CONDENSED AND DRIED MILKS

Module 13. Technology of dried milks

Lesson 28
FREEZE, VACUUM AND FOAM DRYING

28.1 Introduction

The processes other than heat evaporation of milk for manufacture of dried milks are also in practice for some specific purposes. They are freeze, vaccum and foam drying.

28.2 Freeze Drying

The process consists of freezing the product, supplying heat so moisture is removed without passing through the liquid phase, or as is usual, by maintaining a vacuum in the vaporizing chamber. The vapours are removed before they reach the vacuum pump.

28.3 Method

A batch processing or a continuous operation in a high-vacuum belt drier can be applied. Freezing can be done in a separate or the same chamber in which drying is done. Freezing is done to -28.8°C or lower. Multiple chambers are used for commercial operations with the complete drying process in each chamber. A vacuum approaching 2.54 cm Hg absolute is used. The air and vapor are passed over a refrigerated coil to condense the vapor. The quantity of heat supplied must be limited to avoid damage to the product particularly as the conduction properties change with drying. Electric heating platens, which can be decreased in temperature as drying progresses, are usually used for batch processes. Rapid freezing is desirable to provide formation of small ice crystals. Small crystals provide least change in the properties of the product and a product which will reconstitute easily. Low temperature processing is desired to provide a product with minimum change. A thin layer of the liquid is frozen, whereupon the ice is sublimated under a high vacuum. The heat or sublimation must be provided to vaporize the ice. The heat of vaporization is equal to the sum of the latent heat of fusion, sensible heat of liquid, and latent heat of evaporation. Moving heat to the product in the vacuum chamber is a major problem. Heating can be done by radiation, conduction, or convection. A voluminous cake is left (the space of the ice crystals is now occupied by holes) and is subsequently ground.

28.3.1 Advantages

1. Damage due to heating does not occur but that is also true of spray drying if skillfully performed.

2. Suitable for processing in small quantities and is applied to lactic starters, etc.

28.3.2 Disadvantages

1. The method is expensive.

2. The fat globules are subject to partial coalescence; this causes freeze-dried whole milk powder to show segregation after its reconstitution. Freeze drying costs about 5 to 10 times the conventional drying for foods.

28.3 Vacuum Drying

One method of vacuum drying is to meter nitrogen into concentrated whole milk after homogenization. The product is then cooled to 1.7°C in a scraped surface heat exchanger. The product is placed on a solid stainless steel horizontal belt 30.5 cm wide. It goes through first a heating drum, then a cooling drum, each 61 cm diameter for heating and cooling. Heat is supplied by 19 banks of 2 KW radiant heaters. A vacuum of about 50 mm Hg absolute is supplied. Scraper blades remove the product from the belt.

28.4 Foam Drying

Foam spray drying, is new and has caused interest in new products. Common dairy products, skim milk, whole milk, buttermilk, sweet and sour cream (up to 3:1 fat to SNF) whey and emulsified cheese slurry can be foam spray dried. Air is commonly used as the added gas for making foam spray nonfat dry milk. Nitrogen is commonly used for making foam spray dried whole milk.

The product to be dried is first stabilized. Air is fed to the product in a closed mixer where a porous product is formed. The air-product mixture is extruded onto perforated drying trays. The tray of foam passes over air jets catering the foam to increase drying surface. The trays then move into and upward in the drying chamber in the same direction as the air moves. Air at 104.4°C with a velocity of 111 to 203 cm/s is used, traveling co-currently with the tray. At the top section of the drier, after the warm drying air is emitted, cool air enters to cool the product. The dry product is produced in 1.5 to 15 min, is porous and readily soluble.

A foam film up to 40 mm thick is placed on a stainless steel belt. The product is exposed for a minute and temperature does not exceed 76.6 °C. The system can produce 5 to 7.5 kg of dry product per sq.m of belt.

28.4.1 Advantages

1. Using most conventional spray drying equipment for drying liquids up to a maximum of 60% total solids as compared to 50% on a particular drier.

2. The powder quality can be excellent due to the low drying temperature applied.

3. It can be applied to inhomogeneous products.

4. Drying special products, such as malted-milk and cottage cheese whey. But difficulty can be encountered because of sticking to the drier. A product of low density, excellent flow properties, highly hygroscopic, and rapid solubility is obtained using this method.

5. By holding the product, a product with more crystalline lactose is obtained. Holding 52% TS condensed nonfat milk at 35°C for 15 min provides 7.7% crystalline lactose in the powder; for 35 min, 17.1%, for 60 min, 35%. Conventional agglomerated dry milk has from 5 to 25% crystalline lactose. The greater the percentage of crystallized lactose, the less hygroscopic is the dry product.

6. Obtaining an instant type powder, but with characteristics different than the product prepared by agglomerating processes normally used following the spray direr.

7. Provides a procedure for increasing the capacity of conventional equipment. By removing the water ahead of the drier, the cost of operation can be decreased. Approximately twice as much product can be handled by the drier with a feed of 60% TS as compared to 42.5%. The capacity of a conventional drier can be increased appreciably by using higher total solids in the input.

8. Foam spray nonfat dry milk has a bulk density of about 0.35 gm. per ml. or less which is about the same as agglomerated or conventional instantized powder.

9. The product can withstand considerable handling and can be compressed into smaller volumes without appreciably affecting the dispersibility.

10. A more uniform particle size is obtained with foam spray drying.

11. Good dispersibility during reconstitution enhances its prospects for home use.

28.4.2 Limitations

(a) The product on reconstitution remains on top of water for an extended period and needs to be mechanically mixed to form a solution.

(b) During reconstitution, considerable foam is formed.

(c) Providing a liquid milk product at 60 % TS requires special consideration for heat exchangers and evaporation. In general, larger heat exchanger surfaces or continuous forced flow of the product through the heat exchanger is required.

(d) As the holding time increases, the viscosity increases and makes handling difficult and dispersibility decreases.

The product acceptance may be slow for industrial uses because of the low bulk density, hence higher packaging costs.