FREEZING OF FRUITS AND VEGETABLES Presented By: Abhishek Thakur B. Tech(FT) SU13540 Presented To: Dr. Somesh Sharma

WHAT IS FREEZING?? It is a unit operation. Temparature of the food is reduced below its freezing point generally to -18 °C or below.. Change in state. Allows preservation of taste, texture, and nutritional value in foods. Provide a significant extended shelf life. Retards growth of microorganisms.

FREEZING PROCESS The material to be frozen first cools down to the temperature at which nucleation starts. Before ice can form, a nucleus, is required upon which the crystal can grow; the process of producing this seed is defined as nucleation. Once the first crystal appears in the solution, a phase change occurs from liquid to solid with further crystal growth.

CONT... Freezing point is defined as the temperature at which the first ice crystal appears and the liquid at that temperature is in equilibrium with the solid. Freezing time is defined as time required to lower product temperature from its initial temperature to a given temperature at its thermal center. Freezing rate is defined as the ratio of difference between initial and final temperature of product to freezing time.

GRAPH SHOWING FREEZING PROCESS FOR FOODS

TYPES OF FREEZING SLOW FREEZING FAST FREEZING Slow freezing occurs at -24 ºC or above. Ice crystals are big and damage the food cells causing loss of texture, nutrients, colour & flavour on thawing. Quick or fast freezing occurs at –25ºC or less. Ice crystals are small and do not damage food cells.

Water contents and freezing points of selected foods

TYPES OF FREEZERS

AIR BLAST FREEZER Either still air or forced air is used. Air is recirculated over food at between 30ºC and 40ºC at a velocity of 1.5–6.0 m/s. In batch equipment, food is stacked on trays in rooms or cabinets. Continuous equipment consists of trolleys stacked with trays of food or on conveyor belts which carry the food through an insulated tunnel.

BELT FREEZER Have a continuous flexible mesh belt which is formed into spiral tiers and carries food up through a refrigerated chamber. Cold air or sprays of liquid nitrogen are directed down through the belt stack in a counter current flow, which reduces weight losses due to evaporation of moisture.

PLATE FREEZER It consist of a vertical or horizontal stack of hollow plates, through which refrigerant is pumped at 40ºC. Slight pressure is applied to improves the contact between surfaces of the food and the plates and thereby increases the rate of heat transfer.

CRYOGENIC FREEZER Here the food is exposed to an atmosphere below -60°C through direct contact with liquefied gases such as nitrogen or CO2.

LIQUID NITROGEN FREEZER LIQUID CARBONDIOXIDE FREEZER When the CO2 gas is released to the atmosphere at -70 °C, half of the gas becomes dry-ice snow and the other half stays in the form of vapor. This unusual property of liquid carbon dioxide is used in a variety of freezing systems, one of which is a prefreezing treatment before the product is exposed to nitrogen spray

IMMERSION FREEZER The immersion freezer consists of a tank with a cooled freezing media, such as glycol,glycerol, sodium chloride, calcium chloride, and mixtures of salt and sugar. The product is immersed in this solution or sprayed while being conveyed through the freezer, resulting in fast temperature reduction through direct heat exchange.

A general flow chart of frozen fruits and vegetables

Flow diagram of freezing process for fruit-based product.

Flow diagram of freezing process of vegetable-based product.

EFFECTS OF FREEZING Damage caused to cells by ice crystal growth. Freezing causes negligible changes to pigments, flavours or nutritionally important components. When water in the cells freezes, an expansion occurs and ice crystals cause the cell walls to rupture . Consequently, the texture of the produce is generally much softer after thawing when compared to non-frozen produce. Chemical changes that can cause spoilage and deterioration of fresh fruits and vegetables will continue after harvesting. Development of rancid oxidative flavours through contact of the frozen product with air.

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