UHT Pasteurization

There has always been one superficial line between food production and supply, production being less than the required demand of people. Areas enjoying surplus production of food products always tend to wastage because of perishable nature of food, whereas some places face inadequate supply. Thus condition arises where supplier need to provide high quality product with minimal changes, which can only be achieved by preventing spoilage during or after production, as well as in distribution chain. For combating these challenges, various types of food preservation techniques were introduced and implemented in food sector. Now widely used methods of food preservation, Ultra high temperature treatment, is gaining popularity among population as it assures the production of commercially sterile product having similar sensory attributes as possess by the product originally without UHT treatment.

Ultra High Temperature or Ultra Pasteurization processing goes by the name of UHT, one of the major preservation technique in dairy industries as it converts highly perishable milk into a product with shelf life extended from days to month and can be stored and transported without refrigeration condition. Furthermore, marginal nutritional and sensory changes in milk by its processing becomes the driving force for UHT products in market.

UHT processing is associated with aseptic packaging, one of the demanding technology and presents many scientific and technical opportunities.  UHT processing is generally defined as heating milk between 135 and 145 C for 1±10s. The basis of UHT is- sterilization of food before packaging and filling into pre-sterilized containers in sterile environment. The ultimate goal being maximising the destruction of microorganisms and minimize the chemical changes in product.

Component of UHT

UHT treatment requires both a sterilizer and an aseptic unit (for packaging the product) which are described below:

1. Steriliser: A closed continuous chamber where controlled heating of product is done at 135 and 145 ˚C for 1 to 10 seconds for complete elimination of micro-organisms, including thermal resistant spores. Two methods of heating are employed for sterilization process:

• Direct Heating: Product is in direct contact with heating medium such as steam, which need to be of food grade or potable quality. Main advantage of direct heating is that it does less damage to heat sensitive products (milk) as it is held at elevated temperature for a shorter period. Steam is briefly introduced into the product which is rapidly followed by flash cooling. The brevity of the treatment makes it possible to achieve very high product quality but the process becomes costly due to high-energy consumption.
• Indirect Heating: The heating medium and product are not in direct contact, but separated by equipment contact surfaces. For achieving this heat exchangers are used as in case of normal pasteurization process. Heat exchangers can be defined as a system used for transferring heat between two or more fluids. Plate, tubular and scraped surface types of heat exchangers are used which makes the process cost effective as most of the heat energy can be recovered.

2. Aseptic Packaging: Filling of a commercially sterile product into a sterile container under aseptic conditions and hermetically sealing the containers so that reinfection can be prevented. This results in a product, which is shelf-stable at ambient conditions and when UHT is coupled with aseptic packaging technology, then the product can be protected from post processing contaminates thus extending the shelf life of the product.

Advantages of UHT

1. Products with longer shelf life:UHT pasteurization extends product’s shelf life by months.
2. Products with less bacteria:High heat required during UHT pasteurization results in milk that is up to 99.9% bacteria-free and inactivates the highly thermal resistant bacterial spores.
3. Products does not require refrigeration: When the UHT process is coupled with aseptic filling equipment, then the product can be stored and transported at ambient temperature i.e. requirement of refrigerator is not needed thus cutting the extra cost.

Effects of UHT processing on Milk

1. Whey protein denaturation and formation of protein complexes: Β-lacto globulin, which makes the major part of whey protein, is denatured in irreversible manner as they are heated above 70°C. Denaturation of lactoglobulin begins with dissociation of its very compact dimeric structure followed by unfolding of the globular monomer, which leads to exposure of its active free sulfhydryl group, which is normally buried inside the globular structure and forms complexes with caseins and lactalbumin. The extent of whey protein denaturation is greater for indirect heating than for direct heating.

2. Fouling or deposit formation: During UHT processing of milk, a deposit builds up on the heat exchanger, which reduces the operational efficiency of the plant as the deposit acts as a partial insulator and reduces the rate of heat transfer to the milk. The nature of the deposit varies throughout the plant as it depends on the temperature of the heat exchanger. In general, these can be classified into two types :

• Type A: occurs at the lower temperatures in a UHT plant, up to ~110˚C; it is quite voluminous and consists largely of protein.
• Type B: which occurs at temperatures >~110˚C, is hard and compact and consists largely of minerals, principally calcium phosphate.

3. Maillard reactions: During UHT Processing, initial step is reaction between lactose and lysine of milk protein leads to the formation of several intermediate compounds such as hydroxyl methyl furfural (HMF), formic acid, methyl ketones, and formation of advanced Maillard products such as galactosyl-pyranone and brown-coloured melanoidins.
4. Lactose isomerisation: During UHT Treatment, Isomerisation of lactose (galactose ± glucose) to lactulose (galactose ± fructose) occurs which is considered as the most reliable index of heat treatment of UHT milk, as it is not present in unheated milk and it changes little during storage of the milk It has been proposed that the lactulose content of UHT milk should be <600mg/L to enable it to be distinguished from sterilised milk, which should have >600mg/L of lactulose.

Effects on UHT milk during storage

1. Gelation: It is a major defect which limits the shelf-life of UHT milk, where the milk thickens over storage and eventually turns into a gel having custard like consistency. It was reported to be caused by the heat-resistant bacterial enzymes but it can be also caused by plasmin, a naturally occurring protease in milk, which is quite heat stable and can remain active in UHT milk.

 

2. Flavour: UHT milk immediately after production has a sulphury flavour and is characterised by an array of volatile sulphur compounds including hydrogen sulphide, methane thiol, dimethyl disulphide, dimethyl sulphide, carbon disulphide, carbonyl sulphide, dimethyl trisulphide, dimethyl sulphoxide and dimethyl sulphone. Methanethiol, Dimethyl sulphide and Dimethyl trisulphide are identified as major contributors for sulphurous flavour.

UHT processing is a mature technology and now it is also used for processing of other food products such as soups, ketchups, custard, flavoured milk, fruit juices etc. It sterilizes food products by destroying harmful bacteria and pathogenic microorganisms through a tightly controlled thermal process and combines the sterile product with the sterile packaging material in a sterile environment; the end result is a shelf-stable product requiring no refrigeration.