However, these models may not be applicable to powder systems which have moisture absorption

during storage. In this work, the reaction fitted the first order kinetic model up to the 50th day, and then zero order up to the end of the experiment (90 days). For the prediction of the product shelf life of the obtained values for vitamin C degradation between zero and 50 days were considered; thereafter, the vitamin C degradation was considered negligible in relation to time. First order kinetic behaviour is frequently observed for vitamin degradation, whereas zero order kinetic behaviour is observed when the diffusion Selleck Target Selective Inhibitor Library of certain participants of the reaction is limited ( Taoukis & Labuza, 1996). According to Nagy (1980), after consumption of the free oxygen in the packages, anaerobic reactions become predominant, including that of ascorbic acid degradation, but at a reduced velocity as compared to that occurring under aerobic conditions, which can explain the reduction in the oxidation reaction in the end of storage. Under these conditions, the ascorbic acid decomposes

into 2,5-dihydro-2-furanoic acid, which degrades to carbon dioxide and furfural. PR-171 concentration For its part furfural undergoes polymerisation as an active aldehyde, and can combine with amino acids, influencing product browning ( Shaw et al., 1993 and Solomon et al., 1995). Table 1 shows the ascorbic acid degradation kinetics of powdered guavira pulp. The values for the constant (k) indicate that the reaction velocity increases with increase in temperature. At 35 °C the storage time was 45 days, which, multiplied by the factor of 1.09 given by Q10, resulted in a shelf life of 49 days under storage conditions at 25 °C. The moisture content for these conditions was 10.0% and 5.4% for 35 °C and 25 °C, respectively. According to Silva, Gurjão, ever Almeida, Bruno, & Pereira, 2008, the oxidation of ascorbic acid is mainly influenced by an increase in temperature, whereas Lee and Kader (2000) reported that this vitamin was easily oxidised in aqueous media and in the presence of oxygen, metal ions and alkaline pH values, amongst other factors. Galdino et al. (2003) explained that this behaviour could be attributed

to the low protection provided by polyethylene, making the material susceptible to the effects of micro-environments created in the setting up of trials, allowing for the migration of moisture from the environment until reaching equilibrium. Table 2 shows the mean values obtained for the pH and titratable acidity of the powdered guavira pulp stored in polyethylene packages. A decrease in the pH value with time can be seen under both storage conditions, reaching values of 4.17 and 3.94 at the end of the storage period. According to Martins, Jongen, and Van Boekel (2000), non-enzymatic browning reactions are favoured by high pH values, and are inhibited at pH values below 5.0. The influence of pH was also observed with respect to enzymatic browning.