STUDIES ON POLYPHENOL OXIDIZING ENZYMES IN ODOUR GENERATION IN PEARL MILLET MEAL
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Date
1998
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MPKV, UNIVERSITY LIBRARY RAHURI
Abstract
The present investigations were undertaken to study
the role of seed phenolics and polyphenol oxidizing enzymes
such as polyphenol oxidase (PPO) and peroxidase (POD) in the
generation of acidic, mousy odour in pearl millet meal and to
develop suitable seed treatments to improve the shelf-life of
its meal. The pearl millet seeds were obtained from the
Millet Breeder of this University and used for various
experiments.
The characteristic odour of the pearl millet meal was
found to be associated with water - soluble fraction of
methanolic extract from both whole or defatted meals. This
indicated that the phenolic compounds are responsible for the
odour of the meal. Studies on the PPO and POD activities of
pearl millet seeds indicated that these enzymes oxidize a
polyphenolic substrate such as pyrogallol more efficiently
than the monophenolic or diphenolic substrates. The optimum
pH was found to be between 7 and 8 while the optimum
temperature for maximum activity was observed to be 50°C for
PPO and 25 to 30°C for POD activities. During germination of
seeds, the POD activity was found to increase rapidly while
PPO activity remained almost unchanged. These results
indicated that the POD seem to be actively involved in
metabolism of phenolic compounds during germination.
Most of the fat acidity, water-soluble phenolics and
POD activity were found to be concentrated in the germ
fraction of the seed while the PPO activity was observed to
be uniformly distributed in the endosperm and the germ. The
meal of germ fraction showed extreme mousy odour even after
defatting treatment while the endosperm meal exhibited normal
fresh odour. These results indicated that the germ fraction
with water-soluble phenolics and POD activity present in it
are mainly responsible for the undesirable odour. This was
further confirmed by sequential extraction of lipids,
phenolics or both of these constituents from the meal and
testing these meals for odour generation. The meal extracted
with hexane alone produced strong undesirable odour while the
meal extracted with methanol did not produce any off odour.
The defatting treatment of the meal removed only lipids but
the contents of water-soluble phenolics, PPO or POD activity
remained unchanged. The methanol extraction of meal lowered
both water-soluble phenolics and POD activity but not the PPO
activityor total lipids. Generation of a strong undesirable
odour both in whole and defatted meals and absence of any
such odour in methanol extracted meal confirmed the
hypothesis that water-soluble phenolics and the POD activity
are involved in odour generation.
The PPO activity of pearl millet was found to be
relatively heat stable while the POD was observed to be heat
sensitive. A hot water blanching of seeds at 98*C for 40 sec
or steam heating of seeds at 15 psi for 5 min was found to
eliminate most of the POD activity in the seeds. The meals
obtained from such heat treated seeds did not develop
undesirable odour even by odour acceleration treatment during
storage upto 10 days. These results indicated that a simple
heat treatments such as hot water blanching or steam heating
of seeds may be very efficient in destroying the odour
generating reactions and improve the shelf-life of pearl
millet meal. Analysis of several promising pearl millet
hybrids indicated a wide genetic variation for the POD
activity (108 to 426 units/g/min). The cultivars with lower
POD activity and water-soluble phenolics may be desirable in
this context.
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