Phenylalanine Ammonia-lyase, Peroxidase, Pyruvic Acid and Total Phenolics Variations in Onion Bulbs During Long-term Storage

Phenylalanine ammonia-lyase (PAL) and peroxidase (POD) enzyme activities, total phenolics and pyruvic acid (PA) (as pungency indicator) were measured in onion bulb tissues (c.v. Rouge Amposta) during storage at 4 and 20 °C. After 2 wk, PAL activity increased at 4 °C, but decreased at 20 °C, and after 4 wk, PAL activity at 20 °C was higher than activity observed at 4 ° C. Peroxidase activity was high during the first weeks of storage, but at 4 °C this high activity lasted longer and a decrease was noted at both temperatures, coinciding with sprouting appearance. Total phenolics, which rose during the first 12 wk but fell during last weeks of storage, appeared to be linked to PAL activity. An inverse relationship was observed between phenolic content and the amount of sprouting development of bulbs. Pyruvic acid production appeared to be influenced by temperature, but in the last period of storage, the effect of temperature on PA production was less. The data demonstrate that PAL activity, which is linked to phenolic metabolism, and POD are highly involved in the sprouting of onion bulbs and these two enzymatic activities are much influenced by low temperature.