Maintenance of carbon 7 sugar levels and effect on ripening of 'fuerte' and 'hass' avocado (Persea americana Mill.) fruit.
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Avocado fruit are susceptible to a large variety of disorders. These disorders may be a result of an inability of the mesocarp tissue to counteract or tolerate postharvest stress. The C7 sugars D-mannoheptulose and perseitol have been reported to form the predominant portion of antioxidants in the mesocarp and their presence has been associated with avocado fruit quality. It was, therefore, investigated, if mesocarp C7 sugar levels, particularly of D-mannoheptulose and perseitol, can be maintained through infusion of these sugars and further, if this C7 sugar level is associated with fruit quality and shelf life. Avocado fruit, harvested from ‘Hass’ and ‘Fuerte’ avocado orchards in the KwaZulu-Natal Midlands in three different season (early, middle, and late harvest) were infused with 1.5 mL water, 1.5 mL solution of (9.5 mM/fruit; 4.75 mM/fruit D-mannoheptulose), a C7 sugar solution (1.5 mL of 9.5 mM/fruit; 4.75 /fruit; 4.75 mM perseitol/fruit). Fruit quality parameters (firmness, CO2 production, soluble sugar concentrations, moisture content, dry matter, and oil content) were determined over the postharvest ripening period. Early-harvested fruit displayed more severe ripening heterogeneity, with high water loss. The infusion of D-mannoheptulose and perseitol prolonged the shelf life of avocado fruit compared to sucrose-infusion and untreated fruit (control) at different harvesting stages. Water infusion had a considerable effect on mid- and late-season fruit, regarding firmness and respiration rate. Infusion of D-mannoheptulose and perseitol improved the fruit quality attributes flesh firmness and fresh mass retention, and resulted in higher mesocarp C7 sugar concentrations than sucrose- and water-infusion. Regarding the concentration of C7 sugars, water-infused fruit contained the third-highest D-mannoheptulose and perseitol concentration. The oil content was not affected by sugar postharvest infusion, but noticeable differences in oil content were observed through the harvest seasons. Maintaining a certain level of these sugars in the avocado mesocarp tissue seems vital in ensuring a good fruit quality. These C7 sugars could be used as postharvest markers and determining their concentration could become a vital tool in the management of avocado postharvest quality.