Milk Processing

Considerable variations in energy requirements for milk processing have been reported for different plants. Fuel requirements for producing pasteurized bottled milk ranges from 0.25 to 2.65 MJ/L of milk (7). The energy used for processing cheese during the regular plant operation is approximately 2.13 kJ/kg. Excluding drying and evaporation, energy required to produce 1 kg of cheese is in the range of 3.37 to 17.53 MJ (4). Some of this variation is due to a lack of concern to energy conservation, but there are also operational factors that may explain variations among plants.

Considering energy allocation to individual unit operations separately in a typical processing of pasteurized bottled milk, most of the energy is used in pasteurization step (approximately 23% of electricity and 35% of fuel). In the production of pasteurized bottled milk, bottle washing consumes approximately 58% of the fuel energy and 6% of the electricity used in the process.

In butter production pasteurization consumes approximately 30% of the fuel energy. The churning process and cold storage of butter consume approximately 50% of the electricity required for butter processing.

In yogurt production energy for heating and cooling per kilogram of product averages 1,146 kJ/kg (8). The heating of the base yogurt from 10 to 87.8°C uses 80% of the energy, and the electrical equipment used in product handling and packaging consumes only 6%. The manufacturing process of sour cream is similar to that for yogurt, except the amount of heating is considerably less because the highest product temperature is 22.2°C for sour cream compared to 87.8°C for yogurt. In packaging, the shrink wrap machine is not used for holding the sour cream cartons. The thermal energy ratio is 273 kJ/kg of sour cream and packaging uses less electrical energy primarily because of the lack of the shrink wrap operation. Product handling and packaging uses 49 kJ/kg of sour cream compared to 78 kJ/kg of yogurt. Total energy, thermal plus electrical, is 287 kJ/kg for sour cream compared to 1,224 kJ/kg for yogurt. The above values of energy to manufacture sour cream and yogurt exclude the following: the energy required to pasteurize the raw milk, the energy required to pump the base material into the processing vat, and the energy required in temporary cold storage prior to shipping. Yogurt in the processing vat is cooled from 87.8 to 42.2°C partially with well water. Measurements of the energy taken from the yogurt by the well water showed an average of 212 kJ/L of yogurt in 4.16 m3 batch. The cooling could be considered free, except for the cost of the water and pumping energy (8).

Spray drying is the common means of converting fluid feedstocks into solids in the form of powders, granules or agglomerates. It is widely used for dehydration in the manufacture of a wide variety of food, pharmaceutical, and chemical products. Among the foods that have been successfully spray dried are milk, whey, cheese, coffee whit-ener, eggs, soups, baby foods, and fruits. Energy costs constitute a significant fraction of the operating costs. A number of methods have been suggested to decrease energy related costs during the spray-drying process by using insulation to decrease heat losses to the environment and heat recovery from exhaust gases (8). Typically, in the manufacture of spray-dried milk powder, air heating for the spray-drying operation requires between 50 and 80% of total fuel consumption and approximately 30-35% of total electricity consumption in the whole process. The variation in the primary fuel input results from heat recovery steps.

Cleaning in place (CIP) in milk processing plants is one of the operations that takes significant input of energy. In the manufacturing of Cheddar cheese, CIP consumes approximately 31% of the total fuel energy and 18% of the total electricity required for the whole process. In other processing operations such as milk pasteurization, creamery butter, and acid casein processing, the fuel consumption in CIP operation does not exceed 3% of total fuel used in the whole process, and the use of electricity is up to approximately 1% of total electric energy used for the whole process. In spray-dried milk powder production, CIP operation costs approximately 5—8% of the total fuel consumption and 4-5% of the total electricity used in milk powder processing.

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