Food Safety Focus (222nd Issue, January 2025) – Article 2
Vegetable Washing Machines and Produce Washes – Are they Better Choices to Keep Food Safe?
Reported by Mr. Kenneth YIP, Scientific Officer,
Risk Communication Section, Centre for Food Safety
Fresh fruits and vegetables are important parts of a balanced diet. They provide a variety of vitamins and minerals to support health. Washing the produce thoroughly is essential, regardless of whether it will be used for culinary preparation or direct consumption. This article will brief the need of washing produce, discuss some principles and limitations of vegetable washing machines, and explain whether produce washes are preferable to washing fruits and vegetables.
Why should Produce be Washed Before Consumption?
Most fruits and vegetables are grown on farmlands. The produce may be contaminated by bacteria such as Escherichia coli O157:H7 or infested by pests like aphids from surroundings during the growing stage. Besides, farmers may apply chemicals, including pesticides to secure crop yield and quality, and these residues may be left in the produce. Foreign substances including soil particles, animal droppings, and weeds may also be introduced when the produce is collected.
Washing fruits and vegetables can help remove the undissolved physical matter and water-soluble chemicals. Rinsing produce before peeling can also reduce the dirt and bacteria transferring from the knife onto the unwashed fruit or vegetable. Although washing cannot eliminate microbes on the produce, thorough rinsing of fresh produce is effective in reducing microorganisms on their surfaces.
Figure 1: Points to note when using running water to wash fresh fruits and vegetables
Produce Washing by Machines - Technologies adopted
To reduce manual work and expedite the cleaning process, vegetable washing machines adopting different technologies have been developed. One of the popular methods is using water currents for washing. The machines usually contain rotation chambers to generate waves of water to rub on the surfaces of fruits and vegetables, whereas undissolved foreign substances and water-soluble compounds will be removed.
Another cleaning technique adopted by vegetable washing machines is the use of ultrasonic sound waves. By transferring ultrasonic sound waves with frequencies higher than the human ear to liquid media like water, it creates microbubbles that clean food surfaces. The bubbles eventually burst and create impact forces to collide with foreign substances on the produce surface.
Some vegetable washing machines use in-situ water purification technology such as hydroxyl radical water treatment to clean the produce. Because hydroxyl radicals have strong oxidizing capabilities, they can break down organic contaminants through REDOX and free radical chain reactions, which makes disinfection easier.
Figure 2: Some examples of vegetables washing machines
Limitations behind Automatic Produce Washing
While different produce washing machines are equipped with specialised mechanisms, they may have their respective limitations. Mechanical washing is similar to manual washing. Nevertheless, some models store the produce in a closed system that may lack sufficient fresh water supply for transferring unwanted substances out of the containers, in contrast to manual washing under running water. Study findings revealed that a high water flow rate (8 L/min) for 20 seconds is necessary to facilitate significant reductions of total aerobic count and Enterobacteriaceae. The chemicals and microbes that remain in the water may re-contaminate the produce after washing.
For machines utilising ultrasound technology, using improper frequencies may damage tissues and shorten the shelf-lives of the produce. The bursting of unstable bubbles may generate shearing forces on the produce, thus causing damage of plant cells and leakage of cellular contents. These damages could lead to tissue softening and nutrient loss.
Water treatment devices for purifying the water used for produce washing may face capacity issues. It is because only limited amounts of water could be passed into the machine for treatment. A longer time may be required for completion of washing in containers with large water volumes. Also, it may not be efficient to remove sticky substances adhered to the produce surfaces.
Produce Washes – Good for Enhancing Washing Performance?
Some may opt to wash fresh fruits and vegetables with produce washes like special detergents in addition to running water, in the hope that the cleansing work can be improved. However, the produce is usually porous and could absorb the chemicals in the products, which may be difficult to remove completely and remain in the fruits and vegetables as residues after the washing process.
The public is advised to wash fresh fruits and vegetables thoroughly without using soaps, special detergents, or produce washes in the household setting.