Food Freezing & Freeze-drying

Iqbal Nugroho23411660

Quick FreezingThe liquid content of foodstuffs, containing proportions of sugars and salts, has a band of freezing temperatures from 0°C down to (–18)°C and lower. If these liquids freeze slowly, long ice crystals are formed which pierce the cell walls and change the resulting texture.

Quick FreezingIf this damage is to be avoided, the product must be frozen rapidly, so that the crystals do not have time to grow. The process is only applicable to products which are eaten raw or lightly cooked, such as strawberries, peas and beans.

Air Blast Coolers & TunnelsWhere the product shape is irregular, the only way to extract its heat will be by using a cold fluid surrounding it. The most common of these is air. The air temperature will be of the order of (–40)°C and the air speed over the product will be high, to get good heat transfer.

Air Blast Coolers & TunnelsDiscrete pieces of product, such as peas, slices of carrot, beans and items of this size, can be conveyed on a perforated belt, with the cold air blasting up through the holes, to both cool the product and agitate it, to prevent it sticking either to the belt or to other similar pieces.

Air Blast Coolers & TunnelsFlat pieces of product, such as fish fillets, would suffer a change in shape in a free air blast and are better on a flat moving belt. Here, some of the heat goes direct to the cold air and some by conduction to the belt, which is usually of stainless steel. This tunnel can be designed to absorb much less fan power and, since fans input energy which must then be removed by the refrigeration system, the tunnels will be more energy efficient.

Air Blast Coolers & Tunnels

Air Blast Coolers & Tunnels

Air Blast Coolers & TunnelsLarger items, such as tubs of ice-cream, take a long time to harden and a straight conveyor would be too long for convenience. Such conveyors can be wound into a spiral shape and contained within a coldroom with air blast coil.

Contact FreezingProducts in regular-shaped packages, such as ice-cream in flat cartons, are pressed between horizontal, flat, refrigerated plates. These can be opened apart slightly to admit the product and are then closed by hydraulic rams to give close thermal contact. When freezing is complete, the plates open again to remove the packs.

Contact Freezing

Contact FreezingThe vertical plate freezer is used for a loose

product such as wet fish, which is packed into the gaps between the plates. When the freezing is complete, the product is removed as a solid block and may be 75 mm or 100 mm thick.

Contact Freezing

Contact FreezingTrays of product to be frozen can be loaded onto trollies, which are taken through an air blast tunnel. The evaporator coils will usually be in the upper part of the tunnel, with air flow across the trays.

Contact FreezingMaterial to be frozen can be fully immersed in a cold liquid. This might be a brine, in which case the material may have to be wrapped in a plastic bag to avoid contact with the liquid. The sodium chloride and glycol brines cannot be used cold enough to get complete freezing, so this may be a first pre-cooling stage before a final air blast. Alternatively, liquid nitrogen (–196)°C or carbon dioxide (–78.5)°C can be sprayed onto the surface.

Freeze-dryingFreeze-drying is a special form of drying that removes all moisture and tends to have less of an effect on a food's taste than normal dehydration does. In freeze-drying, food is frozen and placed in a strong vacuum. The water in the food then sublimates -- that is, it turns straight from ice into vapor. Freeze-drying is most commonly used to make instant coffee, but also works extremely well on fruits such as apples.

Freeze-dryingThe process is carried out in a vacuum chamber fitted with refrigerated contact freezing plates, heaters and a vacuum pump. Between the chamber and the pump may be a refrigerated separator to prevent too much of the moisture entering the pump. The product is placed in containers on the plates and frozen down to about – 25°C, depending on the product, but sometimes as low as – 50°C.

Freeze-dryingThe vacuum and, at the same time, a carefully controlled amount of heat, is then applied, to provide the latent heat of sublimation (ice to vapour) without allowing the temperature to rise. As the water is driven off, the product collapses to a dry powder. This is extremely hygroscopic and must be packed in air-tight containers as quickly as possible on completion of the cycle.