Spray Drying is a promising means of drying as it can produce powder products with excellent similarity in size even for heat sensitive materials. There are three main steps in spray drying process:
Before converting the product into the powder, the liquid feed should be thickened as much as possible through expelling the extra water. This thickening process occurs in multi-effect evaporators in which, by making a vacuum pressure, boiling temperature is reduced. The greatest advantages of falling film evaporators are heat recovery and mild processing condition. As a general heuristic, their energy consumption is only 100-300 kWh.t-1 of removed water (or even less), whereas it is about 1000-2000 kWh.t-1 for spray drying towers.
Milk enters the evaporation unit from the balance tank. From there, milk is sent to the thermophilic preheater to be preheated up to ~70°C. Then, milk should be pasteurized at 80 – 85°C. The used pasteurizer in this design is a direct pasteurizer in which steam is directly admitted into the feed. After passing through the holding tubes, pasteurized feed, will be directed to the flash vessel in which the injected steam is separated. Actually, the separated steam in this vessel was reused in the thermophilic preheater previously. Then after, the feed enters the calendria of evaporators from the 1st stage to the last stage respectively and gets concentrated. According to the design of the plant, only a small fraction of required steam is provided from the boiler room and the major heat requirement is provided by the reuse of produced steam in each step in the next coming step to enhance economy of the plant. A great milestone in design of evaporators is the design of calenderia. Although reduced boiling point decreases possibility of scales in calendria, but every single extra square meter in area of calendria causes 150 gram extra fouling in a day; the more fouling means more product loss and more production stoppages. In dairy industry, evaporator effluent should include a solid content of 45-55% according to type of the product.
Each stage has a great importance on performance of the constructed unit. Spray Drying in the most complete design, occurs during three steps: 1. Drying Chamber, 2. Internal Fluidized Bed and 3. External Fluidized Bed. As a general guideline, moisture of the feed drops to 10% after the first stage, to 6% after the internal fluidized bed and finally to 3% after the external fluidized bed. A modern spray dryer has the following parts:
. After passing through the holding tubes, pasteurized feed, will be directed to the flash vessel in which the injected steam is separated. Actually, the separated steam in this vessel was reused in the thermophilic preheater previously. Then after, the feed enters the calendria of evaporators from the 1st stage to the last stage respectively and gets concentrated. According to the design of the plant, only a small fraction of required steam is provided from the boiler room and the major heat requirement is provided by the reuse of produced steam in each step in the next coming step to enhance economy of the plant. A great milestone in design of evaporators is the design of calenderia. Although reduced boiling point decreases possibility of scales in calendria, but every single extra square meter in area of calendria causes 150 gram extra fouling in a day; the more fouling means more product loss and more production stoppages. In dairy industry, evaporator effluent should include a solid content of 45-55% according to type of the product.
Applications
There are numerous applications for Spray Dryers in food industry, like Dairy Factories, Instant Tea/Coffee Plant, Coffee Creamer, Honey Processing Plant, Egg Powder Plant, Starch and Glucose Plants, … . Due to the capacity and large number of dairy factories in all over the world, in the upcoming sections different dairy powdery products are described.
Extending shelf life of milk by removing the water phase necessary for micro-organisms goes back to before 1300 as reported in Marco Polo’s accounts. Today Skim Milk Powder (SMP) and Whole Milk Powder (WMP) with maximum shelf life of 36 and 6 months respectively are widely produced in dairy plants. The shorter shelf life of WMP roots in presence of fat; this fat content might oxidize during storage, with a consequent gradual deterioration in taste.
SMP is the most common dairy powder with wide applications in different areas. To provide the best feature, each application dictates the process requirement. A certain application of SMP is in chocolate industry. For this application, the tailor made process will be obtained by Drum Dryer, as a degree of lactose caramelization is highly welcomed in this field. However, if the final product is intended to be used as Instant Milk Powder, Spray Drying is much better since higher degree of agglomeration can be easily achieved. Considering the fact that drying includes a heat treatment, intensity of this heat treatment can cause different categories in SMP; the reason lies in the fact that whey protein gets easily denatured during thermal processes. For evaluating intensity of heat treatment and type of produced SMP, WPNI, Whey Protein Nitrogen Index, has been defined. WPNI expresses degree of denaturation of whey protein as milligrams of undenatured whey protein per gram of powder. WPNI drops if the heat treatment intensity increases. Milk intended to be used for SMP production, should be pasteurized and show a negative phosphatase test at minimum.
Standardized milk is used for WMP production. Unlike SMP, this product is not categorized. Milk intended for whole milk powder is normally pasteurized at 80 – 85°C to inactivate most of the lipolytic enzymes that would otherwise degrade the milk fat during storage. The successful pasteurization process should bring about a negative peroxidase test.
There are two categories of protein concentrated powders, Whey Protein Concentrate and Milk Protein Concentrate. The first one is produced from whey stream produced during cheese production (either sweet or sour whey) and the latter one is obtained from skim milk produced during standardization of milk. To have the protein concentrated, for both products membrane units are required (NF for WPC and UF for MPC).
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