| Bananas are one of the highest yield fruits and one of the most important food sources for many developing countries in the world. During preservation and processing, they easily get browned due to the banana polyphenol oxidase (PPO) which catalyzes the banana polyphenols to be brown. These reactions severely affect the nutrients and flavor of the bananas. We researched the substrates, extraction methods, properties and the catalytic browning mechanism of the banana polyphenol oxidase, in order to find methods to control the enzymatic browning. The main study results are shown as follows:1. The total polyphenol content of banana is36.0±1.9mg catechin equivalents/100g fresh weight banana detected with Folin-Ciocalteu reagent. The polyphenolic contents include dopamine10.6±1.4mg/100g FW, chlorogenic acid9.5±0.1mg/100g FW,(+)-catechin10.6±0.2mg/100g FW, and (-)-epicatechin0.7±0.1mg/100g FW, which were qualitatively and quantitatively analyzed by HPLC.2. The optimal extraction conditions for banana polyphenol oxidase are source of buffer solution (phosphoric acid-citric acid) without potassium chloride, pH (5.58), liquid:solid ratio (1.57), PVPP concentration (1.13%), time (2.5min), through the response surface methodology.3. The special substrate of banana PPO is dopamine, which was determined by the velocity and the color change of the browning reaction. The enzymatic browning on dopamine can be divided into5stages. The initial velocity was higher, and then turned to almost zero in each stage. The colors of the browning products of each stage are orange, reddish brown, violet black, gray, black successively. The experimental data showed that there must be a synergistic reaction which could promote the browning reaction between the polyphenols or the browning products in bananas. When the ammonium sulfate saturation was less than30%, it could remove something and lower the PPO activity or browning reaction in the extraction. 4. Two kinds of isoenzyme are found in banana PPO, whose molecular weights are b=60,000+1,000Da, c=42,000+2,000Da respectively. They have similar properties, like pH stability, optimum temperature and thermal stability, except the optimum pH.Their optimum pH is5.4and6.8respectively. The enzyme activity is strongly stable at pH5.0-9.0, during which the activity could be kept more than70%of the initial one. The banana PPO is strong in alkali-resistance, but weak in acid-resistance.The optimum temperature for banana PPO is30℃. Below60℃, the enzyme activity can still remain at more than50%of the maximum enzyme activity. When the temperature was above80℃, the enzyme activity fell to10%. If the banana PPO was processed over3s,5min,100min under100℃,90℃,80℃respectively, the PPO activity would be completely inhibited.5. When the concentration of ascorbic acid, citric acid, sodium bisulfite reached0.3mg/mL,2mg/mL,0.3mg/mL respectively, they can completely inhibit the banana PPO activity separately.6. Base on the scanning method, the banana juice browning can be detected acutely, effectively and rapidly. The methods of inhibition on banana juice browning are followed:When the concentration of ascorbic acid in banana juice reached0.4mg/mL, it can effectively control the banana juice browning within200min. Ascorbic acid’s browning inhibition property is was reversible at low concentrations, but when its concentration reache over0.8mg/mL, it can form a stable irreversible inhibition to browning. The ascorbic acid can make the brightness of the banana juice to be beyond the initial brightness of the sample, due to its bleaching effects.When the concentration of citric acid was2.5mg/mL, banana juice pH reached3.0, and the banana juice browning was inhibited effectively. Sodium bisulfite can effectively restrain the browning when its concentration is greater than0.4mg/mL. Sodium bisulfite also has the bleaching property during the process... |
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