The Enzymology of Cell Wall Breakdown During Malting and Mashing: An Overview
Peer-Reviewed Paper
MBAA TQ doi:10.1094/TQ-47-1-0309-01 |
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C. W. Bamforth. Department of Food Science &
Technology, University of California, Davis, CA.
Abstract
Enzymolysis of the cell walls of the starchy endosperm of barley can be
considered a two-stage process: solubilization and polymer digestion.
Solubilase enzymes include not only carboxypeptidase acting as an
esterase, but also other enzymes, notably an early-developing xylanase. It
is supposed that pentosan is primarily located on the outside of the cell
wall. Nonetheless, pentosan is less efficiently degraded than beta-glucan,
probably because the chief xylanases are blocked by an endogenous
inhibitor. Although there can be very efficient depolymerization of
beta-glucan, it appears that degradation is incomplete and that the
primary hydrolysis products are beta-linked oligosaccharides. This is
likely because the exo-glucanases that hydrolyze them develop late in
germination and also have a very low affinity for their substrates. The
advantage of this for the brewer is that beer can be legitimately claimed
as a good source of putative prebiotics.
Keywords: arabinoxylan, cell wall, enzymes, fiber, beta-glucan,
prebiotics
S�ntesis
La enzim�lisis de las paredes de las c�lulas del endospermo
feculento de la cebada puede ser considerada un proceso de dos etapas: la
solubilizaci�n y la digesti�n polim�rica. Enzimas solubilasas no solo son
las carboxipeptidasas fungiendo de esterasas, sino tambi�n otras enzimas,
particularmente una xilanasa de desarrollo precoz. Si bien se supone que
el pentosano se encuentra en la parte externa de la pared de la c�lula, el
hecho es que este se degrada m�s lentamente que el beta-glucano,
posiblemente porque las xilanasas principales son bloqueadas por un
inhibidor endovenoso. A pesar de que pueda haber una depolimerizaci�n muy
eficiente de beta-glucano, pareciera que su degradaci�n es incompleta y
que sus productos primarios de la hidr�lisis son oligosac�ridos con lazos
beta. Esto probablemente se debe a que las exo-glucanasas que los
hidroliza se desarrollan tard�amente durante la germinaci�n y tambi�n
porque tienen una muy baja afinidad por sus sustratos. Esto permite decir
leg�timamente que la cerveza es una buena fuente de prebi�ticos putativos.
Palabras
claves: arabioxilano, fibra, beta-glucano, pared de c�lula,
prebi�ticos
