or to maintain a certain temperature in a food process, e.g. in the fermentation and treatment of beer. Cooling is also used to promote a change of state of aggregation, e.g. crystallisation. In the wine industry, cooling (chilling) is applied to clarify the must before fermentation. The objective of cold stabilisation is to obtain the precipitation of tartrates (in wines) or fatty acids (in spirits) before bottling.
Field of application
Cooling is a process step in many food production processes. Chilling for food preservation is widely applied for a lot of perishable foods. The main application of cold stabilisation in the food industry is in the wine and spirit sector.
The supply of chilled foods to consumers requires a sophisticated distribution system, involving chilled stores, refrigerated transport and chilled retail display cabinets. Chilled foods can be grouped into three categories according to the storage temperature (Hendley, B. (1985) Market for chilled foods. Food Process 52, 29-33) and the forth is applied in wine making:
- -1°C to + 1°C (fresh fish, meats, sausages and ground meats, smoked meats and fish)
- 0°C to + 5°C (pasteurised canned meat, milk and milk products, prepared salads, baked goods, pizzas, unbaked dough and pastry)
- 0°C to + 8°C (fully cooked meats and fish pies, cooked or uncooked cured meats, butter, margarine, cheese and soft fruits)
- 8°C to 12°C in the wine industry. The must is kept at this temperature between 6 and 24 hours.
Description of techniques, methods and equipment
Cooling of liquid foods is commonly carried out by passing the product through a heat exchanger (cooler) or by cooling the vessels. The cooling medium in the cooler can be groundwater, water recirculating over a cooling tower, or water (eventually mixed with agents like glycol) which is recirculated via a mechanical refrigeration system (ice-water). Cooling and chilling of solid foods is carried out by contacting the food with cold air, or directly with a refrigerant like liquid carbon dioxide or liquid nitrogen. The equipment used for freezing can also be used for cooling and chilling.
Some typical applications are given below:
a) Cooling of sugar
Sugar to be stored in silos must be dedusted and cooled to the storage temperature. This is done in a sugar cooler, which is a device in which warm and dried sugar is intensively aerated by cold filtered external air to cool the sugar to the storage temperature, approximately 20–30˚C. The most common systems in use are coolers (typically drum or fluidised-bed coolers) with chilling systems with countercurrent or cross-current phase flow.
b) Cryogenic cooling
In cryogenic cooling the food is in direct contact with the refrigerant, which may be solid or liquid carbon dioxide, or liquid nitrogen. As the refrigerant evaporates or sublimates it removes heat from the food, thereby causing rapid cooling. Both liquid nitrogen and carbon dioxide refrigerants are colourless, odourless and inert.
c) Cold stabilisation
Cold stabilisation is a technique of chilling wines before bottling to cause the precipitation of harmless tartrate crystals.
For spirits, this technique consists of bringing the spirit to a temperature of between -1°C and -7°C, depending on the operators, and possibly performing a stabulation (storing at low temperature) in a tank at constant temperature for between 24 and 48 hours. A cold filtration (around -1˚C) allows the fatty acid esters to be retained.
For wines, three techniques can be employed: -stabilisation by batch and stabulation. This is the oldest technique and consists of bringing the wine to a temperature below zero close to the freezing point, then stabulating in an isothermal tank during a period of 5 to 8 days.
But currently the most widely-used techniques are: -continuous stabilisation, where the stabulation tank is replaced by a cylindro-conical crystalliser and an agitator, in which the wine will remain for only between 30 and 90 minutes, stabilisation by crystal seeding consisting of refrigerating at between -1° and -2°C, and seeding at 4 g/l of tartaric crystals with agitation over 2 to 4 hours, and later storage in tank, and decantation after 12 to 48 hours. There can be many variations on these basic schemes.
Cooling can be defined as a processing technique that is used to reduce the temperature of the food from one processing temperature to another or to a required storage temperature. Chilling is a processing technique in which the... read full description