Chilling Techniques

Products distributed in the chilled state must be sufficiently cooled in a reasonable time after cooking and processing. European Economic Community (EEC) Directive 77/99 requires that foods for chilled distribution must be cooled to 10°C or less within two hours of processing and maintained at or below that temperature for the duration of product shelf storage. The U.S. Food and Drug Administration (FDA) requires similar cooling conditions (cooled to less than 7.2°C in less than two hours after processing and maintained at or below that temperature during product shelf storage) for chilled foods.

Several techniques are currently used in the industry that allow for the processed products to be chilled according to the preceding requirements. One of these is the use of air-blast coolers, which circulate chilled air at high velocities (up to 16 m/s for smaller product sizes) to effect a rapid cooling rate. This rapid rate is important for sensitive foods that have been heat processed prior to cooling. The design of these units provides rapid and even chilling, and coolers may also be designed for humidity control in unwrapped product applications. Safety issues such as L. monocytogenes contamination can arise in systems for chilling unwrapped products if conditions such as temperature and humidity are not controlled. Chilling tunnels can also provide good product chilling characteristics by altering the airflow methods in different tunnel sections. Products cooled by this method are usually carried through the tunnel on trays or pallets. Direct-contact refrigerants such as liquid nitrogen and dry ice (carbon dioxide) shavings may also be used to rapidly chill heat-processed products. This equipment is usually designed to prevent products from freezing. Immersion hydrocooling and ice bank chilling are two other common methods of rapidly cooling chilled products; these methods provide more control over the chilling process, as they do not allow for the product to freeze. Hydrocooling is carried out by spraying or immersion of the product and can be included as part of a continuous process. Ice bank chilling is primarily used with vegetables, and systems may be designed to maintain a high humidity to prevent moisture loss. After rapid chilling, products may be held in lower-velocity air system cold rooms before packaging or distribution.

Some products may be prepared frozen and held until they reach the retail or food service market. At the retail outlet, the products are thawed at 4 to 6°C and maintained at this temperature. This approach is referred to as "freeze-thaw" and allows greater flexibility in the distribution chain as well as longer product shelf life and improved safety. Sale of such products requires that the consumer be informed that the product was "previously frozen." Another approach, often referred to as Latent Zone chilling, uses temperatures of — 2 to 0°C for the preparation and storage of products. Such temperatures have been shown to extend the shelf life of a number of products for up to six weeks. The concept was developed to provide more efficient production of prepared foods.

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