GIBBERELLIC ACID APPLIED TO SWEET ORANGES INCREASES JUICE YIELD BY REDUCING PEEL VOLUME

摘要

Application of gibberellic acid (GA_3) at about color break can increase percent juice weight of sweet oranges by 2-10%, but the cause of the increased juice yield is unknown. Some processors believe that GA_3 application increases mechanical integrity of the peel or fruit, and thus improves the efficiency of juice extraction. If juice yield is not related to mechanical properties of the peel or fruit, then GA_3 treatment must increase the pulp:peel ratio. We tested both hypotheses in two separate experiments. In 1998, 'Valencia' orange [Citrus sinensis (L.) Osb.] trees were treated with GA_3 before, at, or after color break or left non-treated (control) and harvested in April and May of the following year. Samples of fruit and peel were subjected to mechanical tests or juice extraction. Application of GA_3, especially before color break, markedly improved the strength of the peel in shear and tension, but had little effect on mechanical properties of the whole fruit. However, there was no obviousrelationship between mechanical properties of the peel or fruit and juice yield. In 2000, 'Hamlin' oranges [C. sinensis (L.) Osb.] were treated with GA_3 at about color break or left non-treated, and juice yield, peel thickness, and fruit diameter were measured every 1 to 2 weeks from application until mid-December. After about 3 weeks, the peel of non-treated fruit became slightly thicker than the peel of treated fruit and remained thicker for the duration of the experiment. Application of GA_3 did notaffect fruit diameter. Because the peel of non-treated fruit was thicker than treated fruit, non-treated fruit had greater peel volume than treated fruit. Neither peel thickness nor fruit diameter was well correlated with juice yield, but the estimatedpeel volume was inversely related to juice yield (r~2 = 0.54). Therefore, it appears that GA_3 application increased juice yield because it increased pulp:peel ratio.