UTH Processing

i. Definition

UHT milk can be defined as a product obtained by heating milk in a continuous flow to a temperature in excess of 125oC for not less than two seconds and immediately packaging in sterile packages under aseptic conditions. In India, UHT milk is generally processed at 140oC for 2 seconds.

 

ii. Theoretical Basis

Heating of milk results in death of microorganisms. While some bacteria are destroyed by pasteurization (71.7OC/15 s) only, some survive this thermal treatment.Bacillus subtilis and Bacillus stearothermophillus spores are very heat resistant.Of the two, Bacillus stearothermophillus spores are most heat resistant. It is therefore, considered index organism for evaluating performance of UHT processing.

Heating of milk at higher temperatures also result in undesirable changes in chemical quality. Browning reactions are particularly important. Higher thermal load results in more browning and therefore loss of flavour and quality. In the temperature range of 100-120oC, time required for death of almost all B. stearothermophillus spores are more. This may therefore result in more browning in the product.However, if milk is treated in the UHT range i.e. 135-150oC for only few seconds,almost all spores may get killed and browning would be minimum. Loss of nutrients and total quality also will be minimum. A product processed in this temperature range will be thus microbiologically safe and yet superior in terms of overall quality.

 

iii. Types of Sterilization Plants

There are two types of UHT plants: Direct type and Indirect type. In direct type plants, heating is done by mixing product and steam. In indirect type plant, product is heated by steam or hot water without the two coming in direct contact. Heating in direct type plant is very rapid particularly between 80-140oC and total heat load is less. Changes in the product quality are therefore minimum. In indirect plant, rise in temperature is very gradual. Therefore, heat load on the product is more. Changes in chemical quality are comparatively more in indirect type than in direct type plants.

(i) Direct Heating Plant: There are two types of direct heating plants (a) Injection type and (b) Infusion type.

Injection type: Processing is through steam-into-milk arrangement. Steam injector is the heart of this plant. Preheated milk at 80-90OC enters the injector nozzles from one side. Steam at slightly higher pressure enters the injector from the other side.As the steam mixes with milk, steam condenses and the product is rapidly heated.Rapid condensation of steam prevents entry of air in holding tube. Air in holding tubes results in improper heating. Backpressure is maintained on the discharge side.Backpressure ensures that product does not boil in holding tube. Boiling may result in fouling and improper heating of milk. Several designs of injector are available.

Infusion type: In this system, milk is heated by milk-into-steam arrangement. The processing unit consists of a chamber filled with pressurized steam. Milk enters the chamber from the top. There are two alternative arrangements for distribution of milk. In the first type, milk flows to a hemispherical bowl with loose circular disc closing the top. When the bowl is full, milk overflows and falls in droplets through the steam environment. In an alternative arrangement, milk flows through a series of parallel and horizontal distribution tubes. These tubes have slits along the bottom and milk flows like a thin film through the chamber. As milk reaches the bottom of the chamber, it is heated to desired temperature. This system is particularly suitable for thicker liquids and for liquids suspended with smaller chunks.

Advantages and Disadvantages of Direct Heating System: During processing in direct type heating systems condensing of steam coming into product contact results in dilution of the product. To remove this excess of water from the product,cooling is done in an expansion cooling vessel. In expansion vessel, along with the evaporating water incondensable gases and undesirable flavour volatiles produced during heating are also removed. The product therefore tastes better. Steam injection induces formation of casein aggregates, which give “chalky” or “astringent” mouthfeel to the product. Aseptic homogenizer, which can safely homogenize the product after final heating section, is generally preferred with direct heating systems to overcome such defects in the product.

Rate of heating is very high (takes less than 1 sec to attain sterilization temperature).Thick/viscous liquid can also be easily processed. Deposit formation is minimum,hence plant can be operated for longer time without cleaning. Undesirable flavours are removed during flash cooling. Oxygen is removed during cooling, hence oxidized flavour defects are delayed during storage.

Cost of processing per unit volume of milk is high. Requires additional equipment(vacuum expansion chamber and aseptic homogenizer) – cost of plant is twice that of indirect type plant. Heat energy requirement is very high. Water and electricity(25-50% more than in direct type) consumption are high. Requires culinary steam and hence special boiler. Creates greater noise during operation.
(ii) Indirect Type Heating System: There are three types of indirect heating systems: (a) Plate heat exchangers (b) Tubular heat exchanger (c) Scraped surface heat exchanger.

Plate heat exchanger: This resembles plate heat exchanger of HTST plants.Several rectangular stainless steel plates with corrugations are arranged in sequence.These plates are then mechanically tightened to hold together. Corrugations on the plates induce turbulence and therefore result in high heat transfer. High temperature processing generates high internal pressure. The gaskets are therefore made of heat resistant materials such as medium nitrile rubber or resin cured butyl rubber. A major advantage of this plant is therefore simple design and comparatively less cost. If deposit formation is more, plates can be removed and manually cleaned.

Tubular heat exchanger: There are two types of tubular heat exchangers – (a)concentric tube, (b) shell and tube type. Concentric tube type heat exchangers comprise two or three stainless steel tube lengths put one inside another. Spacer is placed in each inner tube space to maintain them concentric. Several such multiple tubes are bound together and placed into an outer cylindrical housing. Two tube heat exchangers are used for simple cooling and heating. In triple tube heat exchanger, available heat transfer area is doubled. It is generally used in final cooling section. It is also suitable for processing of thick liquids, which generally reduces heat transfer rate. Product flows through the middle annular space. Heating or cooling medium passes through inner tube and outer annular space. In shell and tube type heat exchangers, 5-7 straight lengths of smaller tubes (10-15 mm internal diameter) are assembled in an outer tube. The smaller tubes are connected to large outer tube at both ends by a manifold. Product passes through the smaller tubes.Heating or cooling medium passes through the space around them in a counter current flow. Tubular heat exchangers are mechanically very strong and can withstand even very high internal pressure generated during homogenization (200-300 bar). Therefore the need for acquiring an aseptic homogenizer to be placed after heating section is totally eliminated. Instead, the high pressure reciprocating pump of an ordinary homogenizer can be placed before the sterile section. The homogenizing valve can be put at any point on the downstream side (even after final heating section). The problem of product contamination arises from the homogenization pump and not the valve. Therefore, with tubular heat exchangers,the product can be homogenized before sterilization, after sterilization or on both the occasions. Fat rich products like cream require homogenization after final heating to prevent re-association of fat globules due to high temperature processing after homogenization.

Scraped Surface Heat Exchanger (SSWE): It is a very specialized type of heat exchanger. It consists of a jacketed cylinder. A shaft passes along the axis of the cylinder. The shaft is supported by bearings at both ends of the cylinder. The shaft also carries several scrapper blades. As shaft rotates, scrapper blades provide turbulence and physically remove the product from the surface of the wall. The colder product subsequently replaces the heated product and the cycle continues.SSHE is used only for heating very thick liquids. SSHE units are very expensive and have poor energy conversion efficiency. The cost of processing is therefore very high.

Advantages and Disadvantages of Indirect Heating System: It is simple in design and requires less pumps and controls. It can regenerate 90% of the thermal energy requirement. It does not require aseptic homogenizer, which is very costly.It does not require culinary steam and therefore special type of boiler. The indirect type plant is less noisy. It requires low initial capital and operational cost is also comparatively less.

In indirect type heat exchanger, rate of heat transfer is low. More heat load results in less acceptable product quality. Deposit formation is more and therefore plant requires frequent cleaning. For removal of dissolved oxygen from milk, additional equipment ‘deaerator’ is required.

 

iv. Changes in Milk during Processing

UHT processing does not cause reduction in biological value of proteins. There is only small loss of available lysine (6-7%). UHT processing changes the casein micelle structure. This slows rennet action during cheese manufacture. Serum proteins are denatured (direct processing – upto 50-75%, indirect processing upto 70-90%). Denatured serum proteins interact with casein and increase casein micelle size. This reflects more light and UHT milk appears whiter. Aggregates of denatured serum proteins and casein also give ‘chalky’ mouth feel to the product.There is no physical or chemical change in milk fat. The total mineral content also does not change during UHT processing. The vitamin content of UHT milk is comparable to pasteurized milk. Losses in B-complex vitamins are not more than 10%. Folic acid and ascorbic acid are destroyed up to 15% and 25%, respectively.Fat soluble vitamins A, D, E and K are not affected by UHT processing. Fresh UHT milk has slightly cooked flavour. The cooked flavour is due to oxidotion of the SH (Sulphydryl) groups from the denatured serum proteins.

 

v. Changes in UHT Milk during Storage

Chemical, physical or sensory changes in stored UHT milk are dependent on storage temperature. Changes are rapid if storage temperature exceeds 30OC.Browning reactions between protein and lactose progress during storage. At higher storage temperature (>30OC) UHT milk may become little brown after 3-4 months.Refrigerated storage of raw milk before UHT processing favours growth of psychrotrophs. They liberate heat resistant proteases and lipases. Proteases that survive UHT treatment act on proteins during storage. Bitter peptides are released causing bitterness in the product. Extensive proteolysis and other physico-chemical changes occurring as a result of interaction of proteins and salts during storage may cause thickening or sweet curdling also referred as age thickening after longer storage (more than 6 months). Lipases surviving ultra-high-temperature treatment act on lipid fraction. Short and medium chain free fatty acids are released. Short chain fatty acids particularly butyric acids contribute to development of rancid flavour in the product. Air in the product or in the packet reacts with unsaturated fatty acids. This auto oxidation reaction causes formation of aldehydes and ketones.These compounds cause oxidative rancidity (flavour defect) in the product. The cooked flavour in UHT milk disappears in first few days and milk tastes best after this period. Few weeks after this, depending on the temperature of storage, oxidized flavour defects appear which becomes more pronounced with progressive storage.In milk stored for considerable period of time, which could be 3-4 months at >30OC,stale flavour is a common defect. Several compounds that form during the progress of Maillard reactions in stored milk are associated with the appearance of this defect. Sometimes coconut like flavour defect also appears in UHT milk stored for longer period. Compounds such as d–dodelactone and d–dodecalactones are responsible for this.