| Summary: | 碩士 === 國立臺灣大學 === 食品科技研究所 === 94 === Cloudy juices, particularly from tropical fruits, are becoming a fast
growing sector of the juice industry. Guava (Psidium guajava L.) juice is an
important part of this sector, and claims a favored place over the other
tropical fruits, by virtue of its high content of vitamin C, its delicious taste,
its distinctive aroma and flavor. A main problem encountered in cloudy juice
production is the maintenance of a stable cloud. Polysaccharides are
commonly used in fruit juice industry to fulfill this goal.
The objective of this study is an attempt to stabilize guava juice by mean
of γ-polyglutamic acid (γ-PGA), a biodegradable polymer that is produced
by Bacillus subtilis. It is water-soluble, polyanionic, biodegradable, and
edible. Its high electronegativity gives it the potential use for cloudy juice
stabilization.
The high molecular weight sodium form (HMS, 841 kDa), the low
molecular weight sodium form (LMS, 326 kDa) and the high molecular
weight calcium form (HMC, 841 kDa) were used in this study at
concentrations ranging from 0.05-0.20% and fixed pH 3.7.
The results showed that all the γ-PGA tested could increase the stability
of guava juice cloud according to their thickening capacity. At all
concentrations, HMS appears as the best candidate in stabilizing juice cloud,
followed by HMC and LMS. However, LMS depicted lesser particle
aggregation after integrated light scattering measurements and HMC
depicted the highest value of particle aggregation, due presumably to the
interaction between pectin and calcium ions.
The results also suggested that not only the viscosity, but also the ζ-
potential influences the cloud stability in guava juice when various
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concentrations of different types of γ-PGA are added. HMS showed the
highest ζ-potential thus, forming the most stable system, followed by HMS.
HMC showed a ζ-potential pattern different from that of the other types of γ-
PGA. The increase concentration of HMC decreases the ζ-potential and
therefore, the stability, as showed by the highest shift of the particle sizes.
Moreover, HMS retains more protein and total phenol in guava juice fine
cloudy phase. However, all types of γ-PGA affected juice cloud Hunter’s L,
a, b color parameters by small decreasing. The increase in concentration
intensified the decrease in the values measured.
The γ-PGAs used in this study were compared to carboxymethylcellulose
(CMC) for the effectiveness to stabilize cloudy guava juice. At a
concentration of 0.05%, juice added with CMC showed the highest viscosity,
cloud stability, total phenol and highest color values.
This study permitted to get more insights in the potential application of γ-
PGA as cloud stabilizer in fruit juice industry. Like CMC, γ-PGA seems to
hold its stabilizing effect by both its viscosity enhancing capacity and its
high electronegativity.
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