Wet and Dry Methods of Cleaning in Food Processing

Before any raw food material reaches a processing line, it must go through a critical step – cleaning. Raw agricultural produce arrives at food processing facilities carrying soil, stones, metal fragments, plant debris, pesticide residues, and microbial contaminants. Removing these unwanted materials is essential for food safety, equipment protection, and final product quality. Two broad approaches are used to get this done: wet cleaning and dry cleaning. Each has a distinct set of techniques, and the choice between them depends on the nature of the food product, the type of contamination, and practical factors like water availability.

Why cleaning is the first line of defence in food processing 🔗

Cleaning in food processing is essentially a separation process. It exploits differences in physical properties – such as size, density, shape, or magnetic behaviour – between the food and its contaminants. According to the University of Florida’s IFAS Extension, the objective of cleaning food contact surfaces and raw materials is to remove substances that support bacterial growth and to eliminate contaminants before they enter the processing chain. A poorly cleaned raw material can damage expensive machinery (think of a stone entering a roller mill), compromise consumer safety, or degrade the sensory quality of the finished product.

In practice, no single cleaning method removes every type of contaminant. Most facilities use a combination of wet and dry techniques, applied in sequence, to progressively clean the raw material to an acceptable standard. The choice of methods must also balance cleaning cost against product quality, especially for delicate materials like soft fruits that can be damaged by aggressive handling.

Wet cleaning methods 🔗

Wet cleaning uses water – sometimes combined with detergents or sanitisers – to wash away dirt, chemical residues, and microorganisms from food surfaces. It is highly effective because water can reach crevices, dissolve soluble contaminants, and physically dislodge particles that stick firmly to food surfaces. However, it also generates large volumes of wastewater and is unsuitable for products that deteriorate when exposed to moisture.

Below are the main wet cleaning techniques used in food processing.

Soaking 🔗

Soaking is among the simplest wet cleaning methods. The food product is immersed in a tank of water for a set period, allowing dirt and adhered soil to loosen and dissolve. It is commonly used as a preliminary step before more intensive cleaning. Root vegetables like potatoes and carrots, which carry heavy soil loads, often go through a soaking stage first. The limitation of soaking is that it can be slow and the soak water quickly becomes contaminated, requiring frequent replacement.

Spray washing 🔗

In spray washing, high-pressure water jets are directed at the product as it moves along a conveyor belt or rotates inside a drum. This technique effectively removes surface dirt, dust, and even pesticide residues from fruits and vegetables. Spray belt washers and rotary drum washers are two common equipment configurations. Spray washing uses water more efficiently than soaking because the mechanical force of the jets does much of the cleaning work, reducing the contact time needed.

Flotation washing 🔗

Flotation washing works on the principle that contaminants and food pieces differ in density. When produce is placed in a water tank, lighter contaminants float to the surface and are skimmed off, while heavier debris like stones and soil settle at the bottom. Sound, good-quality fruit tends to float, whereas bruised or rotten pieces sink and can be removed from the base of the tank. According to a resource on unit operations in food processing, flotation washers are particularly effective for cleaning peas, beans, dried fruits, and similar materials – with water requirements of approximately 4,000 to 10,000 litres per tonne of raw material.

A specialised variant called froth flotation has been used to separate beans from weed seeds. The beans are dipped in a dilute oil-detergent emulsion, air is blown through the mixture, and the contaminants are carried away by the foam that forms on top. The cleaned beans then receive a final spray wash to remove emulsion residues.

Ultrasonic cleaning 🔗

Ultrasonic cleaning uses high-frequency sound waves (typically above 20 kHz) transmitted through water to create tiny bubbles that collapse rapidly – a process called cavitation. The energy released during cavitation dislodges even microscopic particles from food surfaces. This method is especially useful when conventional washing cannot reach surface irregularities or when very thorough cleaning of delicate items is required.

Advantages and limitations of wet cleaning 🔗

Wet methods excel at removing firmly attached soils and a wide range of contaminants, including chemical residues and microorganisms. They also allow the use of detergents and sanitisers for an added level of disinfection. On the downside, wet cleaning consumes large quantities of clean water and generates significant effluent – sometimes as much as 15,000 litres per tonne of canned food, as noted in food engineering literature. If the wash water is not managed properly, it can itself become a source of cross-contamination. Additionally, certain products – grains, spices, powdered ingredients – cannot tolerate moisture without suffering quality loss.

Dry cleaning methods 🔗

Dry cleaning relies on mechanical and physical separation techniques rather than water. It exploits differences in size, shape, density, aerodynamic behaviour, and magnetic properties between the food product and its contaminants. As noted in a guide published by the CEO Water Mandate, replacing wet processes with dry, mechanical processes is a recognised good practice for reducing water use in food manufacturing.

Dry methods are essential for products that are sensitive to moisture – such as flour, cereals, spices, dried fruits, and nuts – or in regions where water conservation is a priority. Here are the key dry cleaning techniques.

Screening (sieving) 🔗

Screening is the most widely used dry cleaning method. It involves passing raw material over perforated beds or wire mesh screens of specific hole sizes. Contaminants larger than the food particles (such as straw, twigs, or pods) are retained on the screen surface – a process called scalping. Conversely, smaller contaminants like sand, dust, or broken fragments fall through the mesh while the good product is retained.

Screens come in several configurations. Flat-bed vibrating screens shake the material across a horizontal or slightly inclined surface. Rotary drum screens (trommels) tumble the material inside a rotating cylindrical mesh. Multiple screens with decreasing mesh sizes can be arranged in series to remove contaminants of different sizes in one pass. Screening is commonly the first cleaning step for grains, pulses, nuts, and seeds.

Aspiration (air classification) 🔗

Aspiration uses a controlled stream of air to separate particles based on differences in their aerodynamic properties – primarily weight and surface area. When a mixture is fed into an aspiration channel, lighter particles such as dust, chaff, husks, and damaged or shrivelled grains are carried upward and away by the air current, while the heavier, denser product falls downward. According to JK Machinery, particles with a lower buoyancy speed than the main crop are effectively sucked out by the air flow in aspiration channels.

Aspiration is frequently combined with screening in grain cleaning equipment. Many commercial grain cleaners include both sieve decks and aspiration channels in a single machine, making the process more efficient.

Magnetic separation 🔗

Magnetic separation specifically targets ferrous (iron-containing) metal contaminants – fragments from harvesting machinery, transport equipment, or preliminary handling operations. These metallic particles pose a hazard both to the consumer and to processing machinery such as cereal mills and grinders.

The setup can vary: food may pass over magnetised drums, travel along magnetised conveyor belts, or flow through magnetic grate separators positioned in chutes and pipelines. Modern separators use powerful rare-earth neodymium magnets that can capture even very fine ferrous particles. Electromagnets offer the added convenience of easy cleaning – simply switching off the power releases the captured metal. Magnetic separation is standard practice in flour milling, cereal processing, and spice manufacturing.

Abrasion cleaning 🔗

Abrasion cleaning removes surface contaminants by rubbing food pieces against each other or against a rough surface. For example, root vegetables can be cleaned by tumbling them in a rotating drum lined with an abrasive material, which scrubs off the outer skin and attached soil. This method works well when the surface dirt is firmly adhered and sieving or air cleaning alone would be ineffective. However, it must be carefully controlled to avoid damaging the product.

Electrostatic separation 🔗

Electrostatic separation exploits differences in electrical surface charge between food particles and contaminants. Under controlled humidity conditions, different materials acquire different charges when passed over a charged surface. Charged contaminants are attracted to an oppositely charged electrode or grounded surface and removed from the product stream. This method is effective for removing fine dust and has been applied in specialised food processing operations, including tea cleaning.

Advantages and limitations of dry cleaning 🔗

Dry cleaning methods conserve water, generate no liquid effluent, and are ideal for moisture-sensitive products. They also carry no risk of waterborne cross-contamination. A Klipspringer guide on dry cleaning in food factories highlights that introducing water into environments handling dry ingredients like flour or spices can create serious food safety risks, including bacterial proliferation and compromised product quality. However, dry methods are generally less effective at removing surface-adhered chemical residues or microorganisms compared to wet cleaning. Some techniques – particularly magnetic and electrostatic separation – also require specialised, costly equipment. Dust generation is another concern that often requires additional control measures like enclosed systems or dust extraction.

Choosing between wet and dry cleaning 🔗

The decision is not always either/or. In many processing facilities, both methods are used in combination to achieve the best results. Several factors guide the choice:

Moisture tolerance of the product: Fruits and vegetables generally tolerate water well and benefit from wet cleaning. Grains, flour, spices, and dried products should be cleaned using dry methods to preserve their quality and shelf life.

Type of contamination: Firmly adhered soil, pesticide residues, and microbial loads are best removed by wet methods. Loose physical contaminants like stones, chaff, metal fragments, and dust respond well to dry separation techniques.

Water availability and cost: In water-scarce regions or facilities aiming to reduce their environmental footprint, dry methods are strongly preferred. The California Department of Water Resources specifically recommends switching to dry, mechanical cleaning processes where feasible to improve water efficiency in food manufacturing.

Product size and shape: Larger, irregularly shaped products often benefit from the thoroughness of wet cleaning. Smaller, uniformly shaped products – like seeds and grains – can be efficiently cleaned with screens and air classifiers.

Equipment and cost considerations: Wet cleaning systems require water treatment infrastructure and wastewater management. Dry systems may need dust control equipment. The capital and operating costs of each approach should be weighed against the value and volume of the product being processed.

Practical considerations for effective cleaning 🔗

Regardless of the method chosen, a few principles apply universally. Cleaning should be performed as early as possible in the processing chain to prevent contaminants from causing damage downstream. The process should be designed to avoid recontamination – for example, ensuring that dust blown off in an aspiration system does not settle back onto cleaned product. After wet cleaning, dewatering is essential. Excess surface moisture can be removed by passing the product over vibratory screens or rotary drainers before the next processing step.

It is also important to recognise that complete cleaning is rarely achievable in one step. A balanced, multi-stage approach – using a sequence of complementary techniques – typically delivers the best outcome while keeping costs manageable.

A quick comparison: wet vs. dry cleaning 🔗

Wet cleaning uses water, detergents, and physical force to remove contaminants. Techniques include soaking, spray washing, flotation, and ultrasonic cleaning. It is thorough and versatile but water-intensive and unsuitable for moisture-sensitive products.

Dry cleaning relies on physical and mechanical separation – screening, aspiration, magnetic separation, abrasion, and electrostatic methods. It conserves water and protects dry products but may struggle with firmly adhered or chemical contaminants.

The best results come from understanding the specific needs of each product and applying the right combination of methods accordingly.

What do you think? Considering the growing emphasis on water conservation globally, how might food processing industries shift their cleaning strategies in the coming years? And for products like fresh-cut salads where both safety and moisture sensitivity matter, what would an ideal cleaning process look like?

References

1. https://ask.ifas.ufl.edu/publication/FS077
2. https://www.slideshare.net/mohitjindal/unit-operation-in-food-processing
3. https://ceowatermandate.org/resources/good-practice-food-manufacturing-mechanical-processes/
4. https://jk-machinery.com/edu_process/cleaning-and-sorting/
5. https://www.eclipsemagnetics.com/products/magnetic-separation-and-metal-detection/magnetic-separation/
6. https://www.food-safety.com/articles/6309-the-importance-of-cleaning-for-food-safety
7. https://www.klipspringer.com/blogs/dry-cleaning-in-food-factories-everything-you-need-to-know/