Food Preparations - Freeze/Chill

 Freeze/Chill

The ready-meals market has grown significantly in Europe over the past decade, with the chilled sector experiencing the most dynamic growth. Demand for ready-meals is expected to grow by 20% from current values to exceed E6.8 billion by 2005.

Chilled ready-meals have a relatively short shelf life and are perceived to be of better quality than frozen meals. Frozen ready-meals are bought more often, however, due to their long shelf life; they also offer better manufacturing and distribution flexibility and a higher level of food safety.

Two emerging technologies are freeze-chilling and sous vide/freezing. In the case of the former, the ready-meals have previously been frozen but are retailed from a chill cabinet. Sous vide cooking, while more traditionally applied to the catering sector, is increasingly being applied to the preparation of ready-meals.

The freeze-chilling of food involves freezing and frozen storage followed by thawing and then retailing at chill storage temperatures, and offers a number of logistical and other advantages. For example, foods can be prepared in bulk, then frozen and stored at deep-freeze temperatures until required. Some or all of the batch can then be thawed as necessary; freeze-chilling enables chilled foods to reach distant markets, in that product can be shipped deep frozen and then thawed when it reaches its destination prior to retail display; freeze-chilling can reduce the level of product recalls as it enables routine microbiological tests to be completed before the product is released from the factory. One of the main thrusts behind current research is the possibility that thawed food could be more conducive to microbial growth because of the presence of nutrients in the drip - the fluid produced during thawing - and also because freezing may open up the cell structure.

First and foremost foods must be suitable for freezing and thawing, i.e. they must not suffer significant structural damage and should give minimal drip on thawing. The cooked products under comparison at The National Food Centre (steamed salmon, a range of potato mashes, steamed carrots, green beans and broccoli, lasagne, starch-based sauces and breaded items) were compared as fresh, chilled, frozen and freeze-chilled. Frozen storage times varied from short (days) to long (months), and time in chill (4ºC) from one to seven days depending on the product. Each product sample was subjected to a wide range of physico-chemical and sensory tests post-treatment and storage.

Sensory scores awarded by a taste panel indicated no difference between freeze-chilling, freezing, chilling or preparing fresh for steamed salmon, broccoli, carrots, green beans, or lasagne, despite differences in shear values - the force required to shear, or 'chew' the product - and more major differences in centrifugal (forced) drip. For example, chilled salmon was the softest, while freeze-chilling or freezing gave softer carrots and potato mash than chilling or preparing fresh. Freeze-chilling and freezing gave much higher centrifugal drip values than chilling or preparing fresh for all products. These and the corresponding shear values are an indication of cell damage due to the freezing stage.

Freeze-chilled mashed potato received a lower sensory score than mashed potato from the other three treatments, a reflection of its soft texture and high centrifugal drip loss. This textural effect can be overcome by the inclusion of a small amount of hydrocolloid (e.g. xanthan gum) in the mashed potato. These laboratory trials have been paralleled by successful industrial freeze-chilling trials by a number of food companies.