If lettuce is packaged when still too moist, it can lead to spoilage. Through a new process, in use in Switzerland, involving an air drying tunnel for processing, the shelf life of prepared salads can be extended. When drying lettuce, people immediately think of centrifuging. This method is really adequate for less sensitive products, provided the right techniques are used. But the market is increasingly asking for tender leafy vegetables for immediate consumption. Similarly, a low water content is desired to enhance the shelf life of the product. These requirements can not always be met by the centrifuges that are commercially used today.
Sormac air drying tunnel
For the drying of washed lettuce, Sormac BV in Venlo NL has developed an air-drying tunnel that extracts water from the product using air currents. Produce can be processed without any damage and a water content of less than 2% is possible. The result is an attractive, fresh-looking product that is longer lasting due to its low water content, without any blemishes such as they can occur during centrifugation.
Reduction of humidity
Before the product enters the drying tunnel, pre-drainage takes place on a dewatering belt. In the drying tunnel itself, the product is dried in an upwardly directed air stream at temperatures between 25° and 30°C. The selected air speed must keep the product in suspension in order to achieve optimum moisture reduction. The circulating air flow is cooled in a heat exchanger to about 5°C. Condensation on the cold fins of the heat exchanger removes part of the water vapour from the air flow. For a renewed absorption of moisture, the air is then heated in a second heat exchanger to a temperature of 25° to 30°C, whereby the relative humidity is reduced.
During the drying of the lettuce, the air cools due to the evaporation of the surface water on the product. This effect keeps the product much cooler than the drying air. The product dried this way, in the drying tunnel, looks fresh and appealing. There is no condensation in the packaging in the supermarket cooler. The product is in a better state microbiologically and has a longer shelf life.
Market leader with tradition
Schwab-Guillod, the third generation of Seeländer family business, is one of the leading Swiss companies in the marketing of fruits and vegetables. Its main activity involves the processing and marketing of the full range of fresh and convenience products. The full range is available all over Switzerland throughout the year and approximately 600 customers daily are serviced, using its own vehicle fleet. The company now has an operating area of 87,000 m2, divided into product-specific climatic zones with a closed cold chain.