enzymatic browning, the rate was measured under the influence of a known concentrations of acid(0. 1 HCl) and base (NaOH). The resultant effect of these concentrations was then measured over an experimentally determined duration. The results observed were noted down and conclusions drawn with reference to the level of inactivation of the enzymatic reactions. Phenyphenol oxidase is the main causative agent of enzymatic browning. It is usually catalyzed by phenolic compounds and quinones. Consequently non enzymatic browning processes involve polymerization reactions that lead to the formation of melanins; determinants of fruit color.
In this experimental set up fresh apple fruits were evaluated for the rates of browning when subjected to acidic or basic medium at physiologically standard room temperature. The rates of browning was found to posses a direct correlation with the polyphenol oxidase activity. Generally fruits that were subjected to a basic medium underwent the process of browning under the shortest duration as compared to the acidic medium that was demonstrated to considerably slow down the process of enzymatic browning. Analysis
Control of phenol oxidase activity by subjecting the fruit to different pH values was done by subjecting the fruit to an acidic solution(HCl) and a basic solution(NaOH), The temperature and time of exposure to these physiological conditions was also taken into consideration. HCl considerably caused inhibition of the enzyme hence reducing its rate of activity. This translated to a longer duration for enzymatic browning . Extrapolation of the resultant effects of different concentrations of HCl acid means that as the concentration of the acid increases, enzymatic activity is considerably lowered at optimum temperature.
Further increase in the concentration of the acid will lead to complete inactivation of enzymatic activity . However, this would negatively impact on the taste of the fruit. Inhibition of PPO is possible because acidic compounds have the capacity to complex with copper, an cation on the enzyme active site. Base on the other hand, create a physiological condition were the pH moves towards the optimal pH for polyphenol oxidase activity(5-7) hence speeding up the process of enzymatic browning as observed in the experimental set up.
Discussion The four fundamental attributes of appearance, flavor, texture and nutritional value are the core elements that guide consumer choice of food. Appearance; the most externally discernible attribute is significantly impacted by the color of the fruit and it remains the first element used in consumer evaluation of quality. Naturally fruit color is determined by pigments such as carotenoids, chlorophyll and athocyanins or color can also be determined by enzymatic and non enzymatic reactions.
In fruits enzymatic browning is the most predominant factor and it is catalyzed by polyphenol oxidase chemically referred as 1,2 benzediol; oxygen oxidoreductase. In some literature polyphenol oxidase is also referred to as monophenol oxidase, phenylase, diphenol oxidase, tyrosinase, phenoloxidase or phenolase(Maurice R. Marshall et al, 2000) However the analysis of the mechanisms of browning have for along time leaned towards the detrimental effect of browning as opposed to some of the beneficial effects of this enzymatic mechanism.
As fruits ripen their susceptibility to disease and pest and insect infestation usually decrease due to the biochemical decline of phenolic content. Phenoloxidase enzymes are endogenous constituents in fruits. They play a primary role in the catalysis of quinolone production from phenolic constituents. Once quinolones are formed, they undergo polymerization reactions that lead to the production of melanins. Melanins exhibit antibacterial and antifungal activity hence keeping the fruit physiologically wholesome.
Economic loses caused due to browning of fruits have elicited a new wave of research targeting enzymatic browning and proposing mechanisms of controlling these enzymatic reactions particularly the phenyl oxidase enzyme. The control of browning is therefore very critical to ensure that throughout the production and handling of agricultural products the appearance, flavor and nutritional value is maintained. Enzymatic browning severely limits the shelf life of minimally processed agricultural products such as apricots, apples, pears, peaches, bananas and grapes.
In the determination of the rate of enzymatic discoloration, the concentrations polyphenol oxidase (PPO) and other phenolic compounds present. The pH, temperature and the availability of oxygen have also been proven to be determinants of the rate of enzymatic browning. Additionally, the oxygen availability and the pH also influence non enzymatic browning. Physiologically,the optimum pH of polyphenol oxidase activity is between 5-7. In the processing of apples, this pH is usually adjusted to 4 by the use of citric acid, malic acid or fumaric acid.
However, this is only applicable so long as the resultant acidity can be effectively tolerated taste wise. When this pH is reduced further to levels below 4, the tight binding of copper onto the active sites of enzymes are loosened hence chelators such as citric acid can be used to remove copper from the binding sites(Murat Ozdemir, 1997). On the other hand, non enzymatic browning results from reactions of carbonyl groups such as reducing sugars, ketones, aldehydes, lipid oxidation products with amino acid compounds.
Caramelilisation of the carbohydrate in food due to the use of heat in the treatment if the heat used is above the melting point of sugar in physiological alkaline or acidic conditions. Another mechanism is through the spontaneous thermal decomposition process of ascorbic acid under aerobic or anaerobic conditions in the presence or absence of amino compounds. Lipid browning can be caused by oxidation of unsaturated glyceride components. This is followed by polymerization accelerated through the presence of amines, ammonia or proteins.
In fruits non enzymatic browning is inhibited through refrigeration, reduction of reducing sugar content, control water activity or glucose oxidase treatment. The severity of browning is more pronounced at peeled surfaces. This is because damage to surface tissues during the peeling or cutting procedures causes the cell wall or its cellular membranes to lose integrity. Exposure to oxygen accelerates PPO activity hence the duration of browning. Conclusion
It was successfully proven that hydrochloric acid has an inhibitory effect on the enzymatic activity of polyphenol oxidase enzyme and hence a resultant effect on the biochemical rate of enzymatic browning. This is supported by the fact that the reduction of pH (increase in acidity) generally lowers the resultant enzymatic activity because the physiologically optimum Ph for PPO activity is between the pH of 5 and 7. Sodium hydrochloride on the other hand reduces acidity hence increasing the activity of PPO activity. New approaches to help people understand the process of enzymatic browning are currently under intense study.
Successful initiatives aimed at biochemically controlling the rate of fruit browning should be evaluated on the basis of cost, effectiveness and regulatory status. Inhibitors chosen should not have adverse effects on the flavor, texture or color of the agricultural product. References Maurice R. Marshall, Jeongmok Kim and Cheng-I Wei (2000)Enzymatic Browning in Fruits, Vegetables and Seafoods. Food Science and Human Nutrition Department University of Florida Murat. Ozdemir. (1997) Food Browning and its Control. www. okyanusbilgiambari. com