Physical and chemical changes during growth, maturation, and ripening of saskatoon (Amelanchier alnifolia) fruit

Both ongoing and future studies of saskatoon (Amelanchier alnifolia Nutt.) fruit would benefit from a maturity index that could be referred to as as an indicator of fruit development. Accordingly, two cultivars of saskatoon fruit (Northline and Smoky) were sorted subjectively into nine maturity and ripeness stages based on differences in size and color. The physicochemical attributes of each stage were then characterized in detail. Color ranged from mostly green (stages 1-4) to white-pink (5) to pink (6) to red (7-8) to purple (9). The fastest gains in fresh and dry weights occurred over the later stages of development (from stages 5 to 9), while increases in fruit diameter between stages 1 and 9 were linear. Fruit firmness declined substantially between the first and the fifth maturity stages and then only slightly as ripening progressed to maturity class 9. Fruit nitrogen concentration decreased throughout development as did pH, chlorophyll, and phenolics. Fruit soluble solids, the ratio of soluble solids to titratable acidity, and anthocyanins increased from the fourth to the ninth stage of development. Glucose and fructose concentrations were about equal, increased about eightfold from class 4 to 9, and along with sorbitol were the major carbohydrates in fruit. Starch was nondetectable in fruit at all stages. Succinate was the predominant organic acid in immature fruit; however, levels declined with advancing maturity, and malate was the most concentrated organic acid in mature fruit. Changes in the relative concentrations of quinate, galacturonate, citrate, pyruvate, cis-aconitate, fumarate, and oxalate were also characterized with advancing maturity. In total, the concentration of organic acids dropped at least twofold as fruits developed from class 1 to 3, then declined relatively slowly through the remaining developmental stages. Cultivar variability in physicochemical attributes was relatively small and insignificant with regard to sorting and classifying fruits according to maturity. It is proposed that this nine-point maturity index be adopted for future studies on saskatoon fruit growth and development.