How Do Can Washers Work? A Detailed Overview

Every day, thousands of milk cans move through dairy plants – collecting raw milk at farms, transporting it to chilling centres, and eventually arriving at processing facilities. By the time a can reaches the plant, it carries more than just residual milk. It may harbour fats, proteins, mineral deposits, and microbial contaminants that, if left unchecked, can compromise the safety and quality of every subsequent batch. Can washers solve this problem systematically. These machines put milk cans through a defined sequence of cleaning and sterilization steps that leave them physically, chemically, and bacteriologically clean – and ready for reuse. Understanding how they work explains why they are indispensable to dairy hygiene.

Why milk cans need specialized cleaning 🔗

Milk is chemically complex. It contains fats, proteins, carbohydrates, and minerals, each of which behaves differently on equipment surfaces. Fats and proteins are a delicate substrate and protection for bacteria, and once milk residues are left on surfaces, microbial multiplication begins quickly. The situation worsens when deposits dry out. A white film formed on cold surfaces will dry and solidify if not cleaned immediately, making removal significantly harder.

Beyond organic residues, hard water introduces another problem. Calcium and magnesium in water react with milk proteins and fats to form milkstone – a stubborn, chalky deposit that standard rinsing cannot remove. Milkstone is a layer of scale mainly formed by cations like calcium and magnesium, and it can harbour bacteria within its porous structure. These characteristics make a simple rinse-and-dry approach completely inadequate for milk cans.

There are four levels of cleanliness that any dairy cleaning operation must address: physical cleanliness (removal of visible dirt), chemical cleanliness (elimination of microscopic residues detectable by taste or smell), bacteriological cleanliness (achieved by disinfection), and sterile cleanliness (destruction of all microorganisms, required for products like UHT milk). In dairy cleaning operations, the objective is nearly always to achieve both chemical and bacteriological cleanliness. Can washers are engineered to achieve all of these in a single, automated sequence.

The can washing process: stage by stage 🔗

Regardless of the type of can washer used, the cleaning sequence follows a consistent and scientifically established order. Each stage has a specific purpose, and skipping or shortchanging any step reduces the effectiveness of the entire process.

Stage 1: Draining milk residues 🔗

Before a can enters the washer, any remaining milk must be drained out. This is not merely a preparatory step – it directly affects cleaning efficiency downstream. Excessive residual milk increases the organic load on the detergent solution, reducing its effectiveness and shortening its useful life. It is necessary to remove residues from the production equipment to minimize the loss of products, help in the cleaning process, and reduce sewage produced. In automated can washers, cans are inverted or positioned at an angle during entry to allow complete drainage before the washing stages begin.

Stage 2: Pre-rinsing with water 🔗

The first active cleaning stage involves rinsing the can with water to flush out loose milk residues. Critically, this rinse must use cool or lukewarm water, not hot water. Organic soils consist of the major organic constituents of milk – fats, proteins, and sugars – and their adhesion to surfaces increases with time, dryness, and heating. Using hot water at this stage causes milk proteins to denature and bond firmly to the can surface, creating a layer that is far harder to remove in subsequent stages.

Around 90% of un-encrusted residues and almost 99% of the total residue can be effectively washed out in the pre-rinsing process. Pre-rinsing should continue until the water exiting the can runs clear. This stage greatly reduces the organic load before the detergent is applied, making the chemical wash more effective.

Stage 3: Detergent washing 🔗

This is the core cleaning stage. The can is subjected to a detergent solution – typically alkaline – that chemically breaks down the residues that water alone cannot remove. Alkaline cleaners dissolve milk fats, proteins, and carbohydrates, and loosen and suspend other soil particles so that they can be removed by mechanical action.

According to the Dairy Practices Council guidelines, can washers should be operated with a non-foaming alkaline detergent solution at 0.15% to 0.3% alkalinity. Milk cans typically clean effectively at 0.15% to 0.25% alkalinity, while cans used for cream or chocolate milk may require the higher end of this range. A meter-controlled proportioner helps maintain consistent solution strength in large-scale operations.

The detergent solution is recirculated and reused across multiple cans. However, it must be drained and replaced after every 150 cans (or at a frequency determined by practical observation), as the solution becomes progressively contaminated and loses its cleaning potency. Temperature is equally important: the wash solution should be maintained at an appropriate temperature to keep fats in suspension and support chemical activity without denaturing proteins prematurely.

Stage 4: Post-rinse with clean water 🔗

Once the detergent wash is complete, the can must be thoroughly rinsed to remove all traces of detergent. Any detergent left in the system after cleaning can contaminate the milk. This rinse is typically done with warm water. Where water hardness is a concern, softened water is preferable for this stage – hard water can deposit limescale on cleaned surfaces, partially undoing the work of the detergent wash. Softened water prevents the deposition of limescale on the cleaned surfaces.

In some operations, an acid rinse (using dilute phosphoric acid) is introduced at this stage to neutralize any residual alkalinity and dissolve mineral deposits that alkaline detergents cannot address. This step is particularly important in hard-water regions where milkstone buildup is a recurring problem.

Stage 5: Sterilization by steam or hot air 🔗

The final stage eliminates any microbial contaminants that survived the chemical wash. Dairy equipment can be disinfected by thermal disinfection (boiling water, hot water, steam) or chemical disinfection (chlorine, acids, iodophors, hydrogen peroxide, etc.) In can washers, steam is the most widely used method.

Two types of steam injection are commonly used: intermittent and continuous. The intermittent type delivers steam in pulses, which is more economical in steam consumption while still achieving effective sterilization. The continuous type maintains a constant steam flow for the duration of the sterilization period. Cans must be loaded into the sterilization zone before the steam valve is opened, and operators are required to use hand gloves to protect against burns. For products like UHT milk and sterile milk, sterilisation of equipment frees it from all bacteria that might be present.

Types of can washers and their working mechanisms 🔗

Can washers are available in several configurations, each suited to different scales of operation. All of them execute the same cleaning sequence but differ in how cans are fed through the machine and how much manual intervention is required.

Can scrubbers 🔗

The simplest form of can cleaning equipment, can scrubbers use rotating brushes to clean the interior and exterior surfaces of cans. They are manually operated and best suited to small dairy operations. While effective for physical cleaning, they typically require a separate sterilization step using a steam block. Their main advantage is low cost and ease of use.

Steam sterilization blocks (can steaming blocks) 🔗

These are standalone units used specifically for sterilizing cans after they have been manually washed. The can is placed over a steam outlet on the block, and steam is injected into the can for a set duration. They serve as a dedicated sterilization station and are often used alongside can scrubbers in smaller plants.

Rotary can washers 🔗

Rotary can washers automate the process by using a rotating mechanism to carry cans through successive cleaning stages. Cans are loaded onto the machine, which rotates them through pre-rinse, detergent wash, post-rinse, and steam sterilization zones in sequence. These machines handle a significantly higher throughput than manual methods, with consistent results across every can. They are well-suited to medium and large dairy plants. For heavily soiled or dented cans, a preliminary scrubbing in a can scrubber is recommended before loading into the rotary washer to prevent damage to the machine and ensure effective cleaning.

Straight-through can washers 🔗

In straight-through washers, cans move in a linear path along a conveyor through each cleaning stage. This design allows for very high throughput and is common in large-scale dairy processing plants where hundreds of cans must be cleaned per hour. The linear flow ensures that every can receives the same treatment duration at each stage. The development of automatic milking and cleaning systems have been great time-savers for both farms and processing plants, and straight-through washers represent the most advanced end of this spectrum for can cleaning specifically.

Key operational factors that determine cleaning effectiveness 🔗

Even with the right equipment, the quality of can washing depends on several variables being correctly managed. The British Columbia Centre for Disease Control’s guidelines for dairy plant cleaning identify seven key factors: solution temperature, duration of application, mechanical action, chemical concentration, soil solubility, water hardness, and other water impurities. These variables interact with each other, and an imbalance in one can reduce the effectiveness of the entire process.

Temperature is particularly critical. Fats are easier to remove above their melting point, while proteins must not be exposed to excessive heat during initial rinsing. Chemical concentration must be maintained within the specified range – too little and the detergent fails to break down residues; too much is wasteful and can leave residues on can surfaces. Water hardness directly affects detergent performance: in hard water, calcium and magnesium ions neutralize detergents and reduce their effectiveness. When water hardness exceeds 10 grains per gallon, it may be necessary to increase detergent concentration, and in very hard water, a water softener should be used.

Maintenance and quality assurance in can washing 🔗

A can washer that is not properly maintained will not clean effectively regardless of the chemicals used. Routine maintenance includes checking the effectiveness of cleaning after each washing cycle, changing worn-out brushes promptly, and inspecting the motor and power transmission system (including chains or belts) for proper function. The detergent solution must be monitored for concentration and replaced at the prescribed intervals.

Verification of cleaning effectiveness should go beyond visual inspection. Improper cleaning and sanitizing protocols have resulted in high costs, product spoilage, equipment damage, and hospitalization of personnel – a reminder that can washing is not a routine chore but a food safety-critical operation. Swab testing of cleaned surfaces, along with checks for detergent and sanitizer residues, provides objective data to confirm that the process is working as intended. Cleaning operations must be performed strictly according to a carefully worked-out procedure in order to attain the required degree of cleanliness, and the sequence must be exactly the same every time.

Why getting can washing right matters 🔗

Milk is among the most tightly regulated food products in the world. There is more legislation concerning milk – its production, handling, processing, packaging, storage and distribution – than any other food product. Can washers sit at the intersection of all these concerns. A can that is not properly cleaned reintroduces bacteria, residual fats, and mineral deposits into a fresh batch of milk, undermining pasteurization and shortening shelf life. By contrast, cans cleaned and sterilized through a properly operated can washer present a reliably safe, contamination-free surface for every new consignment of milk.

The process is not complicated in concept – drain, pre-rinse, wash, rinse, sterilize – but its success depends entirely on disciplined execution: the right temperatures, the right chemical concentrations, the right timing, and regular equipment maintenance. Chlorine is the most common chemical sanitizing agent used in the milk industry for the final disinfection phase when chemical sanitization is preferred over steam, further underlining how well-established and standardized these procedures are across the global dairy sector.

What do you think? Given that milk can washing involves such a precise sequence of steps, which stage do you think is the most likely to be compromised in a busy dairy plant – and what consequence would that have for milk safety? Is fully automated can washing always preferable over semi-automated systems, or are there situations where human oversight adds value?

References

1. https://dairydynamics.com/cleaning/
2. https://www.neologicengineers.com/blogs/how-to-clean-dairy-equipment
3. https://dairyprocessinghandbook.tetrapak.com/chapter/cleaning-dairy-equipment
4. https://www.thedairysite.com/articles/686/cleaning-and-sanitizing-milking-equipment
5. https://health.maryland.gov/phpa/OEHFP/OFPCHS/Milk/Shared%20Documents/DPC029_Cleaning_Sanitizing_Fluid_Milk_Plants.pdf
6. https://www.bccdc.ca/resource-gallery/Documents/Educational%20Materials/EH/FPS/Food/DairyProcessingCleaning.pdf
7. https://www.ndvsu.org/images/StudyMaterials/LPT/cleaning_and_sanitation_of_milk_plant.pdf