Cleaning lettuce and vegetables using ultrasound saves resources

By the time lettuce makes it to a customer's plates, it has often gone on a long journey with many washing cycles. This usage of drinking water though is neither environmentally friendly nor sustainable. The joint project "MultiVegiClean" is developing a cleaning facility which cleans heads of lettuce with the lowest possible use of water and at the same time lowering bacterial exposure, by using ultrasound and pulsing streams of water.

Freshly harvested salads are either going to retail ready-to-eat or as a raw product. Raw products always have to be washed and sorted before eating. Pluck salads such as lamb's lettuce, arugula and such, in particular, are washed after being cut in the fields to clean off dirt and lice. After being washed, they are packed into trays or bags. The lettuce is cleaned in two steps. In the first step, dirt is loosened and cleaned off in a pool, working similarly to a whirlpool. In the second step, the lettuce is then buzzed off with clean water again. Then it is put into its packaging. There is a note on the packaging, informing the consumer that the lettuce has to be washed and sorted once again before consumption.

Since the water used for washing is not entirely free of bacteria, there is a possibility of an increase of bacteria for prepared as well as raw products throughout the full supply chain. This can be contained within limits if the cold chain is constantly upheld from the point of packaging at the producers' facilities, via the transport and in the stores. At the point of sale though, the salads are often not refrigerated and bacteria can multiply quickly. In a worst case scenario, this could lead to diseases. The most efficient way to minimize the spread of bacteria is a thorough cleansing of the product after the harvest and again right before consumption. This causes an immense use of drinking water, which makes this practice neither environmentally friendly nor sustainable. This is the reason why "MultiVegiClean" is trying to find a way to clean salads more efficiently by using ultrasound, which would result in significantly lower water usage and better cleaning.

The Pfalzmarkt eG is one of the biggest producer organisations for vegetables and is therefore very interested to be able to provide a technology like this to their members. The less bacteria gets into the packaging, the longer the lettuce stays fresh, it tastes better and there are fewer complaints. Therefore the efficient cleansing of salads has direct economic benefits.

The main idea of this efficient process is the usage of ultrasound. This is a widely known practice for cleaning industrial components with liquid cleaning media. The main cleaning mechanism is a deliberate generation of acoustic cavitation, hence the development and the dynamics of gas bubbles in the liquid. The sudden implosion of these bubbles is producing microjets, which remove the dirt. Depending on frequency and intensity of the ultrasound, the cleansing effect can be regulated. This mechanism can be used in idle as well as laminar flowing cleaning media.

Another cleaning method is based on the generation of modulated water jets. To achieve this, a continuous water jet is combined with ultrasound. This is causing periodically disturbed fluid structures, which imply a bundled force pulse on the surface that needs to be cleaned. This method is already being used for high-pressure water jet cleaning. When it comes to organic products, this method can be used with lower pressure which is still efficiently cleaning the foods. The components that are used to create the ultrasound are developed by Weber Ultrasonics AG, one of the European market leaders of industrial ultrasound technologies.

Aside from the changes to the cleaning processes, other important factors are the design and the construction of the cleaning tanks with the necessary conveyor technology are built at the German Institute for Food Technologies e.V. (DIL). The DIL can use their long time experience in the field of plant development in the sector of food production and processing. This way, an ideal handling of the lettuce heads in the basins is implemented so by using a defined control system, other kinds of vegetables can be cleaned in the same tank as well.

In addition to the technical realisation of the new cleaning process and the overall facility, the analysis and evaluation of the outcome is invaluable. This is another part of the project which is taken care of by the DIL. Experts from the field of food physics are examining the influence the ultrasound treatment has on the lettuce, to rule out changes in structure in the leaves. This should reduce losses of quality in the new system. The same goes for the microbiological examinations of the heads, in order to compare the new system to traditional ways of cleaning. By specifically analyzing the findings, the necessary intensity and duration of the cleaning process can be determined. This will in turn make sure the least possible amount of water is used to achieve the most bacteria free environment with stable or improved product quality.

The joint project MultiVegiClean, for the "Development of a resource-saving lettuce and vegetable cleaning system" is supported by the Ministry for Education and Research as part of the funding initiative "KMU-innovativ" within the "Research for Sustainable Development"-program. 

Dr. Thomas Dreyer