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Wet Blending Spray Drying

In the wet blending – spray drying process, ingredients are blended together, homogenized, pasteurized and spray dried to produce a powdered product. The pasteurization step destroys harmful bacteria that may be present in the ingredients. Therefore, this process is much less dependent on the microbiological quality of ingredients. This process also has the advantage of ensuring a uniform distribution of nutrients throughout the batch. However, the wet blending – spray drying process requires that processing equipment, including the spray dryer and fluidized bed, be regularly wet cleaned. This frequent wet cleaning provides the moisture needed by bacteria to grow and become established in the plant environment. If not controlled, these bacteria can be a source of product contamination. Manufacturers that use the wet blending – spray drying process typically partition their plants into wet process and dry process areas and strictly limit the movement of people and equipment between the wet and dry areas. Also, they strictly enforce sanitation practices, particularly in the dry processing areas.



The wet blending – spray drying process begins with the receipt of ingredients from suppliers. As with the dry blending process, ingredients are stored until they are tested for conformance to specifications. Ingredients are blended with water in large batches (1,000 – 5,000 gallons) then pumped to a heat exchanger for pasteurization. The severity of the pasteurization process varies among manufacturers, but it is always sufficient to destroy the vegetative cells of harmful bacteria. After pasteurization, the liquid is homogenized. [Note: Some manufacturers homogenize product prior to pasteurization.] After pasteurization, certain heat sensitive micro nutrients (e.g., vitamins, amino acids and fatty acids) are added to the product. The microbiological quality of these nutrients is critical, since the product may not receive further heating sufficient to destroy harmful bacteria.

After the addition of micro-nutrients, the liquid may be concentrated by passing it through an evaporator or it may be pumped directly to the spray dryer. If the product is evaporated, it is heated to 145 – 170°F during the process.  In some cases, the concentrated liquid may be cooled to less than 45°F and stored in a large tank (10,000 – 30,000 gallons) until needed.

Prior to spray drying, the product is pre-heated to 160 – 200°F and passed through a high pressure pump to the spray dryer nozzles. The product is atomized into the spray dryer where the inlet air temperature ranges from 280 - 400°F, depending on the dryer model and process requirements. The spray dryer may be either a horizontal box-type dryer or a vertical funnel shaped dryer. As the droplets of product pass through the dryer, water is evaporated and the dry powder falls to the bottom of the spray dryer. The outlet temperature of the dryer is approximately 220°F. The temperature of the powder at the spray dryer outlet ranges from 110 – 175°F. The warm powder is passed through a fluidized bed where it is cooled by a stream of chilled air.

At the end of the fluidized bed, the powder has cooled to approximately 70°F. Typically, HEPA-filtered air is supplied to the spray dryer and fluidized bed to minimize the risk of product contamination. After spray drying, the product may be agglomerated to increase the particle size and to improve its solubility. The finished powder is passed through a sifter then transferred to bags, totes or silos for storage. In some cases, the powder may be transferred directly to the powder packaging line. At the packaging line, the powder is transferred to a filler hopper that feeds powder into the can filling line. Filled cans are flushed with inert gas, seamed, labelled, coded and packed into cartons. Typically, finished product is held until it undergoes a final check for conformance to specifications, including microbiological contaminants.

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