Control of Enzymatic Browning in Green Bananas for Freezing

Abstract

When green bananas are peeled by heating in steam at 80 lb pressure for 30 s the polyphenol oxidase is inactivated in a heat ring to a depth of 1/8 in (3.1 mm), but not in the interior pulp of the fruit. Increasing the length of the peeling treatment, alone or in combination with hot water or steam blanching, could not be used to inactivate the enzyme system due to the excessive softening of the pulp. Dipping the steamed fruit in citric acid solutions 1 to 5% w/w reduced browning in the exposed surfaces only. When slices 1 1/2 in thick were sulfited to 412 p/m SO₂, a high  SO₂ content was reached in the heat ring but the core had a resultant sulfite content of 22-32 p/m. The polyphenol oxidase in an area around the degenerated ovules and immature seeds was not inactivated. When the whole fruit was sulfited, no sulfite reached the inner tissue. Sulfiting by dipping in K₂S₂O₅ solutions for 240 min at an  SO₂ level of 1,289 p/m failed to inactivate the polyphenol oxidase in the interior tissue. During thawing, diffusion of sulfite to the inner tissue was slow, and even after thawing for 6 h, complete inactivation of the polyphenol oxidase was not achieved. Sulfited bananas with the enzyme system active in the interior pulp showed browning when thawed and exposed to the air. Complete enzyme inactivation and absolute control of the browning reaction was achieved by heating the raw fruit in water at 200° F (93° C) for 30 min. Besides controlling browning, the hot water treatment loosens the peel, facilitating its subsequent removal by hand.