Grain-associated xylanases: occurrence, variability, and implications for cereal processing

Xylanases (EC 3.2.1.8) hydrolyse the backbone of cereal cell wall arabinoxylans and often have a significant impact on cereal-based processes and end-products. The use of microbial xylanases as processing aids in this respect is well established and has been extensively studied. Much less research has focused on inherently present cereal-associated xylanases and their possible impact. Cereals produce xylanases for remodeling and expansion of cereal cell walls during normal cell growth and for more drastic cell wall degradation during seed germination. Besides these endogenous xylanases, cereals also contain microbial xylanases from micro-organisms populating the outer grain kernels layers. Unfortunately, these microbial xylanases are often inhibited by wheat proteinaceous xylanase inhibitors and they hence escape standard xylanase activity measurements. It is more correct to refer to these activity levels as 'apparent' xylanase activity levels. As a result, the occurrence of cereal-associated xylanases might have been largely underestimated in the past and hence unjustly been disregarded. The levels and the types of cereal-associated xylanases differ strongly between grain species, varieties, and tissues, and are largely affected by grain growing conditions. These variations in the levels of grain-associated xylanase activity affect several cereal-based food and feed applications. This paper provides an overview of the occurrence and variability of cereal-associated xylanases and of their potential impact on bread making, shelf life of refrigerated doughs, brewing, animal feed efficiency, pasta production, and wheat gluten-starch separation.