Freezing and dehydration：冻结和脱水.pptVIP

Freezing and dehydration Learning objectives To understand the different types of processes used in freezing and dehydration. Freezing Immersion freezing Traditionally foods were immersed in solutions of salt and ice for several hours, e.g. brine, freezing of fish at sea. However, modern methods of freezing have meant that this process is rarely used. Refrigerants are now sprayed directly onto the food. Plate freezing The food is prepared as normal, then packed between flat, hollow, refrigerated metal plates. These are adjusted to press tightly on the food and reduce any air gaps. The plates may be horizontal or vertical, the latter being used for many bulky products, such as blocks of fish for fish fingers. This system is ideal for freezing large blocks of product, but cannot easily freeze irregular shaped items. Blast freezing Batches of food are subjected to a constant, steady stream of cold air (-40oC or lower) in a tunnel or large cabinet. This process can freeze irregular shaped foods, including those which have already been packaged, e.g. battered fish pieces. Fluidised bed freezing Vertical jets of refrigerated air are blown up through the product, causing it to float and remain separated. This is a continuous process which takes up to 10 minutes. The product, e.g. peas, beans, chopped vegetables or prawns, move along a conveyor belt. Scraped heat exchangers Products such as ice cream are frozen using this method in order to stir and freeze simultaneously. It reduces large ice crystal formation, producing a smooth end product. Cryogenic freezing Liquid nitrogen or carbon dioxide is sprayed directly onto small food items such as soft fruit and prawns. Due to the liquids’ extremely low temperatures (-196oC) and -78oC respectively) freezing is almost instant. The nitrogen gas is removed by fans. Carbon dioxide is used for larger products. The carbon dioxide system is more economical and the gas can be recycled into the system. Benefit of rapid freezing