An effective plant-cleaning regime for the milking equipment is essential to maintain milk quality and reduce bacterial cross infection between cows. Effective cleaning and disinfection requires adequate volumes of hot water at a suitable temperature, with a suitable sanitising product, at the appropriate rate, circulated for the correct time. The routine needs to be monitored and any procedural drift identified and addressed.
Satisfactory plant sanitisation (a combination of cleaning and disinfecting) is essential after every milking to help control Bactoscans and to reduce the risk of cross infection of mastitis pathogens between cows from contaminated milking equipment. Effective cleaning requires a correctly designed and installed milking system, adequate volumes of water at an appropriate temperature, with cleaning chemicals added at the correct rate. It is also critical that a regular, monitored routine is followed by everyone involved in the plant-cleaning operation.
While effective plant sanitisation is an essential component of a Bactoscan control programme, effective cleaning is also essential to control levels of thermoduric bacteria. Thermodurics are heat-resistant bacteria that can survive the pasteurisation process and cause serious problems for milk processors, especially in the cheese-making process. Many milk processors now monitor thermoduric levels in milk routinely, and it seems likely that this will soon form part of some milk-supply contracts.
PRINCIPLES OF PLANT CLEANING
The vast majority of milking systems in the UK are sanitised after milking using a method described as circulation cleaning. This form of cleaning comprises three distinct phases:
1Pre-rinse– to remove milky residues
2Circulation– to remove fats, proteins and minerals
3Final rinse– to remove chemicals from the plant to prevent contamination of milk; certain disinfectants can be added to the final rinse
For the cleaning to be successful, the plant-cleaning solution needs to be distributed equally around the plant with all internal surfaces receiving the pre-rinse, circulation and final rinse.
The basic system of plant cleaning is the same regardless of type or design of milking parlour. While there are some specific issues with recorder-jar plants, as these installations are becoming less popular over time, this article focuses on milking pipeline plants.
Prior to cleaning
Prior to commencing the washing cycle, all external surfaces of the liners, clusters and long milk and pulse tubes should be cleaned thoroughly. Unfortunately, in many cases, this component of the cleaning regime consists of a cursory spray with a hose, which is unlikely to remove all external soiling. Once the clusters have been cleaned, they should be attached to the jetters. This transfers vacuum to the wash manifold and leads to wash solution being drawn into the plant. Any in-line mastitis detectors should be checked and cleaned prior to rinsing.
The jetters should also be cleaned as they may have become soiled during the previous milking and can introduce contamination into the solution (Fig. 1). Over time, studies have shown that the best method of cleaning clusters and jetter assemblies thoroughly is brushing clean with a warm/hot detergent solution.
OPERATOR-SUPERVISED CLEANING SYSTEMS
The following refers to plant cleaning overseen by the operator, i.e. not automatic plant cleaning systems, although the same principles apply.
Ideally warm water at 30–40°C should be rinsed through the milking system. This warm solution removes the majority of the residual milk in the plant and is pumped from the receiver vessel directly to waste. Some farmers use this first rinse water to ‘chase’ residual milk from the plant into the bulk milk tank. However, this is a high-risk strategy which can lead to contamination of the milk with water and ensuing problems with freezing point depressions (FPD).
Typically, 14 litres of rinse water is required for each milking unit to ensure adequate rinsing. The milk filter should be removed after this first rinse and replaced with a clean filter.
When hot water is limited, a cold rinse should be employed. If inadequate volumes of water are employed in the rinse phase, milking residues will remain in the plant. This can reduce the efficacy of the detergent-disinfectant product.
Allow a minimum of 14 litres of hot water (at 85°C) per cluster for the main wash – unless stated otherwise by the manufacturer of the detergent disinfectant. Fill the wash trough with the correct amount of hot water. Leave the delivery line out of the trough at this stage to allow approximately 4 l per unit discharge to waste.
When the discharge water has achieved a temperature of at least 55°C, place the delivery pipe into the trough and start circulation. The detergent-disinfectant should be added at this stage. Circulate the main wash water containing the detergent-disinfectant for 5–10 minutes. The temperature of the wash solution at the end of circulation should be at least 55°C. Where the circulation time of the solution exceeds 5–10 minutes, the temperature is at risk of falling below this target temperature.
Circulation should be followed by a final rinse of cold water. Typically 14 litres of water are required per unit. Depending on the quality of water provided (there is a legal requirement to use potable water), there may be a need to add sodium hypochlorite to the final rinse water at the rate of 25 ml per 40 litres. This final rinse water is discharged to waste. Many farms are now adding products based on peracetic acid to the final rinse as an alternative, although there is concern that the concentrations recommended may adversely affect plant components. Once the rinse is completed, the vacuum pumps should be switched off and the plant drained.
Milk contains fats, proteins, sugars and minerals. The water used for cleaning the milking machine can also contain varying levels of minerals, depending on the water hardness. Fats, proteins and minerals are deposited on the internal surfaces of the milking equipment.
Alkaline detergents break down fats and proteins from the milk and remove them from the smooth surfaces of the milking equipment. However, the alkaline conditions produced can encourage the deposit of milk-stone (protein-calcium complex), limescale and other mineral deposits. These deposits can only be removed by acid detergents.
Traditionally, alkali detergent-disinfectants were used for the majority of the hot circulation cleaning cycles. Periodically, the frequency depending on the hardness of the water, an acid milk-stone product was used as an alternative to the regular alkali product. These concentrated acidic milk-stone products (often based on phosphoric acid) tend to be extremely aggressive on rubber components in the plant. This has seen the emergence of cleaning systems which rely on the alternate use of acid and alkali. More frequent use of acid means that the acid concentrations can be reduced for each wash.
The alkaline components are typically combined with surfactants, water-hardness suspending agents and chlorine to produce a chlorinated alkaline cleaner. The surfactants assist the cleaning solution to penetrate and remove soils and milk-film components from internal surfaces, while the water-hardness control agents (phosphates and chelating compounds) keep minerals and soils in suspension and prevent them being re-deposited.
Alkaline detergents can come in either powder or liquid form. Powders are unsuitable for use in many automatic cleaning systems and they tend to be available in smaller pack sizes. For many producers with large parlours, the most cost-effective detergent-disinfectants are available in a liquid form. However, powdered alkaline detergents are often more effective, particularly in hard-water areas or where borehole water is used.
Detergent-disinfectant cleaning products are often based on sodium hydroxide (caustic soda) and are extremely aggressive if they come into contact with the skin or eyes. All cleaning chemicals must be treated with caution and appropriate safety equipment worn when handling and working with such chemicals. At a minimum, aprons, gloves and eye protection should be used.
Acid and alkali products should never be allowed to mix as this will generate chlorine gas, which is extremely harmful and can lead to serious injury and death.
DIRECT TO PIPELINE PLANTS
The majority of new milking parlours employ a stainless steel milk line. Not only does a closed pipeline make observation of cleaning more difficult, the bore of these lines poses a challenge to cleaning.
The diameter of milk lines typically ranges from 70–105 mm. It is not possible to wash all internal surfaces of a 70 mm line adequately simply by circulating a sanitising solution. The top of the milk line will not come into contact with the circulating solution, which can lead to the development of deposits on the upper surfaces (Fig. 2).
Parlours with milk lines with a diameter greater than 50 mm rely on the formation of a wash slug to scour around the plant and clean all the internal surfaces of the milk line. The wash slug is formed by the controlled admission of air (Fig. 3). It is crucial that the slug thus formed maintains its integrity along the complete length of the pipeline until it enters the receiver vessel. Without an injection of air, the wash solution will continue to flow along the base of the pipeline and the upper surfaces will remain un-cleaned.
If the duration of the injection of air is inadequate, the slug is not correctly formed. If the duration of air injection is excessive, the vacuum level in the plant will fall sharply, the wash slug will deteriorate and solution flow rates will fall, compromising the effectiveness of cleaning.
It is important that the slug moves at around 10 m/s. Manufacturers have developed various design features to ensure the correct formation of the slug, but it is important that the duration of the air injection and the interval between slugs are optimised.
There are two basic rules of thumb with air injection:
1The duration of the air injection should be equivalent to the period of time the wash slug takes to reach the receiver vessel. If the milk pipeline is 25 m long and the wash slug is moving at 10 m/s, the air injection duration should be 2.5 seconds.
2There should be at least 15 wash slugs per wash cycle. If the hot solution circulates for seven minutes, the interval between injections should be 28 seconds.
AUTOMATED PLANT CLEANING SYSTEMS
There are many automated systems available to clean the equipment after each milking. When the automated system is correctly set up and maintained, it can be highly effective. However, the automated systems should be checked regularly to ensure that the correct volumes of water and chemical are being used and that the temperatures are adequate. Many have a variation on the standard recommended washing routine, such as a cold followed by a second but warm/hot rinse, with some having two final rinses. The first is clean water to remove alkaline cleaning chemicals from the plant and the second comprises a peracetic acid disinfection rinse.
MONITORING PLANT CLEANING
Plant cleaning can be assessed at two levels.
At the most basic level, once circulation has been established, it is important to check that all clusters are receiving equal amounts of circulating solution. Touching the external surface of the milk line in a number of places should confirm solution is reaching all parts. It is also important to check that there is always sufficient water in the wash trough to ensure the pick-up pipes are submerged. If air is sucked into the pick-up pipe the flow of wash water is dramatically reduced and the temperature of wash solution also drops significantly. Listening for the operation of the air injector and feeling the receiver shake as a slug of wash solution hits the receiver is also essential.
Higher level cleaning check
At a slightly higher level, a basic cleaning check can be carried out. It is important that appropriate safety equipment is used for a cleaning check as the combination of hot water and aggressive chemicals can be dangerous. An apron, gloves and eye protection are the minimum requirements.
Measuring the volume of water at each stage of the wash is important. This can be done by catching the water in buckets at the end of each stage as it is pumped through the delivery line to the drain. This can prove challenging on many new installations as the delivery line is piped directly into the drain to contain the products.
The temperature of the water should be taken as it leaves the boiler, enters the plant, prior to commencement of circulation and at the completion of circulation.
Measuring the volume of chemical used will allow a check to be carried out on the concentration of the product. The container of each product should provide information on the appropriate concentration.
If the system employs an automatic cleaning system, it is likely that the concentrated product will be picked up by a peristaltic pump. Rather than try to catch the solution within a closed system, it is acceptable to fill a large jug with water, place the chemical pick-up pipe in the jug and measure the amount of water used. Cleaning a plant with hot water and no sanitising product once when chemical calibration is calculated will not have an adverse effect on milk quality.
As with the basic check, once circulation is established, a visual inspection should be made of solution distribution and operation of air injection.
Effective plant cleaning is an essential component of a milk-quality programme. Poorly cleaned milking equipment can lead to elevated Bactoscans, increased thermoduric counts and an increased risk of cross contamination, particularly in situations where mastitic cows are separated and milked last.
The principles of cleaning are quite basic but it is important that a routine is developed and followed and that all people associated with the operation take responsibility for monitoring effectiveness, identifying problems and correcting deficiencies.
CONTINUING PROFESSIONAL DEVELOPMENT
In order to test your understanding of this article, answer these multiple choice questions, or if you are a subscriber, go online at http://www.ukvet.co.uk, and find many more multiple choice questions to test your understanding.
1How much hot water is required for circulation cleaning a 36 point herringbone parlour:a. 405 litresb. 504 litresc. 600 litres
2What is the minimum target temperature at the completion of circulation cleaning:a. 55°Cb. 40°Cc. 60°C
c. What type of product is most effective at removing protein films:a. Acidb. Milk-stone removerc. Alkali