Researchers in Sweden have developed a method of freezing fresh vegetables that preserves their firmness and taste after defrosting. The team at Lund University says their method could allow farmers to freeze their produce for sale all year round, while producing crisp and tasty pre-frozen salads every time. Jim Drury went to find out more.
When frozen, then defrosted, spinach leaves wilt like the one on the right, into a lifeless, unappetising mulch. But Lund University scientists say they have the answer. The sturdy leaf on the left was injected with trehalose, a substance found naturally in many species of fungi and grass that helps them to survive cold winters. SOUNDBITE (English) EDA DEMIR, RESEARCHER FOR OPTIFREEZE, SAYING: "Trehalose is a sugar, it's a natural sugar, it's like five times less sweet than sucrose, and it is actually protecting the cells against freezing." And now, it's working to protect common vegetables as well. First, researcher Eda Demir dunks individual leaves in trehalose-based solution for treatment in a vacuum machine. Air is then removed from the plant tissue, and replaced by the trehalose, according to Associate Professor Federico Gomez. SOUNDBITE (English) FEDERICO GOMEZ, ASSOCIATE PROFESSOR OF FOOD TECHNOLOGY AT LUND UNIVERSITY, SAYING: "This vacuum is forcing the air out of the tissue. Once the air has been exhausted under pressure then we come back to the atmospheric pressure.....then the surrounding liquid will be impregnated, will be replacing the air that was exhausted in the structure of the tissue." Next, electric pulses are applied, to penetrate the outer cell membrane and achieve cryoprotection against cold-induced damage. SOUNDBITE (English) FEDERICO GOMEZ, ASSOCIATE PROFESSOR OF FOOD TECHNOLOGY AT LUND UNIVERSITY, SAYING: "Cryoprotection can be possible only if these substances are also inside the cell. So we use electric pulses because electric pulses will open pores inside the cells and then these pores are kind of gates that allow this solution to get also inside.... The key feature of the method is that we keep the cells alive after thawing. It means that all the fresh-like characteristics will be intact in the leaf." The treated vegetables are then stored in the freezer. Once defrosted, the researchers say they look and taste as though they've just been harvested. So far they've tested vegetables frozen for up to a month. As well as spinach, they've successfully treated parsnips and strawberries. Potatoes were less successful, because of their tissue density. The team says their method could allow farmers to freeze large quantities of vegetables for sale year round. Optifreeze, the group's start-up firm, hope to scale up production methods, so that trehalose-treated frozen vegetables available for sale within a year.