Starch breakdown in ethylene-and ethanol-treated bananas: Changes in phosphorylase and invertase activities during ripening.

Abstract

Green Cavendish bananas were placed in 6-l glass jars and sprayed with 10 ml of water (control) or 20, 40 or 60% ethanol or exposed to 1000 p.p.m. ethylene for 24 h. The jars were covered with perforated plastics film and stored at 20 degree C. Total sugar, starch and phosphorylase and invertase activities were determined in fruits at different stages of ripening based on peel colour (colour index ranging from 1 (green) to 8 (overripe)). Extracts for enzyme analysis were prepared by homogenizing banana pulp with cold 0.2M citrate-phosphate buffer (pH 7.5) containing 10mM EDTA and 15% polyclar AT (w/w), centrifuging at 13 000 rev/min for 15 min, dialysing the supernatant (fraction containing invertase) in 5mM buffer and extracting phosphorylase from the residue using the same extraction medium. Sugar was extracted from the banana pulp with 80% hot ethanol and starch was estimated in the sugar-free residue. Starch content decreased and sugar content increased during ripening. The most rapid decrease in starch occurred between colour index values 1 and 3 and by value 6 starch content was less than 1%. Max. total sugar content (20%) was reached at colour index value 6-7, and the increase was fastest in ethylene-treated fruits. Non-reducing sugars accumulated before and faster than reducing sugars. Phosphorylase activity increased at the onset of peel yellowing (colour index 2) and then decreased in parallel to the decrease in starch content. Invertase activity paralleled the accumulation of sucrose, activity increasing 10-fold between colour index values 1 and 3. Invertase activity was max. at colour index 5 and decreased slightly thereafter. A mechanism is proposed, whereby, during ripening of bananas, starch is transformed into sucrose via the action of enzymes, including phosphorylase, and sucrose is then hydrolysed into glucose and fructose by invertase. The pathway for starch transformation appeared identical in control, ethanol- and ethylene-treated fruits.