Phytochemical variation during blueberry juice processing : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology at Massey University, New Zealand
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Date
2015
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Massey University
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Abstract
Blueberry is regarded as a ‘super fruit’ by many consumers and believed to offer
health benefits for humans. It is well known for its high antioxidant levels and for
the diversity of its anthocyanins. Blueberries can be eaten fresh but are very
perishable, so are commonly kept frozen and available all year round. Frozen
blueberries are suitable for a range of products including juice. During juicing, there
are likely to be changes in phytochemical constituents arising from the various
processing steps. These changes lead to variable composition of the finished juice
and uncertain impacts on the ‘health value’ of the product. Therefore, this study
focused on evaluating three major phytochemicals (anthocyanins, chlorogenic acid
(CGA), and procyanidin B2) throughout juice processing in order to model
compositional change.
Blueberry juice processing involves a series of unit operations: thawing, blanching,
mincing, enzyme treatment, separation of juice from pomace, pasteurisation, and
bottling. Enzymatic degradation occurs during thawing of blueberries as they still
contain ‘live’ oxidases. Prolonged thawing at warm temperatures would therefore be
particularly bad for phytochemical degradation. If these oxidases are destroyed by
blanching, thermal degradation also occurs but was found to be less aggressive than
polyphenoloxidase (PPO) activity. Blanching at high temperature (= 70 °C) for
3 min eliminated PPO and significantly increased the phytochemical concentration in
the juice but it induced pectin gel formation which reduced juice recovery.
Depectinisation is essential after berry blanching to dissolve pectin gel and to avoid
juice volume penalty. Significant losses of phytochemicals were also observed
during pressing of the berries into juice, due to physical associations between the
phytochemicals and the berry matrix, and entrapment. Blanching at 90 °C for 3 min
followed by pectinase enzyme treatment at 50 °C for 2 h was the best way to deliver
high phytochemical concentration in the juice with high juice volume recovery and
acceptable viscosity. There is a risk that juices with high phytochemical
concentration will seem bitter or astringent. This was found not to be the case in
sensory trials, with consumers consistently preferring the high-phytochemical juices;
it seems sugars in the juice masked any adverse perceptions.
Because of the complexity of blueberry juice processing, the processing model
developed in this study was simplified into three components: a defrost model, a
recovery model and a thermal model. In short, the defrost model was used for the
whole berry phase during thawing when PPO was still active; the recovery model
accounted for losses into the pomace; and the thermal model covered the subsequent
liquid phase. These processing models were able to predict anthocyanin and CGA
changes throughout processing (particularly in blanched products) but procyanidin
B2 behaviour was not predictable.
This modelling approach provides the ability to predict variations in composition
arising from changes in the juicing process and offers manufacturers the opportunity
to produce consistent blueberry juice with a high phytochemical concentration.
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Keywords
Blueberries, Fruit juices, Analysis