Neutral detergent fibre values from two sources, Van Soest and Mahoney, show interlaboratory variation. It should be noted that assays were performed on different food samples. However, inter-laboratory differences between different food samples are less than those observed between different analytic methods. Such differences point out the need for suitable standard reference materials that can be distributed to laboratories as part of the quality-control process. The effects of processing may significantly alter the nutrient content of foods.
Processing includes harvesting, mechanical and heat treating, packaging, and storage procedures.
Processing food products together also alters the nutrient content of the products. For example, deep frying potatoes in vegetable oil increases the fat content of the product as eaten. Table 5 shows the effects of boiling on selected cruciferous vegetables. Although calories remain constant, vitamins A and C decrease with cooking. Cruciferous vegetables have characteristics as a group that appear to be protective for some types of cancer, including bowel cancer.