29E-19

JUSTIFICATION:

Nonenzymatic browning is one of the main reasons for the loss of commercial value in citrus products as it is the first visible quality defect to be detected at ambient temperature storage. As browning is related to ascorbic acid degradation (among other compounds) it would be very important to see how the addition of ascorbic acid would influence the formation kinetics of brown pigments under aerobic conditions.

 

OBJECTIVES:

To study the kinetics of brown pigments under aerobic conditions, in orange juice with added L-AA, in a range of temperatures.

 

METHODS:

Single strength orange juice with different added L-AA concentrations (A=0, B=100, C=200, D=300, E=400, and F=500 mg/L) was protected from light, continuously aerated for up to 48 hours and monitored for browning, O₂, and pH. Experiments were conducted at 20, 25, 30, 35, 40 and 45 ºC.

 

RESULTS:

A zero-order model was used to fit the data. The model produced fair fits to the experimental data (0.782<adj.R²<0.995). The rate constants showed to increase with temperature according to an Arrhenius type equation. At a reference temperature of 32.5 °C the rate constants are statistically independent of the concentration of L-AA added (with a value of 0.17´10^-6±0.01´10^-6 s^-1), while the activation energy increased linearly with added L-AA content (46±4 kJ/mol for batch A and 83±4 kJ/mol for batch F). As temperature increases the effect of adding L-AA is more perceptible, which can explain the increase of the activation energy values. Oxygen content was maintained fairly constant throughout the experiments, ranging from 5.22 to 5.93 ppm, depending on the batch and temperature.

 

SIGNIFICANCE:

The data show that, under aerobic conditions, addition of ascorbic acid to orange results in increasing browning, being this increasing more noticeable for higher temperatures. The zero-order model may be used for predictive purposes.