Enzyme Technology
Enzymes in the fruit juice, wine, brewing and distilling
industries
One of the major problems in the preparation of fruit
juices and wine is cloudiness due primarily to the presence of pectins. These
consist primarily of a-1,4-anhydrogalacturonic acid polymers, with varying
degrees of methyl esterification. They are associated with other plant polymers
and, after homogenisation, with the cell debris. The cloudiness that they cause
is difficult to remove except by enzymic hydrolysis. Such treatment also has the
additional benefits of reducing the solution viscosity, increasing the volume of
juice produced (e.g., the yield of juice from white grapes can be raised by 15%),
subtle but generally beneficial changes in the flavour and, in the case of
wine-making, shorter fermentation times. Insoluble plant material is easily
removed by filtration, or settling and decantation, once the stabilising effect
of the pectins on the colloidal haze has been removed.
Commercial pectolytic enzyme preparations are produced
from Aspergillus niger and consist of a synergistic mixture of enzymes:
- polygalacturonase (EC 3.2.1.15), responsible for the random hydrolysis of
1,4-a-D-galactosiduronic linkages;
- pectinesterase (EC 3.2.1.11), which
releases methanol from the pectyl methyl esters, a necessary stage before the
polygalacturonase can act fully (the increase in the methanol content of such
treated juice is generally less than the natural concentrations and poses no
health risk);
- pectin lyase (EC 4.2.2.10), which cleaves the pectin, by an
elimination reaction releasing oligosaccharides with non-reducing terminal
4-deoxymethyl-a-D-galact-4-enuronosyl residues, without the necessity of pectin
methyl esterase action; and
- hemicellulase (a mixture of hydrolytic enzymes
including: xylan endo-1,3-b-xylosidase, EC 3.2.1.32; xylan 1,4-b-xylosidase, EC
3.2.1.37; and a-L-arabinofuranosidase, EC 3.2.1.55), strictly not a pectinase
but its adventitious presence is encouraged in order to reduce hemicellulose
levels.
The optimal activity of these enzymes is at a pH between 4 and 5 and
generally below 50°C. They are suitable for direct addition to the fruit pulps
at levels around 20 U L−1 (net activity). Enzymes with improved characteristics
of greater heat stability and lower pH optimum are currently being sought.
In brewing, barley malt supplies the major proportion of
the enzyme needed for saccharification prior to fermentation. Often other starch
containing material (adjuncts) are used to increase the fermentable sugar and
reduce the relative costs of the fermentation. Although malt enzyme may also be
used to hydrolyse these adjuncts, for maximum economic return extra enzymes are
added to achieve their rapid saccharification. It not necessary nor desirable to
saccharify the starch totally, as non-fermentable dextrins are needed to give
the drink 'body' and stabilise its foam 'head'. For this reason the
saccharification process is stopped, by boiling the 'wort', after about 75% of
the starch has been converted into fermentable sugar.
The enzymes used in brewing are needed for
saccharification of starch (bacterial and fungal a-amylases), breakdown of
barley b-1,4- and b-1,3- linked glucan (b-glucanase) and hydrolysis of protein
(neutral protease) to increase the (later) fermentation rate, particularly in
the production of high-gravity beer, where extra protein is added. Cellulases
are also occasionally used, particularly where wheat is used as adjunct but also
to help breakdown the barley b-glucans. Due to the extreme heat stability of the
B. amyloliquefaciens a-amylase, where this is used the wort must be boiled for a
much longer period (e.g., 30 min) to inactivate it prior to fermentation. Papain
is used in the later post-fermentation stages of beer-making to prevent the
occurrence of protein- and tannin-containing 'chill-haze' otherwise formed on
cooling the beer.
Recently, 'light' beers, of lower calorific content, have
become more popular. These require a higher degree of saccharification at lower
starch concentrations to reduce the alcohol and total solids contents of the
beer. This may be achieved by the use of glucoamylase and/or fungal a-amylase
during the fermentation.
A great variety of carbohydrate sources are used world
wide to produce distilled alcoholic drinks. Many of these contain sufficient
quantities of fermentable sugar (e.g., rum from molasses and brandy from grapes),
others contain mainly starch and must be saccharified before use (e.g., whiskey
from barley malt, corn or rye). In the distilling industry, saccharification
continues throughout the fermentation period. In some cases (e.g., Scotch malt
whisky manufacture uses barley malt exclusively) the enzymes are naturally
present but in others (e.g., grain spirits production) the more heat-stable
bacterial a-amylases may be used in the saccharification.
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This page was established in 2004 and last updated by Martin
Chaplin on
6 August, 2014
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