ETHANOLIC FERMENTATION OF BARTLETT PEARS AS INFLUENCED BY RIPENING STAGE AND ATMOSPHERIC COMPOSITION

Changes in fermentation volatiles and enzymes were studied in preclima cteric and postclimacteric 'Bartlett' pears (Pyrus communis L.) kept i n air, 0.25% O2, 20% O2 + 80% CO2, or 0.25% O2 + 80% CO2 at 20C for 1, 2, or 3 days. All three atmospheres resulted in accumulation of aceta ldehyde, ethanol, and ethyl acetate. The postctimacteric pears had hig her activity of pyruvate decarboxylase (PDC) and higher concentrations of fermentation volatiles than those of the preclimacteric fruit. For the preclimacteric pears, the 0.25% O2 treatment dramatically increas ed alcohol dehydrogenase (ADH) activity, which was largely due to the enhancement of one ADH isozyme. Exposure to 20% O2 + 80% CO2 slightly increased ADH activity, but the combination of 0.25% O2 + 80% Co2 resu lted in lower ADH activity than 0.25% O2 alone. For the postclimacteri c pears, the three atmospheres resulted in higher PDC and ADH activiti es than those of air control fruit. Ethanolic fermentation in 'Bartlet t' pears could be induced by low O2 and/or high CO2 via 1) increased a mounts of PDC and ADH; 2) PDC and ADH activation caused by decreased c ytoplasmic pH; or 3) PDC and ADH activation or more rapid fermentation due to increased concentrations of their substrates (pyruvate, acetal dehyde, or NADH).