Effect of storage conditions on methanol content of fruit and vegetable juices

Methanol contents of fresh squeezed juices from 10 fruits and 5 vegetables were determined and the relation of methanol release from stored juices on the physico-chemical properties including degree of esterification (DE), total pectin content, pH value, titratable acidity and the hydrolytic activities of pectinesterase (PE), polygalacturonase (PG), cellulase (CE) and pectate lyase (PAL) of fresh squeezed juices were investigated. The range of methanol content in fresh squeezed juices of fruits and vegetables were 1.14–6.77 and 2.04–10.92 mg/100 mL, respectively, but increased to 1.13–14.82 and 4.73–24.08 mg/100 mL after 3 h storage. In most of the juices, the increase of methanol content was significant (p<0.05) after 3 h storage, except grape (4 and 30 °C), guava (4 °C), lemon (4 and 30 °C), and star fruit (4 °C). The methanol levels above 10 mg/100 mL (the given limit of alcoholic beverages) were found in fresh tomato juices (squeezed and stored) and several other stored juices including pea shoot juices (after 60 min storage), star fruit juices (after 180 min storage), papaya juices (after 180 min storage), pineapple juices (after 180 min storage) and Valencia orange (after 30 min storage), and alfalfa sprout juices (after 60 and 120 min storages for 4 and 30 °C, respectively). After analyzing with multiple regressions, methanol release was positively associated with enzyme activities of PE and PAL, while negatively associated with total pectin content in fruit juices. In vegetable juices, methanol release was positively associated with PE activity, pH value and titratable acidity, while negatively associated with PG activity.