Minerals and bulk nutrients like total fat, protein and carbohydrates are rather stable food components, but many phytochemicals are quite sensitive to harvest and post-harvest sample handling before the food sample is actually being analysed. Fresh, unprocessed plant food materials are living organisms: various stress-related enzymatic and chemical reactions may occur in response to cutting (= wounding), cooling, heating or drying. Some of these reactions can be very fast and detectable within a few seconds, e.g. oxidation of ascorbate and unsaturated fatty acids.
Textbox 4.1: Example of a high-Fe report in a tropical cereal crop (Abebe et al. 2007), which was mainly due to the adhering Fe from the soil.
For analysis of reliable nutrient levels in food, these undesirable reactions need to be prevented as much and as quickly as possible. Especially leafy vegetables can easily lose water by evaporation, and thereby cell integrity, after removing leaves from the plant of by cutting the entire shoot from their root system. The best is freezing food samples in liquid nitrogen directly or at least as quickly as possible after their harvest. Foodstuff growing in or in contact with soil, like potatoes and carrots but also vegetables or cereals (Textbox 4.1), needs to be washed firstly to remove contamination from adhering soil. Perhaps some products may need to be peeled as well, depending which plant tissue needs to be analysed. Large food products, e.g. tomatoes, potatoes, cucumber, bell peppers etcetera, should be quickly cut into workable pieces before freezing in liquid nitrogen, as complete fruits or large pieces are hard to handle subsequently. Once frozen in liquid nitrogen, the sample can be stored at -20°C or for a longer time at -80°C in ultralow freezers. Alternatively, the frozen food may be freeze-dried. It is key that a water-containing food sample stays well frozen all times, also during freeze-drying. After completely freeze-drying, the material can be stored at room temperature but always in the dark and at low humidity, preferably in a desiccator. However, some loss of nutrients may occur upon freeze-drying, depending upon food matrix and specific nutrient. Note: freeze-drying cannot be applied to materials which also needs analysis of volatile phytochemicals, e.g. for determining flavour (aroma) compounds.
Both frozen and freeze-dried material should be ground into a fine and homogenous powder, in order to be able to take representative aliquots and for an efficient extraction of the nutrients. Frozen samples can be manually powdered using a liquid nitrogen-cooled pestle and mortar. However, this frequently is though work and time-consuming, and thus rather impractical in cases of large series of samples. Dedicated food grinders exist that can be pre-cooled with liquid nitrogen and enable quick and efficient grinding. Again, it is essential that frozen material should never be allowed to thaw and always kept well-frozen during the entire grinding procedure; especially thin leafy food materials, like spinach, lettuce and endive, can thaw rapidly if not sufficiently cooled. After grinding, the powder obtained should be stored frozen, preferably at -80oC. Likewise, freeze-dried materials can be powdered either manually or by using grinders; the freeze-dried powder can again be stored at room temperature, dry and dark.
Drying food in an air flow, either at room temperature or with heating, is detrimental to most phytochemicals and thus should never be applied for their analysis if the air-drying effect has not been investigated previously. The effect of drying can be determined by comparing frozen, freeze-dried and air-dried samples of the same original plant material. In contrast, air-drying does not affect the levels of minerals.
Abebe Y, Bogale A, Hambidge KM, Stoecker BJ, Bailey K, Gibson RS. 2007. Phytate, zinc, iron and calcium content of selected raw and prepared foods consumed in rural Sidama, Southern Ethiopia, and implications for bioavailability. Journal of Food Composition and Analysis 20: 161–168. DOI: 10.1016/j.jfca.2006.09.003.