Freezing and Chilling Revision: Chilling & Freezing

Freezing Freezing is a longer-term method of preserving food. Freezing uses two controls to prevent the growth of bacteria The liquid is converted to a solid (ice) and is therefore unavailable to bacteria The low temperatures reduce the rate of bacterial growth Together these controls allow food to be stored for long periods. It is important to remember that the bacteria have not been destroyed. On thawing they will become active again

Transferring to ice water Blanching While freezing reduces the growth rate of bacteria, it does not prevent enzyme action. For this reason, fruits & vegetables which are to be frozen are blanched. Blanching is a process which involves dipping fruit of vegetables first into boiling water, then ice cold water for a few seconds. This deactivates the enzymes and reduces the number of bacteria present on the surface of the food. Boiling water Transferring to ice water

Slow freezing = large crystals Before freezing, anti-oxidants may be added to foods to prevent fats present in the products from becoming rancid as this chemical reaction is not prevented by freezing. During freezing the time factor is critical. If freezing is slow, large ice crystals will form, damaging the cell walls of the food Slow freezing = large crystals Quick freezing = small crystals

Quick Freezing Food quickly frozen from 0 > -18°C in approximately 12 minutes. Creates smaller ice crystals & less damage to the food. The food retains its shape & texture with this method. There is little change in the nutritional value of foods & in some cases the vitamin content could be higher than in fresh foods. Vegetables are frozen very quickly after harvesting, while fresh fruit & vegetables lose ascorbic acid (vit. C) during transportation and storage

Freezers The freezer star ratings are as follows * Ice Box -6°C * * * * Freezer -18°C Frozen food should be transported home quickly, using a cool box to prevent defrosting. They can be stored using the star markings on the freezer, which are a guide to the length of time they may be stored * = one week ** = one month *** = 3 months Temperature Length of storage

Immersion Freezing In immersion freezing, food is placed in a refrigerant prior to freezing. Brine is often used for fish, and a sugar solution for fruits. This provides a layer which protects the food from the dry atmosphere of the freezer

Cryogenic Freezing Uses liquid nitrogen which is very cold (-196ºC) Food passes through a tunnel where nitrogen gas is sprayed downwards. A beefburger will be frozen in 1 minute at these extreme temperatures. This produces small crystals, and little moisture loss. This method is used when freezing prawns. The prawns are first dipped in liquid nitrogen to freeze the outside layer. This prevents the prawns sticking together and from sticking to the freezer belts.

Plate Freezing Ideal for thin, flat foods such as steak, fish fillets or burgers. The food is placed between two “plates” which make contact with the food’s surface. This speeds up the freezing process & freezing occurs evenly throughout the food

Accelerated Freeze Drying (AFD) This is a combination of freezing and drying. Foods are quickly frozen which produces small ice crystals in the cells. The food is then placed in a vacuum under reduced pressure and heated gently. In these conditions the ice crystals change directly to water vapour. This is known as sublimation. Products treated this way weigh very little, are porous and look like fresh products. They can be easily reconstituted with water Coffee, tea, shellfish and food for camping expeditions are treated in this way

The market has been driven by several factors including consumer demand for convenience foods, a rise in freezer and microwave oven ownership, fewer families sitting down to eat together and a rise in one and two-person households. The UK chilled ready meals sector is the most highly developed in Europe in terms of the range of products available and total sales. Sales are dominated by the major supermarket chains, which accounted for two-thirds of sales by value in 2001. As many as half of all new ready meal launches use international or ethnic recipes. Low-fat and low-calorie only account for a small percentage of new product developments. Most of these types are within existing well-known brands such as Weight Watchers from Heinz. New product developments are likely to focus on healthiness, although not necessarily low-fat/low-calorie. The emphasis is more on wholesome fresh ingredients. The first chilled ready meal to go on sale in the UK was developed by Northern Foods for Marks & Spencers in 1970 – Chilli Con Carne. Foods Standards Agency

Chilling Chilling is a short term process of preservation. Chilled foods are prepared foods which, for reasons of safety or quality are designed to be stored at or below 4ºC for their entire life (e.g. salads). The optimum temperature for storage is 4ºC . Cook-Chilled products are dishes which are cooked first, rapidly chilled to below 4ºC within 90 minutes then stored in a fridge. The product needs to be reheated prior to eating. Cook-chilled products can only be stored for 5 days & this includes the day of manufacture Collins Real World –Food Technology