32-3 |
Freshness parameters of vegetables |
H. STEINHART, Institut für Biochemie und Lebensmittelchemie, Univ. Hamburg, Grindelallee 117, Hamburg, 20146, Germany Food is called fresh if it is brought directly after production into commerce for consumption. Fresh foods should not differ essentially from the same food directly after harvest and should have obtained no treatment for longer shelf life. Freshness of vegetables represents a quality aspect that is dependent on the time as well as temperature. Loss of fresh character expresses itself in changes of sensory qualities like taste, smell, color and consistency and in nutrient content. After harvest, physiological processes occur in the vegetable like respiration, ripening, perspiration and ageing. Ageing processes include water loss, oxidation, enzyme reactions and sensitivity to putrefaction-bacteria and moulds. These processes lead to fading of the vegetable color, the loss of aromas and other content materials and to softening by the decomposition of the cell wall-substances with enzyme release contributing to oxidative decomposition. As a result of oxidation, the loss of vitamin C is considerable during storage. The loss of vitamin C content is 20 to 40% with green peas, green beans and broccoli, and 90% with spinach within three days at room temperature (RT). Therefore, the current content of vitamin C is a possible freshness parameter. It is observed that other vitamins behave in storage analogously to vitamin C; e.g. losses of folic acid at RT from 50 to 70% with most types of vegetable. However, storage at 4 - 6 °C prevents the losses almost entirely. The reduction of nitrate to nitrite also is an enzyme-supported oxidation or decomposition reaction. All types of vegetables pick up nitrate for the synthesis of proteins where enzymatic reduction to nitrite or hydroxylamine follows. For this reason it is possible that for spinach that the nitrite content may be a negative indicator of freshness. Spinach has a high sensory fresh attribute with absence of nitrite.
Session 32, Refreshing the fresh concept
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