Abstract
The apple cv. Hokuto is very susceptible to a new skin-browning disorder called stain during storage. To study the cause of 'Hokuto' stain, young fruits were individually covered with commercial double paper bags from early June to late September. These shaded fruits were harvested in late October and stored at 0°C for up to six months. Incidence of stain between bagged and unbagged (control) fruits during storage at 0°C was recorded and volatiles in their skins were determined by gas chromatography-mass spectrometry and gas chromatography. There was less stain in bagged fruit than in unbagged ones. The same 50 volatiles were identified in the skin of bagged and unbagged fruits during storage. Differences in content of volatile compounds in the skin of bagged fruit were compared to those in unbagged fruit, noting the compositions whose contents were smaller in the skin of bagged fruit than those in unbagged ones. The difference in content of trans-2-hexenal was the greatest among the volatiles, followed by 2-methyl-butan-1-ol, n-hexanal, n-propanol, trans-3-hexenol, n-hexyl propionate and n-amyl acetate, in that order. No difference was found in (E, E)-α-farnesene whose oxidation products are believed to cause superficial scald. Exposure of healthy 'Hokuto' apples to seven volatiles and farnesene (mixed isomers) at 20°C for 20 hours showed that both trans-2-hexenal and n-hexanal induced stain-like injury at the lowest concentration, followed by n-amyl acetate, n-hexyl propionate and 2-methyl-butan-1-ol, in that order. No symptoms of stain appeared on fruits exposed to n-propanol, trans-3-hexenol and farnesene. Comparison in stain-like injury between C6-aldehydes and its C6-alcohols i.e. trans-2-hexenal and trans-2-hexenol, n-hexanal and n-hexanol, revealed that the injury was greater in C6-aldehyde than in its respective C6-alcohol. Application of linolenic and linoleic acids, respective precursors of trans-2-hexenal and n-hexanal, on 'Hokuto' apple skin at 0°C for four days induced stain-like injury in both unsaturated acids. The injury was greater in linolenic acid than in linoleic acid. However, the degree of the injury decreased remarkably in nitrogen gas. Results suggest that shading of the apples by double paper bags reduces concentration of trans-2-hexenal characteristically and that the shading is related to metabolism of linolenic acid to trans-2-hexenal, which were used in these experiments to induce stain-like injury to 'Hokuto' apples.
