Foundation and optimization of protocol for blueberry fruit firmness measurement under mix mode of deflection distance and pressure weight

Abstract: 【Objective】Fruit firmness of blueberry (Vaccinium spp.) at different developmental stages were measured under diverse force and deflection testing modes. We aimed to found and optimize the protocol for blueberry firmness determination. In addition, we also wanted to summarize the breeding tendency of blueberry for high fruit firmness.【Methods】Assuming that the sample size was an important element for study design and validation of fruit firmness, we assessed how the mean fifirmness varied with different sample sizes using the ripe fruits of four blueberry cultivars Emerald, Jewel, O’Neal and Sharpblue. The optimal sampling amount for blueberry fruit firmness measurement was statistically analyzed. The fruit firmness was analyzed at six developmental (from S3 to S8) stages using different deflection thresholds and force threshold modes using the fruits of blueberry cultivar O’Neal. Optimized measurements was constructed for firmness detection of blueberry fruits. Using the complementary methods, we determined the fruit firmness 36 blueberry cultivars. The breeding tendency of fruit firmness of different blueberry groups was proposed by combing the correlation coefficient between thefruit firmness and the release year of blueberry cultivar, and by considering the cultivar phylogeny.【Results】The sample sizes showed pivotal an effect on variance in the mean fifirmness values of blueberry genotypes. Taking discrete degree of fruit firmness, the values out of the confifidence intervals, and the coefficient of variation of fruit firmness under different sample sizes into account, we found stable and reliable observation value of fruit firmness could be obtained when the sampling amount of blueberry fruit exceeded 45 berries. However, a sampling amount ranging from 60 to 75 was advised to stand for the characteristic fruit firmness of the blueberry cultivar. The formation process of blueberry fruit firmness could be divided into two stages, with the first stage from S3 to S4 and the second stage from S5 to S8. Fruit firmness at stages S3 and S4 was significantly higher than that at stages S5 to S8. In the deflection measurement mode, the correlation coefficient between the fruit firmness and the process deflection distance at stages S3 and S4 was significantly higher than that at other stages when the fruit deflection thresholds were 0.5 and 1.0 mm. When the deflection thresholds were 1.5 and 2.0 mm, the correlation coefficient with process deflection distance and fruit firmness at S5 to S8 stages were significantly higher than that of fruits at S3 and S4 stages. The deflection threshold of 1.0 mm was more suitable for measuring the firmness of blueberry fruits at the S3 and S4 stages with higher firmness, and 1.5 and 2.0 mm were more suitable for measuring the firmness of the blueberry fruits at the S5 to S8 stages with gradually decreasing firmness. In the force measurement mode, the correlation coefficients between the measured value of fruit firmness and the value of process force at the force threshold of 200 g were significantly lower than those of 300 g and 400 g at the six developmental stages. The correlation coefficient between the fruit firmness and the process force value at different stages showed an upward trend with the increase of the pressure thresholds, and the largest correlation coefficient was seen when the force threshold was 400 g. There were significant differences in fruit firmness at stages S3 and S5 measured under the three force measurement modes. The measured value of the mode 400 g was significantly larger than that of the modes 200 and 300 g, and the correlation coefficient was the highest. Therefore, the 400 g force threshold mode was suitable for the determination of fruit firmness from stages S3 to S8. The 36 blueberry cultivars used for fruit firmness detection included 18 southern highbush blueberry (SHB) cultivars, 9 northern highbush blueberry (NHB) cultivars and 9 rabbiteye blueberry (REB) cultivars. The fruit firmness of those blueberries ranged from 161.7 to 263.1 g·mm^-1. Blueberry Draper showed the highest fruit firmness (258.5 g·mm^-1 ) and Elizabeth had the lowest fruit firmness with a value of 161.7 g·mm^-1 . The average fruit firmness of SHB, NHB and REB were 205.0, 203.9, and 202.6 g· mm^-1, respectively. The fruit firmness of recent bred highbush blueberry cultivars was generally greater than that of old cultivars, especially for those cultivars that belong to southern highbush blueberries. The fruit firmness showed a release-time based increment tendency among the different blueberry cultivars.【Conclusion】A sampling amount of 75 berries is suitable for measuring blueberry fruit firmness by considering the confidence and the detecting cost. The mixed mode of 400 g force threshold and 2.0 mm deflection threshold, or the combined deflection threshold mode of 1.0 and 2.0 mm could be used to determine the fruit firmness of blueberry during developmental stages. Cultivars with firmness would be the tendency of blueberry breeding. The southern highbush blueberry cultivars Star and Misty, Northern highbush blueberry cultivar Draper, rabbiteye blueberry cultivars Vernon and Tifblue probably could be used as outstanding parents for the selection of firm-flesh blueberry cultivars in the future.