CHARACTERISTICS OF WHEAT AND RYE BRAN FRACTIONS OBTAINED BY DEBRANNING

Debranning, an essential technological step for covered cereals, is used in wheat milling as an additional operation for removing the outer bran layers. Furthermore, debranning by-products (DBP) hold great promise as novel food ingredients with physicochemical and nutritional properties that differ from those of traditionally available milling fractions (Dexter and Wood, 1996). On the basis of the interesting results obtained with the debranning of common wheat (Triticum aestivum L.) (Bottega et al., 2009), this work aimed at thoroughly evaluating the characteristics of the bran tissues sequentially removed from common wheat and rye (Secale cereale L.) by using debranning. Pre-hydration of kernels, keeping wet grains continuously mixing, applying short and repeated processing passages and using superabrasive elements covered with fine particles were the technological parameters used in this research. The starch losses in the DBP collected at each step and their amount, expressed as debranning level (DL), were evaluated. The abrasive effects on each DBP were observed by Fluorescence Microscopy and quantified by evaluating several chemical and physical indices. Both for wheat and rye, particular attention was paid to the fiber content of DBP. The bran fractions collected from wheat had a significantly lower starch loss than that obtained for rye. In particular, DLs lower than 12% were associated with a starch content of DPB lower than 10% db, values comparable to the quantity of starch lost in bran in a conventional milling process (Saunders, 1980). The higher the DL, the higher the starch content of the DBP, indicating the removal of endosperm tissues by debranning. At 25% of DL (observed only for rye), the starch amount present in the DBP was higher than 45% db, suggesting that the fiber component is strongly "diluted" from excessive endosperm abrasion (Pagani et al., 2002). The fiber content of DBP samples collected from wheat was higher than those obtained for rye. In fact, about 80% of the fraction removed during the first passage of wheat debranning is made up of fiber, and the soluble fractions represent only a small percentage (1.3÷2.4% db). As expected, the total fiber content decreases significantly as the value of DL, due to the accumulation of other constituents in the material removed (Laca et al., 2006). Nevertheless, at the same time, there is a gradual increase in the soluble fiber fraction. As for the rye, the DPB presented the lowest fiber content, although the ratio of soluble fiber : insoluble fiber was significantly higher than that observed for wheat. This result is related to the presence, in the outer rye regions, of cell-walls thick and rich in beta-glucans and arabinoxylans (Vinkx and Delcour, 1996).