Alicia R. Chaves

Polyphenoloxidase activity from strawberry fruit (Fragaria ananassa, Duch., cv Selva): characterisation and partial purification

In this work, polyphenoloxidase (PPO) from Selva strawberry fruit (Fragariau2009×u2009ananassa, Duch) was extracted, characterised and partially purified. The activity of PPO was analysed in crude extracts obtained from either fresh fruits or acetone powder. The presence of NaCl and Triton X-100 in the extraction buffer caused a marked increase in enzyme extractability. The enzyme showed an apparent Km value of 11.2u2009mM with pyrocatechol as substrate. The maximum enzyme activity was observed at 50u2009°C and pH 5.3–6.0 without SDS and pH 7.2 in the presence of SDS. The presence of SDS increased PPO activity at pH 7.2 but diminished it at pH 6.0. The enzyme showed high thermal stability and maintained activities equal to or greater than 50% of its maximum activity in the 2.6–9.3 pH range. One polyphenoloxidase isoenzyme was detected in crude extracts of all ripening stages, showing an isoelectric point of 7.3. The specific activity of PPO decreased continuously through fruit ripening. Maximum specific activities were found at the ‘small green’ and ‘large green’ ripening stages. A total enzyme extract was partially purified by means of (NH4)2SO4 precipitation and cationic exchange chromatography in an FPLC system. The purification grade achieved was near 25. The partially purified enzyme showed an isoelectric point equal to 7.3 and a molecular mass of 135u2009±u20094u2009kDa for the native protein. n n n n© 2000 Society of Chemical Industry

Effect of exogenous application of gibberellic acid on color change and phenylalanine ammonia-lyase, chlorophyllase, and peroxidase activities during ripening of strawberry fruit (Fragaria x ananassa Duch.)

The effect of exogenously applied gibberellic acid (GA3) on the postharvest color change of strawberry fruit was evaluated through their external color and surface color parameters. A significant delay on color evolution was observed in fruits treated with GA3. The evolution of activities of phenylalanine ammonia-lyase (PAL), chlorophyllase, and peroxidase was also analyzed. PAL activity increased during strawberry ripening, but in fruits treated with GA3 the increase in such activity was slower, and, probably as consequence, the development of red color was delayed. Moreover, the activity of chlorophyllase and peroxidase, enzymes possibly involved in chlorophyll metabolism, decreased during strawberry ripening. However, a delay was observed in the decrease of such activities in GA3-treated fruits.

Kinetics of colour change of raspberry, sweet (Prunus avium) and sour (Prunus cerasus) cherries preserves packed in glass containers: light and room temperature effects

Abstract This work studied the kinetics of colour change in preserves of raspberries, sweet and sour cherries exposed to different lighting conditions (light and darkness), at constant temperature, as well as stored at several temperatures. In all the preserves and treatments, the colour diminished noticeably during the first 90 days of storage to remain constant from then on. The kinetics of colour change was evaluated using the concept of fractional conversion, and a first-order kinetics was found in relation to both the effect of lighting conditions and temperature. The corresponding reaction rate constants were determined by regression analysis of the experimental data. For samples stored at different lighting conditions, the resulting constants of the different preserves were not alike: constants were higher in samples kept in the presence of light than the corresponding samples stored in the dark. However, the statistical analysis of the results indicated those not significant differences among the different light treatments. The statistical analysis of reaction rate constants obtained for the samples stored at constant temperatures, of 4°C, 20°C and 40°C, did indicate significant differences between treatments. In this case, applying one (A. Arabshai, D.B. Lund, Journal of Food Process Engineering 7 (1985) 239) and two-step (Arrhenius model) methods, the corresponding values of the pre-exponential factor (ln k 0 ) and the energy of activation ( E a ) were obtained and compared for the three preserves analysed.

Estimation of Chlorophyll Contents by Correlations between SPAD‐502 Meter and Chroma Meter in Butterhead Lettuce

Abstract Chlorophyll content of butterhead lettuce leaves was estimated by correlations between the Minolta SPAD‐502 and Minolta chroma meter CR 300. The SPAD‐502 readings and extractable chlorophyll (Chl a, Chl b, and total Chlorophyll) contents were related. High linear correlations were obtained for chlorophyll a (R2=0.90), chlorophyll b (R2=0.85), and total chlorophyll (R2=0.92). After that, during the growth cycle of butterhead lettuce grown in soilless culture, leaves were selected and SPAD‐502 and chroma meter readings were made. Finally, correlations between SPAD‐502 readings and leaf color (L*, a*, b*, C*, and hue angle) determined by the chroma meter were calculated. Hue angle (H°; R2=0.75) and lightness index (L*; R2=0.68) were the parameters that better relate with SPAD‐502 readings, so these parameters can also be used for rapid and nondestructive estimates of leaf chlorophyll in situ.

Effect of gibberellic acid on ripening of strawberry fruits (Fragaria annanassa Duch.)

Effect of GA3 on postharvest ripening in strawberry fruit was evaluated through different biochemical parameters. Strawberry slices at different ripening stages were incubated with GA3. A significant decrease on respiratory activity depending on GA3 concentration was obtained. Also GA3 was applied to whole and deachened fruit at white and green ripening stages. Our results show that GA3 has an inhibitory effect on strawberry fruit ripening, evidenced by a decrease in the respiratory activity and a delay in anthocyanin synthesis and chlorophylls degradation.