B.B. Mishra

Free phenolics and polyphenol oxidase (PPO): the factors affecting post-cut browning in eggplant (Solanum melongena).

Polyphenol oxidase (PPO) catalyses oxidation of phenolics, which results in instant but differential browning in many cut fruits and vegetables, including eggplant. Eight cultivars of eggplant were characterised by their PPO specific activity, phenolic content, browning index, and PPO polymorphism. In fresh eggplant, browning was found to be dependent on both the phenolic content and PPO specific activity, whereas, total phenolic content played a major role in browning of stored fruits. Interestingly, although browning index increased in stored eggplant fruits, PPO activity reduced in four out of eight cultivars studied. Phenolic level was found to increase in all these cultivars during storage. Although a significant level of homology was observed in PPO nucleotide and conceptually translated protein sequence, two cultivars, which displayed highest PPO specific activity, differed in the 38 amino acid stretch in the peptide region 301-338.

Purification and characterisation of polyphenol oxidase (PPO) from eggplant (Solanum melongena)

Eggplant (Solanum melongena) is a very rich source of polyphenol oxidase (PPO), which negatively affects its quality upon cutting and postharvest processing due to enzymatic browning. PPO inhibitors, from natural or synthetic sources, are used to tackle this problem. One isoform of PPO was 259-fold purified using standard chromatographic procedures. The PPO was found to be a 112 kDa homodimer. The enzyme showed very low K(m) (0.34 mM) and high catalytic efficiency (3.3×10(6)) with 4-methyl catechol. The substrate specificity was in the order: 4-methyl catechol>tert-butylcatechol>dihydrocaffeic acid>pyrocatechol. Cysteine hydrochloride, potassium metabilsulphite, ascorbic acid, erythorbic acid, resorcylic acid and kojic acid showed competitive inhibition, whereas, citric acid and sodium azide showed mixed inhibition of PPO activity. Cysteine hydrochloride was found to be an excellent inhibitor with the low inhibitor constant of 1.8 μM.

Shelf Life Extension of Sugarcane Juice Using Preservatives and Gamma Radiation Processing

Preserving raw sugarcane juice is a challenging problem. Sugarcane juice turns brown soon after its extraction and gets spoiled due to fermentation within hours. A combination of gamma radiation (5 kGy) with permitted preservatives and low temperature storage (10 °C) could preserve raw sugarcane juice for more than a month. The preservatives used were citric acid (0.3%), sodium benzoate (0.015%), potassium sorbate (0.025%), and sucrose (10%). The treatment helped in extending the shelf life to 15 d at ambient temperature (26 ± 2 °C) and 35 d at 10 °C. The microbial load was found to be below detectable limit within this period. The biochemicals like phenolics and flavonoids were not found to be affected by addition of these preservatives. The antioxidant activities including free radical scavenging activity, nitrite scavenging activity, and reducing power were also not significantly affected. The sensory evaluation scores showed that the juice with this combination treatment was highly acceptable.

Polyphonel Oxidases: Biochemical and Molecular Characterization, Distribution, Role and its Control

The polyphenol oxidase (PPO) catalyzes oxidation of phenolics and responsible for enzymatic browning in plant products, seafood, and melanin formation in skin. The enzyme has been reported to be universally distributed in animals, plants, fungi and bacteria. It has received significant attention from researchers due to its involvement in the post-processing enzymatic browning i.e. a major problem for various food industries particularly dealing with cut fruits and vegetables and juices. The enzymes purified form different plant sources showed differences in the biochemical characteristics. The PPO protein structure has been solved and reported in few plants. The enzyme is a copper protein. The actice site of the enzyme undergoes transitions among met-, oxy-, and deoxy-forms in a cyclic manner for catalysis. The gene sequences from different sources show homology among close family members and introns are peculiary found to be absent in many cases. Various physical, chemical, and genetic methods have been proposed for inhibiting or preventing PPO activity for the control of undesirable enzyme activity, and also to achieve disease resitance in certain cases. The current review aims to consolidate the understandings about the PPO enzyme and possible means of its control.