Modern Dairy Processing
The primary objective of a heat-treatment process is to ensure food safety, to comply with hygiene requirements and to facilitate those ingredients that require heat to activate and initiate functional properties. This applies to water continuous emulsions. Milk separation may be carried out as part of the heat-treatment process (e.g. pasteurisation). In many continuous heat-treatment applications on farms, a separator is linked to the heat-exchange unit either in the heating cycle or after the final heating stage and during the cooling cycle. In either arrangement the separation temperature used is in the region of 40–45oC. In the former method the separated fractions, skimmed milk and cream are further heat treated to pasteurise and are then cooled.
Pasteurisation methods can be either of the batch type or of the continuous high-temperature short-time (HTST) type.
Batch method. In this method the product is heated to a temperature of not less than 65.6oC and held at that temperature for at least 30 min.
High-temperature short-time method. In this method the product is heated to not less than 72.0oC and held at that temperature for at least 15 s (continuous method), or it is heated to some other temperature for some other time regime, to have an equivalent effect to eliminate pathogens.
Ice-cream batch method. In this method the product is heated to not less than 71.1oC for not less than 10 min.
Ice-cream high-temperature short-time method. Here, the product is heated to 79.4oC for not less than 15 s and is cooled to less than 7.2oC within 1.5 h before freezing. If frozen ice cream reaches above -2.2oC at any time during storage it must undergo pasteurisation again before sale.
In each method the ice cream must be cooled to less than 10oC, preferably less than 7oC.
- The pasteurisation temperature sensor is located in the early part of the holding tube.
- A divert valve is fitted at the end of the holding tube.
- A continuous recording method is used for probes monitoring the temperatures of pasteurisation, hot water and the final product cooler.
- A pressure differential measurement and indicating device should be fitted to monitor the raw and pasteurised products. If the pressure differential is not monitored a valid pressure-test certificate should be available for inspection by the licensing authority (every 12 months).
Ultra high-temperature treatment
Long-life creams and ice-cream mixes have been produced successfully for many years in Europe. Long-life ice-cream mixes have become popular in many other parts of the world. The ultra high temperature (UHT) process was originally applied successfully to milk. The same basic processing methods were later adapted for creams and ice-cream mixes.
The minimum heat treatment required is defined as follows:
- heat the product to not less than 140oC and hold that temperature for not less than 2 s
- heat the product under other temperature and time regimes having an equivalent lethal effect on vegetative pathogens and spores
The normal heat-treatment regimes used in commercial operations are 136–145oC for 2–6 s. For ice-cream mixes it is necessary to raise the temperature to not less than 148.9oC for at least 2 s. The regulations may vary according to the food process control measures introduced in individual countries.
Process plants have been developed and mechanised to be able to withstand heat-treatment duty and to satisfy hygiene and food safety requirements and the organoleptic quality preferences of the consumer. Heat-exchange systems are divided into three main types based on the method of construction: plate, tubular and scraped-surface. The most commonly used in the dairy industry is the plate-type heat exchanger. Plate heat exchangers are efficient in terms of energy usage and some plants have been designed to operate with 95% energy efficiency. Modern shell and tube heat exchangers are designed for similar energy efficiency and to handle more viscous liquid products. The method of heat transfer is also divided into two main groups based on how the transfer is carried out: direct or indirect. The direct method may involve steam injection, in which high-pressure steam is injected into a stream of product, steam infusion, in which the product is injected into a chamber containing steam under pressure, or it may involve electrical heating, in which a high voltage passes between two electrodes placed inside a stainless steel tube carrying the product, the resistance to electrical conductance producing the necessary heat.
Extended shelf-life processes(ESL)
A systematic investigation into the poor keeping quality of milk and creams commenced in the early 1970s. At that time it was well known that the storage life of cream, milk and other fresh liquid milk products was very short. For example, when these products are stored at ambient temperature (e.g. 10–20oC) for a few hours the microbiological quality can reach unacceptably high levels even when they are subsequently stored under refrigerated conditions. The shelf-life of such products can be prolonged by storing the products under refrigerated conditions (5oC–8oC) immediately after heat treatment. Such low-temperature storage conditions prolong the shelf-life from perhaps 1 or 2 days to perhaps 4 or 10 days, but prolongation of shelf-life by this extent is of limited value industrially. The deterioration in the quality of milk, cream and other fresh liquid milk products is due to microbiological activity that generally develops within a few days of storage to such a level that the product takes on unacceptable flavour characteristics and frequently undergoes unacceptable physical changes.
Microbiological quality of raw milk. Cows’ milk is an almost perfect food for human beings and is the perfect food for calves. Unfortunately, it is also a good source of food for microorganisms. Milk from the udder of a healthy cow contains very few organisms (not more than about 300/ ml) and these are of no danger to the consumer. Therefore, milk is contaminated generally by post-production handling, including milking equipment and the general hygiene of operatives. Therefore, the quality of raw milk will vary depending on general production hygiene, the equipment used, the environment, and organism population and type.
High-temperature pasteurisation. Pasteurisation of milk and milk products by the continuous flow method applies a heat-treatment regime of 72oC for 15 s. These conditions destroy pathogens in milk, in particular Mycobacterium tuberculosis. However, a pathogen Mycobacterium paratuberculosis was found to survive this pasteurisation regime, and many milk processing dairies have already extended the holding time from 15 s to 25 s in order to destroy this organism.
One drawback of the present pasteurisation process is that the whole system is vulnerable to post-process contamination. Thus the shelf-life of the product may vary from one day up to 10days, depending on the total counts in the final product after heat treatment and depending
on fluctuation in storage temperature. It is difficult to guarantee the hygiene standard of milk holding tanks, filling machines, packaging materials and the packaging environment. If contamination is high then the shelf-life may be reduced significantly. Therefore, a method to extend the shelf-life of creams and milks is desirable in order to minimise losses through microbiological spoilage.
In flash pasteurisation heat-treatment temperatures in the range 80–90oC are used commercially with a holding time of 1 s or less. Latest development in heat treatment showed that 120oC for 1 second and following aseptic filling technology can prolonged the shelf life of milk and cream to 30 days at refrigerated storage.
Homogenisation of emulsions
A homogeniser is simply a high-pressure pump usually designed to operate with a three-piston arrangement. It was invented in 1899 by a Frenchman, August Gaulin. The general method of homogenisation of creams and other dairy products involves pumping the liquid by a positive displacement pump arrangement into a homogeniser valve chamber or head. The head encloses the valve arrangement, with a very narrow gap allowing the product to exit. When the product is allowed to exit through the narrow slit the product particle velocity undergoes a sudden increase, which breaks down the coarse material and incoming fat globules (up to about 20 μm in diameter) to a much finer particle size (0. 1 μm).
With the inclusion of a homogenisation step in the process one would expect to achieve a stable emulsion as a result of particle size reduction, a smoother mouth feel as a result of the smaller fat globules, the need to use less stabiliser, a shorter ageing time, better overrun and a decreased tendency to churn fat in the freezer in ice-cream mixes. Therefore, even small variations from the optimum process conditions can lead to significant deterioration of consistency and texture.
In long-life milks, creams and ice-cream mixes the homogenisation can be done after the sterilisation stage and the cooling section of the process (downstream). To be able to homogenise in this downstream position, the pistons and pressure-adjustment devices must be fitted with steam tracing to protect the product from post-process contamination.
Double homogenisation. Information on double homogenisation or multiple homogenisation is limited. Information on double homogenisation showed that fat-globule dispersion in ice cream was better with the liquid-whirl valve design compared with the flat-valve and conical valve designs in single-stage homogenisation. Two-stage homogenisation did not improve the degree of dispersion in comparison with single-stage homogenisation, whereas double homogenisation gave a higher degree of dispersibility than single-stage or two-stage homogenisation.
The transfer of food subsequent to UHT or ESL treatment has to follow in sterile environment to avoid microbiological contamination. The food must receive this sterile environment from the end of the holding tube of the process plant until the sealed packaging container. The techniques involved in this part of the procedure are defined as the aseptic technology.
Aseptic filling machines are available to fill plastic cups, metal cans, flexible cartons and pouches, bag in box bulk packs, glass and plastic bottles. The most commonly used sterilizing agent for these containers is hydrogen peroxide at a concentration of 35%. Steam can be used for metal cans and some plastic materials. Bag in box system uses irradiated containers and the filler sterilise the mouth of the container prior to filling. After filling the opening is re-sealed. A large proportion of aseptic fillers used for milk are defined as form fill seal type designed to fill 20 ml portion packs to 2 liter family packs. The packing material is a composite multilayer paper, plastic and aluminium. These machines are also available to fill pre-formed containers.
Majority of plastic containers are filled using form fill and seal type and pre formed container filling systems. The preformed containers offer greater flexibility in filling variety of products with minimum cost.
Most bottle filling machines are pre formed container type. These bottle fillers use hydrogen peroxide or other chemical sterilents for bottle sterilisation. However, in-line bottle forming systems are also available for very high through put production requirement.
The food manufacturers today comes under pressure from various food authorities to conform to accepted standards for safe production methods. BRC (British Retail Consortium) is one such globally accepted standard that maintain high level of food production in every sector. It is important to maintain food establishments to high standard as BRC which gives the clients a guarantee of safety and quality of products made.
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