Quality monitoring in milk processing — Part 2
In Part 1 the necessary steps in the quality monitoring of whole milk production were introduced and the issues around milk delivery, storage and minimisation of product loss discussed. In this article, the issues of food safety and shelf life maximisation through heat treatment are described.
The main components of milk are water (85–88%), fat, proteins, lactose (milk sugar) and minerals (salts). There are also trace amounts of other substances such as pigments, enzymes, vitamins, phospholipids (substances with fatlike properties) and gases. The relative amount of these constituents is variable, due to the natural origin of the raw product and its dependence on natural biological variation between animals, and both seasonal and locational (regional and farm-specific) variation. The process of milk standardisation is intended to produce whole milk for retail sale that has a standardised minimum fat content, while also producing the cream (fat) by-product that can be used for secondary products such as butter, cream, yoghurt and cheese.
The composition of milk in Australia is regulated by the Food Standards Code (Standard 2.5.1), published by Food Standards Australia and New Zealand. According to FSANZ1:
The standard for packaged cows’ milk for retail sale requires that it contain at least 3.2% fat and 3.0% protein. Skim milk must contain a maximum of 0.15% of fat and a minimum of 3.0% protein.
The Code allows milk processors to adjust the components of milk, such as lactose, protein, fat or vitamins and minerals by adding or removing those components to produce a standardised product.
Typically raw cows’ milk delivered from the farm contains approximately 4% fat. The standardisation process therefore involves producing ‘full cream’ milk and skim milk that meet the requirements of the standard, the remaining cream/fat being available for the manufacturing of other products.
The general process of standardisation involves separating the raw milk into skim milk and cream using a centrifugal separator. Typical (rounded) figures for fat content (see Figure 1) may be that the skim milk from the separator has a fat content of 0.05%, and the cream has a fat content of 40%. Some of the cream is then remixed with the skim milk to produce full cream milk standardised to the required level of fat (in the figure 3% and in Australia at least 3.2%).
For precision in the process it is not only necessary to measure the fat content during the re-mixing process, but also to measure other variable parameters such as fluctuations in the fat content of the incoming milk, fluctuations in throughput and fluctuations in preheating temperature. As most of the variables are interdependent, variations in any one stage of the process can result in deviations in all stages.
The raw whole milk is heated to 55–65°C in the pasteuriser/heat exchanger before being passed to the separator. Reliable performance of the separator also depends on the pressure in the skim milk outlet being kept constant — this pressure must be maintained regardless of downstream variations in flow or pressure, so it is necessary to monitor the outlet pressure to control a constant pressure valve.
Online density measurement is commonly used for the control of the milk standardisation, at the re-mixing stage. The densities of skim milk and milk fat are known, although temperature-dependent (see Table 1). The density change on the mixture addition of skim milk and cream is used to monitor and control the fat content of the standardised milk.
Instruments for monitoring flow and density
Measuring milk flow and density can be accomplished by any number of technologies, but can be combined accurately in a single instrument by utilising a Coriolis meter. These types of meter are ideal for virtually all fluids, measuring several process parameters — including both mass flow and density directly in the pipeline. Coriolis meters designed for the dairy industry can also measure temperature and viscosity, and be configured to viscosity, °Brix, and % Milkfat.
Additionally, electromagnetic flow meters are ideal for batching, and for the receiving and feed lines of product or CIP flows. They can also handle pulsating flow and can be used for empty pipe detection. Those with integrated conductivity measurement allow for continuous monitoring of phase changes and product identification.
Instruments for monitoring pressure
Pressure conditions in the feed line and the outlet of a separator require reliable pressure monitoring. Pressure instruments that offer a flush stainless steel or ceramic diaphragm, and a wide variety of hygienic process connections, provide the highest flexibility to retrofit into existing applications. Not only should they provide reliable and accurate pressure measurement, but they should be condensation tight and easy to clean.
Raw milk can contain a range of pathogenic microorganisms, the most resistant to treatment of which is the tubercle bacillus (tuberculosis). Fortunately they can all be killed by heating the milk to a minimum of 63°C for 10 minutes, and general practice is to maintain the temperature for 30 minutes.
In addition to pathogenic microorganisms, milk also contains other substances and microorganisms that can spoil the taste and shorten the shelf life of the milk. A secondary purpose of heat treatment is to destroy as many of these as possible, which requires more intense heat treatment than is needed to kill the pathogens. This secondary purpose for heat treatment has become more important in recent decades, as dairies have become fewer in number and larger, and are located at greater distances from both the farms and the consumers — a significant factor in the Australian dairy industry. Despite chilling milk throughout transport and storage, the time delay in processing and consuming allows more time for these bacteria to multiply — chilling milk to 4°C slows the process but doesn’t stop it completely, and once the temperature rises above 4°C, the number of bacteria multiply rapidly (see Figure 4).
Because heat treatment has potential detrimental effects on the milk product, such as changes to flavour and appearance, or can impair secondary processing such as cheese making, the management and control of heat treatment processes is a critical factor in maintaining product quality as well as safety.
The minimum temperature and time mentioned above are used in a process known as LTLT (low temperature, long time) pasteurisation. Modern dairies in contrast use a process called HTST (high temperature, short time) in which the milk is heated to 72–75°C for a shorter time of 15–20 seconds. It is easier to maintain the higher temperature for this shorter time in a holding tube than it is to maintain a lower temperature for longer. After pasteurisation, the milk must be cooled to 4°C for packaging and transport.
The most energy-efficient way to achieve the various stages of heating and cooling is to use a multiple stage heat exchanger in which the hot pasteurised and raw cold milk are used as part of the heat transfer process. By this method, the hot pasteurised milk is cooled in the heat exchanger by transferring some of its heat to not only a cooling fluid such as cold water, but also to the cold raw milk in an adjacent section of the exchanger. In this way, less steam heating is required to bring the raw milk up to the required temperature. Regenerative heat exchange in this way can recycle as much as 95% of the heat from the pasteurised milk.
The risk with regenerative heat exchange is that any leak that may occur internally in the heat exchanger could lead to raw milk contaminating the pasteurised milk. To mitigate this risk, the flow of hot milk entering the heat exchanger for cooling must be pumped through the heat exchanger at a higher positive pressure than that for the raw milk by using a booster pump.
In some cases it is not possible to pasteurise and process all the milk immediately after reception, and the storage of raw milk for days or even hours, despite chilling, can result in degradation of the milk before it is even processed.
Thermisation is a process carried out by many dairies to temporarily inhibit bacterial growth. It involves preheating the milk to a temperature just below the pasteurisation temperature for about 15 seconds.
To prevent bacteria from multiplying after thermisation, the milk must be rapidly chilled to 4°C or below and it must not be mixed with untreated milk. This process should be applied only in exceptional cases — ideally pasteurisation of the incoming milk should be completed within 24 hours of receiving.
As the most critical process in relation to food safety, the milk heat treatment process must be accurately and continuously monitored and recorded, with correct temperature being the most primary consideration. Accurate and fast-responding temperature instruments are required at a number of points in the process:
- Raw milk stored in the balancing tank and flowing into the heat exchanger, as well as the pasteurised milk after cooling must be monitored to be sure it remains at 4°C.
- The milk leaving the heat exchanger and entering the holding tube as well as the milk leaving the holding tube must be monitored to ensure the pasteurisation temperature is achieved and compared to ensure that the temperature is maintained for the required holding period. This data must be continually logged for food safety audit purposes. If the temperature is not maintained for the required time, then the milk must be diverted back to the balancing tank to avoid compromising food safety. Fast temperature response is essential.
- The heat exchange process also requires the right flow of water for cooling and steam for heating to maintain the correct temperature flows, so temperature instruments are also required to monitor and control the temperature of these fluids.
If a thermisation process is being used for stored raw milk, then fast-responding temperature instruments will also be required to monitor the heating and cooling process, as well as the storage temperature.
Instruments for monitoring temperature
Due to the rapid heating and cooling used in milk processing, temperature sensing in milk heat treatment must be fast and accurate. It is essential that temperature instruments that are used have the fastest possible response. Today, temperature instruments are available that have a t90 response time of under 1.5 seconds. It is also important that they have a high vibration resistance (better than 60g) for plant safety.
To minimise calibration time (and maximise plant uptime) newly designed temperature sensors provide for two-piece construction, whereby, the first piece (thermowell) is permanently welded into the process and the second piece (temperature measuring element) is inserted into the thermowell by means of a simple bayonet connection.
The next most important parameter to measure is pressure. As stated above, when using regenerative heat exchange, the differential pressure between the raw milk and pasteurised milk sides of the heat exchanger must be measured and maintained to ensure there is a higher pressure on the pasteurised milk side.
Under normal continuous process conditions, the pressure would normally be constant and within a certain range. Abnormal variations in the pressure and flow rate anywhere in the process may be an indication of a leak or potential failure in the heat exchanger or elsewhere. The same applies for heating and cooling fluid lines, where a leak could result in these fluids also contaminating the milk.
Instruments for monitoring pressure
Monitoring pasteurised milk pressure will require a high level of hygienic safety, and so an appropriate instrument designed to be food safe with a ceramic or stainless steel sensor membrane and tolerant of CIP wash-down chemicals is required (Figure 6).
Level measurement in the balance tank is also important to monitor, and is typically performed using hydrostatic pressure measurement, in the same way as for the raw milk receiving tanks.
As a continuous process, empty lines could create problems for the heat exchanger, whether it be the raw and pasteurised milk product, the heating steam or the cooling fluid. It is therefore necessary to make sure that the flow rate through the heat exchanger is constant and balanced, and also to monitor the flow and consumption of steam.
Instruments for monitoring flow
The requirements of flow measurement in dairy heat treatment (sanitary, accurate and robust) can all be fulfilled with the use of a magnetic flow meter.
The accuracy of a typical magnetic flow instrument used for heat treatment flows is unaffected by large flow variations. Since the magnetic measurement principle is virtually independent of pressure, density, viscosity and temperature, it is ideal for monitoring flow monitoring in the heat treatment process and can provide empty pipe detection.
- Food Standards Australia and New Zealand (FSANZ) 2012, <http://www.foodstandards.gov.au/consumer/generalissues/milk/Pages/default.aspx>
- Tetra Pak Processing Systems AB, Dairy Processing Handbook.
- Goff H D, Hill A R 1993, Dairy Chemistry and Physics, Dairy Science and Technology Handbook, VCH Publishers, vol.1.
- Tetra Pak Processing Systems AB, Dairy Processing Handbook.
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