Quality monitoring in milk processing — Part 1

In modern dairy production, in which food safety and quality are paramount, accurate and fast control of each step of the process requires appropriate instrumentation for measuring flow, mass, temperature and pressure.

The milk of various animals has been used a source of human nutrition for thousands of years; however, it has only been in the last 200 years that techniques have evolved to produce milk for sale in high volumes. With the discovery of pasteurisation in the 19th century, it became possible to produce milk for human consumption on an even larger scale while ensuring it was bacterially safe.

Modern dairy production is now a highly automated and complex process involving many stages of processing to produce not only whole milk of standardised quality, but also secondary products such as skim milk, cream, butter, cheese and yoghurt, along with a broad range of processed food products.

This article describes the general processes and instrumentation required in the production of whole milk, and describes the three main processes of raw milk receiving and storage, standardisation of fat content, and heat treatment for food safety. Accurate and fast control of each step of the process requires appropriate instrumentation for measuring flow, mass, temperature and pressure — instrumentation that is fast, accurate, resistant to CIP washdown, and suitable for food and beverage applications.

Milk receiving and storage

The process of milk receiving at a dairy involves the emptying of the milk delivery trucks to raw milk storage tanks to await processing. The delivery necessarily requires measuring and recording the quantity of raw milk delivered, which is challenged by the fact that the transportation will have caused some frothing of the milk. Therefore, there will be entrained air in the milk (bubbles), making accurate measurement difficult. The milk reception process therefore needs to be designed well to minimise bubbles.

Figure 1: The milk receiving and storage process.

An air eliminator is used prior to measurement to ensure that the majority of the large coalescent foaming bubbles are removed. The larger the buffer size, the greater the pressure available to collapse bubbles. This also has the advantage of a longer holding time, allowing the maximum number of bubbles to escape the process before passing through the meter.

Milk receiving and storage measurements

The reception and storage of raw milk requires the elimination — as far as possible — of entrained air in the form of bubbles, and finding an accurate mass flow reading of the milk quantity, compensating for the small bubbles that remain after the air elimination stage.

Accurate mass balance and volume measurements throughout the diary process are critical for understanding such things as:

• the mass of cream or fat taken into the process, as compared with after standardisation
• accounting for the consumption and use of the raw milk through subsequent processing
• control of product losses at all following steps in the process.

As for all aspects of milk processing, temperature control is critical and so the accurate monitoring of delivered and stored milk temperature needs to be carried out. The storage silo will also require high/overfill level detection and low-level detection when emptying, to find a balance between wasting milk and allowing air to be pumped into processing lines.

Measurement of milk pH at the receiving stage also ensures that milk that has been spoiled in transit is not introduced into the process.

In storing the milk in the storage tank, it should be remembered that the temperature of the milk will probably have risen above 4°C and will need to be chilled and maintained at 4°C while in storage. The milk must also be gently agitated to prevent cream separation and so it is important that the agitator is only operated when the milk level is higher than the agitator blades.

Instruments for monitoring flow

As mentioned previously, the largest challenge in accurately measuring received milk quantity is the presence of bubbles. While a well-designed milk receiving process should eliminate most of the bubbles, it is also advantageous to choose a measurement technology that can compensate for the remaining entrained air.

In the case of measuring delivered milk quantity, the best type of instrument is a Coriolis flow meter, because it is capable of measuring the milk as a mass quantity and also measuring its density, which is directly related to its fat content (see below). Such a meter can therefore provide qualitative as well as quantitative information. Recently, dual-frequency Coriolis meters have been developed that are capable of greater accuracy by being able to compensate for bubbles.

Figure 2: Typical Coriolis flow meter for sanitary applications.

Instruments for monitoring level

There are two places in the milk receiving and storage process where level measurement is necessary. The first is in the air eliminator and the second is the raw milk tank itself.

The air eliminator, being essentially a smaller short-term storage vessel, is best equipped with a capacitance level probe instrument, since fast changes in temperature and pressure will not affect its accuracy and it offers a fast response time.

In the (often very large) milk storage tanks, a hydrostatic pressure instrument (Figure 3) provides best performance with high accuracy and stability. The instrument needs to be hermetically sealed and resistant to CIP washdown chemicals. An instrument available with remote electronics also helps alleviate issues with access to hard-to-reach areas.

Figure 3: A hydrostatic pressure instrument.

The final — and most important — type of level measurement is point level sensing, which has a number of applications, including:

• overspill protection in air eliminators
• overspill protection and filling pump control for storage tanks
• minimum level in air eliminators
• minimum level in storage tanks for pump regulation and agitator control.

Detecting spoilage

While the conditions under which the milk is stored and processed at a dairy can be well controlled, what happens to the milk at the farm and in transport to the dairy is not within the dairy’s control, and there are potential opportunities for the milk to begin to spoil before it reaches the dairy.

It is known that the spoilage of milk causes its pH to change.¹ The pH of unspoiled milk is approximately 6.7, and as the milk spoils it becomes more acidic as lactic acid is formed. In most dairies the measurement of the pH is a manual step that is labour-intensive and time-consuming, but automation of the process can be achieved with a suitable instrument.

Instruments for monitoring pH

One of the problems with pH monitoring is the need to clean the pH electrode, so the ideal choice in this instance is an automated self-cleaning pH system utilising an ISFET glass-free pH electrode that can be installed in the receiving line to monitor and record the pH of every batch of milk.

Technologies are also readily available to allow for the automation of the cleaning of the pH electrode. In order to access the electrode, a retractable sterile assembly must be used, which seals the hygienic process from the outside world as the electrode is extracted and inserted. Such assemblies can be manually operated or pneumatically driven for full automatic control. When used in conjunction with an automatic electrode cleaning system, such an assembly reduces the time and labour needed to maintain the pH measurement system.

Figure 4: A retractable sterile assembly for pH electrode extraction and insertion.

Minimising product loss

Due to the complexities of the dairy process, with the various processing steps (to be described below) and the many pumping and storage steps along the way, there is always the chance of product loss due to various changes and malfunctions that can occur in the system. The majority of milk losses in a dairy occur during the transfer of product from one production step to the next. For example, built-in safeguards designed to prevent pumps running dry can cause pumps to stop operating and milk to be dumped to a drain. As a result, interface measurements (milk/air or milk/water) are usually performed in the pipework between processing steps. These measurements are traditionally performed by timers that are triggered by a low limit switch — a method that is not precise and is subject to process issues.

Instruments for minimising product loss

Minimising product loss can be achieved by using instruments that incorporate a robust, hygienic and fast sensor that can quickly detect changes in the composition of the sample. For this application, the most suitable sensor is an optical sensor, since the measurement is instantaneous and therefore provides for real-time monitoring of what is flowing through the pipe. Ideally the sensor is located in the line as close as possible to the tank or process vessel that it is feeding into or before the transfer pump to protect against the pump running dry. As soon as an interface is detected, the sensor can send a signal to the control system.

Optical sensors typically suitable for this purpose utilise near-infrared or visible light to detect the product interfaces or suspended solids. A sensor with a glass-free hygienic design that can withstand high temperatures during CIP processes or in heat treatment phases is most appropriate, and needs to be coupled with a matching multiparameter process transmitter.

Figure 5: A glass-free NIR/VIS optical sensor suitable for interface detection between process steps.

Milk standardisation and heat treatment

Milk is mainly a suspension of various constituents in water. The relative amount of the natural constituents is variable due to the natural origin of the raw product and its dependence on natural biological variation between animals, as well as both seasonal and locational 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.

In its natural form, milk can also contain a range of pathogenic organisms that are harmful to humans, as well as other microorganisms that cause the milk to spoil and shorten its shelf life. Heat treatment is used to eliminate the harmful bacteria, and also to maximise shelf life of the final milk product.

In Part 2

The standardisation of milk and its heat treatment are the more complex areas of milk quality management and will be discussed in Part 2 of this article.

References

1. Lu M et al 2013, ‘Milk Spoilage: Methods and Practices of Detecting Milk Quality’, Food and Nutrition Sciences, vol. 4, no. 7A, pp. 113-123.

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