Home > Applications of water in the dairy

Water as an ingredient:

Water is used extensively as an ingredient in the dairy industry. For example, most largescale production sites will use milk or whey powder as an ingredient that is brought in, rather than having one large site that processes liquid milk, and then uses it in a variety of applications. A typical example of this approach would be in yoghurt manufacturing. Much of the work relating to this is dealt with elsewhere in this publication, and therefore it will not be reviewed in this chapter, but it is worth emphasizing that any water that is reused must be to a high quality, and should be checked on a regular basis.

Water as a cooling agent:

Most dairy products – apart from cheese – are classed as short life, and therefore will need to be kept cool after processing to prevent the build-up of micro-organisms. Some dairies will use ammonia or other gases as a refrigerant; however, the other common process used is a cooling tower. The tower is usually constructed as a loop system (Figure 3.3), where water will be used as coolant within the factory, for example as the cooling fluid in a plate evaporator. After use, it passes out of the factory, either directly or indirectly, to the cooling tower; this cools the water down before it is reused in the factory or, in older systems, before it is discharged from the dairy.

Because of the heat of the water coming out of the evaporator, some of the water evaporates to the atmosphere, and this is normally topped up from mains water. The efficiency of this system is measured in terms of cycles: this is based on the amount of water in the system, and how often it is totally replaced. In this method of water cooling, the desired aim is a total of six cycles, but the amount will vary depending on the season: in winter the efficiency tends to be greater owing to the colder ambient climate.

Because of the evaporation and aerosols generated in this system, chemical treatment is normally applied (e.g., biocide) to help prevent bacterial build-up: in the worst cases, if not treated correctly, this can be seen as slime. More importantly, chemical treatment is used to control viruses such as Legionella spp. Legionella was fi rs-identified in 1976 in an outbreak in Philadelphia in the USA amongst delegates attending an American Legion convention – hence the name. Surprisingly, there is no legal requirement in the USA to take measures to control Legionella, but most of Europe has adopted the UK government’s guidelines, which are published by the Health & Safety Commission (Anonymous, 2000). These guidelines, termed ‘L8’, outline the requirements for all water systems used in a dairy – not only the cooling systems, but also hot and cold services, such as showers and sinks – along with guidelines as to the frequency of monitoring and cleaning of water systems. They are important guidelines: as the attached report shows from an outbreak in the UK, the consequences of Legionella infection can be quite tragic.

Because of the water evaporation from the cooling tower, the concentration of solids in the tower may increase, and sometimes, because of the absorption of atmospheric carbon dioxide (CO2), these solids can become acidic. Therefore, it is sometimes beneficial to add other chemicals as well as biocides to help control corrosion and scale. To ensure that the system is working efficiently, it should be monitored regularly; simple tests, such as conductivity of water, give a rapid assessment of the build-up of solids. Total bacterial count (TBC) using a dip slide is also a good indicator, although with this method of analysis there is a time delay (two days at room temperature).

Water in heating applications:

Water for boilers

As with cooling water, water designated for boilers needs to be treated to prevent damage to the system from scaling and corrosion. In a boiler, the treated water is converted to steam, which can then be carried around the factory for a variety of applications, such as office heating, use as heat transfer medium in a plate heat exchanger or evaporator, or to maintain heat in a jacketed vessel, such as a cheese vat.

Boilers come in a variety of sizes, depending on the demand for steam in the dairy – small dairies may even use a steam generator – but the basic principle is the same in all cases. When the water is converted to steam and is taken off as vapour, the water left behind in the boiler becomes more concentrated, because the solids in the water are heavier, and therefore are not evaporated as vapour. On most modern boilers, the level of total dissolved solids (TDS) is monitored automatically: once the TDS reaches a predetermined figure, the boiler automatically goes into a programme called blow-down. This removes the heavier solids in the water, and the boiler is then refreshed with additional water, either from condensate return (that is, returned boiler water from the factory) or from fresh water.

Water for use in boiler applications needs to be pretreated to remove as much of the solids and dissolved oxygen as possible. Normally the solids are removed by a softener unit. The cation exchanger, regenerated with sodium chloride (salt), removes the calcium and magnesium hardness from the water. If borehole water or rainwater is being used, it might be necessary to remove metals, such as iron and manganese. In most cases demineralisation of water is used, particularly in high-pressure boilers; this also helps to remove silica, which is found in nearly all natural water. Finally, it is important to remove any gases that might be present, particularly oxygen, in order to prevent corrosion of metal equipment. In most cases this can be done by the addition of chemicals known as oxygen scavengers. Alternatively, de-aeration of water might also be employed.

Having taken a lot out of the water, it is then conditioned by the addition of other chemicals, such as phosphates or polyphosphates. These compounds react with the alkalinity of the boiler water to form heavy insoluble compounds, which can be taken out in the blow-down procedure. Natural and synthetic polymers might also be used as they help to increase the dispersion of the phosphates when applied. Furthermore, in some cases anti-foam might also be used to prevent the carry-over of fi ne water into the steam.

Regular checks have to be made, at least daily, on a boiler to ensure the water quality is maintained. Normally, the TDS and alkalinity are checked daily to ensure that the water quality is maintained: this provides an early indication if there are any problems in areas such as the chemical addition or automatic TDS blow-down valves, thus preventing scale or corrosion.

Water as condensate return

When the steam vapour has been around the dairy, if it does not pass out into the atmosphere it can be returned to the boiler house: this is termed condensate return, and it is estimated that steam gives up 610–670 kcal kg−1 of water as it condenses. However, it is still valuable and, if possible and practical, it should be reused not only for its heat value, but also because it does not need too much treatment as it will be free from the contaminants that are found in raw water. Another source of condensate in the dairy industry is evaporators: in a larger system, such as a seven-stage evaporator, it is estimated that as much as 12 kg of water can be evaporated from milk for every 1 kg of steam used; the concentration of skimmed milk solids in such an evaporator rises from 8–10 g 100 g−1 to 50 g 100 g−1. Such condensate would require some form of treatment, normally to correct the, and this could be achieved by introducing a neutralising or film-forming amine. These types of product would also help to protect the pipework and tanks used to transfer and hold the condensate return.

Water for general use:

Throughout the dairy, there will be many areas where water is used so routinely that it is almost unnoticed, such as wash stations prior to entering the process areas, toilets, and canteens. Many factories will operate control systems to flush areas such as toilet bowls and urinals; however, it is estimated that for every person on site the average consumption of water per day associated with the site services will be in the region of 10 L. This can increase dramatically if taps are left running, or pipework is leaking. For example, if a tap is left dripping and loses water at 1 L min−1, this equates to 1.4 tonnes of water per day.

Water for cleaning purposes:

Water is the commonest form of cleaning material used in the dairy industry. From open plant cleaning to cleaning-in-place (CIP), the main medium for carrying the chemicals used will be water. Most chemicals are used at concentrations between 1 and 5 g or 1 and 5 mL per 100 mL, and therefore the rest of the make-up will be from water. The principles of CIP are reviewed in Chapters 1 and 2. As with water for general use, water in these areas can easily be lost, despite sites having trigger hoses and automated dilution systems. Most people can recall walking through a dairy where either hose pipes are running at full bore so the floor ‘looks nice and shiny’, or the workers, knowing a visit is due, have disappeared for another break and have tied the trigger hose so that it is running full bore into the drain.

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