Kinetic Modelling of Vitamin C Degradation in Selected Fruits under Market Storage Conditions

The degradation kinetics of vitamin C in three fruits namely; orange, banana and apple under different market storage conditions prevalent in Nigeria were investigated. Orange and banana samples were stored in sacks and open air at ambient conditions, while open air and refrigerator were used for apple samples. Storage was for 6 days duration. .Iodometric titration was used to determine vitamin C content of the fruits on a daily basis. Regression analysis was employed to fit the variations in vitamin C concentration in the different samples with time, to three kinetic models, to determine which model best describes the degradation trend. Results showed that vitamin C concentration in all the fruit samples reduced over time following zero order kinetics. Kinetic studies obtained the following degradation rate constants: orange 0.44g.L.s and 0.29 g.L.s, for sack and open air storage respectively, banana 0.316 g.L.s and 0.264 g.L.s for sack and open air, and apple 0.122 g.L.s and 0.188 g.L .s, for refrigerated and open air respectively. The study indicated that with respect to vitamin C retention, open air storage is preferable to sack for oranges and bananas while apples are preferably stored in refrigerator. Keywords— Diet, samples, rate constant, open air, biological activity, anti-oxidant.


INTRODUCTION
Fruits such as orange, banana, mango and apple are a major component of a healthy diet. They are low in fat and provide significant levels of some micronutrients. Most fruits when ripe have a short durability, which is further reduced when the fruits are exposed to unfavourable conditions during transportation, processing and storage. Fruits are important sources of vitamin C. (Giannakouurou et al., 2003). Vitamin C is defined as the generic term for all compounds exhibiting the biological activity of L-ascorbic acid (AA) (Lee et al., 2000). Vitamin C or ascorbic acid is a nutrient that is valuable for its antioxidant effect (Giannakouurou et al., 2003). It is a water soluble vitamin, and very essential to human beings (Uddin et al., 2002). Most plants and animals have the ability to synthesize vitamin C. The only mammals that are unable to synthesize vitamin C are primates, including man, and guinea pigs. Therefore, humans depend on exogenous sources of the vitamin which include fruits and vegetables as well as food supplements and pharmaceutical preparations (Parviainen, 1995). Vitamin C is required for the synthesis of collagen, an important structural component of blood vessels, tendons, ligaments and bone. It is also important in the synthesis of the neurotransmitter norepinephrine, which is critical to brain function and can affect mood. Vitamin C content of fruits vary depending on fruit size and species. Oranges contain about 50mg/100g, while banana and apple contain 9mg/100g and 6mg/100g respectively (Bellow and Krebs, 2007). Vitamin C is easily destroyed and various studies have been carried out on the rate of vitamin C degradation in foods and fruits alike, and various kinetic models adopted. However, the vitamin C degradation mechanism is specific to a particular system, as it depends on several factors (Tannenbaum, 1976), such as temperature, water activity, pH and metal ions (Masamba et al., 2013). Vitamin C is most sensitive to destruction when the commodity is subjected to adverse handling and storage conditions. Losses are enhanced by extended storage, higher temperatures, low relative humidity, physical damage, and chilling injury (Lee and Kaddar, 2000). Ascorbic acid is recognized as one of the most heat sensitive nutrients in foods, therefore, it is a marker of the loss of other nutrients (Esteve et al., 1999 ). Numerous analytical techniques are available for the determination of the vitamin C content in various fruits and vegetables, amongst them are iodometric titration, chromatographic methods, enzymatic methods, and electrochemical methods (Abraha et al., 2014; Gunjun and Mangla, 2012; Iwase, 2000). This study was carried out to (i) determine the rate of degradation of vitamin C in orange, banana and apple under the market storage methods prevalent in Nigeria namely; ambient storage in open air, s ack storage, and refrigeration, so as to recommend the best option; (ii) develop kinetic models for predicting vitamin C degradation in the selected fruits under the studied conditions.

II.
MATERIALS AND METHOD Source and preparation of samples Ripe and fresh fruits were purchased from Relief market in Owerri, Nigeria. The fruit samples were divided into two groups and stored using the different market storage methods. The first group of oranges, apples and bananas were kept on a tray and exposed to open air under ambient conditions, the second group of oranges and bananas were packed in two separate jute bags and kept on concrete floor, while the apples were stored in a refrigerator. The fruits were stored for a period of 6 days. The fruit juice from the orange and banana samples was prepared by blending 100g of the fruit sample with 50ml of distilled water and the mixture strained. Distilled water was then added to make a final solution of 100ml in a volumetric flask. The apple samples were squeezed to extract the juice.

Preparation of standard solution
Starch solution was prepared by dissolving 0.5g soluble starch in 50ml near boiling distilled water, while vitamin C solution was prepared by dissolving 0.25g of standard vitamin C in 100ml of distilled water, and the solution was diluted to 250ml with distilled water. 5g of potassium iodide (KI) and 0.268g of potassium iodate (KIO3) were dissolved in 200ml of distilled water, and 300ml of 3M sulphuric acid (H2SO4) was added to obtain standard iodine solution.

Sample analysis
The iodine solution was titrated with 25ml of the standard vitamin C solution, and then with 25ml of prepared fruit juice samples using starch indicator. Triplicate samples were analyzed and the average titre used for calculation.  Tables 1 -3. Tables 4-6 summarize the results of kinetic model regression analysis, while Table 7 compares vitamin C zero order degradation kinetic parameters for the samples under different storage methods. As can be observed in Tables 1-3, the concentration of vitamin C decreased steadily with time during storage in all the samples. This confirms the fact that vitamin C degrades during storage in fruits. The vitamin C concentration of the fruit samples decreased during storage, but in different degrees, depending on the method of storage. This is in agreement with the report of earlier workers on citrus and strawberry fruit juices (Burdurlu et al., 2006;Derossi et al., 2010,) and in accordance with the degradation kinetics of ascorbic acid in model systems as put forth by Liao and Seib (1988). Besides, it is also evident that the ascorbic acid content in a given mass of orange is greater than that in an equal mass of banana and apple. A visual inspection of the kinetic plots of models (4a), (5a) and (6a) respectively, presented in Figures 1-3  values and the lowest RMSE values. Thus, the vitamin C degradation kinetics in orange, banana and apple can be best described by zero order kinetics. . Furthermore, the degradation rate constants of fruit samples stored in sacks were generally higher than those kept in open air and refrigeration. Refrigerated samples exhibited lower rate constants relative to open air for apple. Since the magnitude of the rate constant is a reflection of the rate of reaction, the inference is that degradation of vitamin C occurred faster in samples stored in sacks than in those exposed to the air, and slower in refrigerated samples for apple. This trend manifested in the half life of the samples. The time at which the concentration of vitamin C in the samples reduces to half of its original amount (half life) was shorter in sack stored samples for orange and banana, and longer in refrigerated samples for apple. This implies that the air exposed and refrigerated samples will be expected to have longer shelf life than the sacked ones. Refrigerated apple samples recorded the longest half life of 10.79days, with a proposed zero order model C = C0 -0.1223t, while sacked banana exhibited the shortest half life of 2.40days with the proposed model C = C0 -0.3166t. These models and others presented in Table 7 are proposed for monitoring vitamin C degradation for the respective fruits under the indicated market storage conditions.

IV. CONCLUSION
The influence of market storage conditions on vitamin C degradation in orange, banana and apple samples was evaluated in this study. Vitamin C content of these fruits decreased with time following zero order kinetics. It was found that open air storage and refrigeration helped to significantly reduce the rate of degradation when compared to storage in sacks as shown by the degradation rate constants and half lives. Thus, refrigeration for apples and open air storage for oranges and bananas are preferable in terms of vitamin C retention to sack storage, and hence, recommended for the storage of the indicated fruits in Nigeria local markets. However, in situations where refrigeration facilities are not available, properly managed open air storage can go a long way to retain vitamin C in apples for extended periods.