Postharvest Keeping Quality of Blueberry Cultivars in Cold Storage: Texture and Appearance

Rachel A. Itle, Rion T. Mooneyham and Savithri U. Nambeesan, Department of Horticulture, University of Georgia

This article is the first of a series on work comparing fruit quality traits of major commercial cultivars in southern highbush (SHB), rabbiteye (RE), and northern highbush (NHB) cultivars. 

Georgia’s blueberry industry consists of southern highbush (SHB, Vaccinium corymbosum L. and V. darrowii Camp complex) and rabbiteye (RE, V. virgatum Aiton). There exists a subjective bias in the blueberry industry that SHB has higher fruit quality than RE. Their quality is also compared to northern highbush (NHB, Vaccinium corymbosum L.), which is perceived as superior. This often leads to reduced price points received by growers by third party distributers, and has even resulted in cultivar and/or type exclusion from some commercial purchasers.  However, there is limited information that supports this preconceived perception at best. The objective of this study was to examine the physicochemical postharvest keeping quality of major SHB, RE, and NHB cultivars in commercial postharvest cold storage (4°C) and determine how each type maintained fruit quality over time.

Fruit Collection

Fresh fruit was collected from commercial packers from May to August in the 2018 and 2019 blueberry harvest seasons. In 2018, seven SHB (‘Camellia’, ‘Farthing’, ‘Keecrisp’, ‘Meadowlark’, ‘Legacy’ from Georgia and Michigan, ‘Star’, and ‘Suziblue’), five RE (‘Alapaha’, ‘Austin’, ‘Brightwell’, ‘Powderblue’, and ‘Vernon’) and five NHB (‘Bluecrop’, ‘Draper’, ‘Elliott’, ‘Liberty’, and ‘Nelson’), were collected for a total of seventeen cultivars. In 2019, seven SHB (‘Abundance’, ‘Camellia’, ‘Farthing’, ‘Meadowlark’, ‘Legacy’ from Georgia and Michigan, ‘Star’, and ‘Suziblue’), five RE (‘Alapaha’, ‘Brightwell’, ‘Powderblue’,’Premier’, and ‘Vernon’), and three NHB (‘Aurora’, ‘Elliott’, and ‘Liberty’) were collected for a total of fifteen cultivars. SHB and RE cultivars were collected from commercial packers in Georgia during both harvest seasons. NHB cultivars were collected from commercial packers in Michigan and Canada during the 2018 harvest season, and from Michigan and Indiana during the 2019 harvest season. All SHB and RE was placed into half pint clam shells, and coolers with ice and were transported to the UGA Griffin campus for evaluation. All NHB fruit was received via refrigerated truck. All fruit was harvested within a week of the fruit collection.  Fruit were kept at 4°C and fruit quality attributes were measured over a 30-day period at four timepoints during storage: 1) 3-4 days, 2) 10-11 days, 3) 20-21 days, and 4) 30-31 days after collection.

Fruit Quality Measurements

The major fruit quality attributes for fresh fruits and vegetables are appearance, texture, taste including aroma and flavor attributes, and nutritional value (Barrett et al., 2010).  For this portion of the study, attributes of texture (fruit firmness and fruit skin strength) and appearance (berry size and percent healthy fruit) and were measured.

Fruit firmness determination

Samples of multiple berries were used to measure fruit firmness using a TMS-Pro Texture Analyzer (Food Technology Corporation, Sterling, Virginia). The measurement used is called the Kramer Shear Press, which used ten blades to cut the fruit. This measurement recorded the maximum force needed to shear/cut the fruit, and it simulated the force needed to bite down on a handful of berries and fully crush them with the molars.  For each cultivar at each of the four timepoints, nine measurements were done [N=9; 3 reps x 3(50.0-51.0g berry sample) subreps].

Fruit skin strength

Individual berries were used to measure fruit skin strength using the same texture instrument with a measurement called the puncture test, which uses a single thin probe to pierce the skin from the top of the berry. This measurement records the maximum force needed to puncture the fruit skin, and it simulated the force needed to bite down on an individual blueberry with the two front teeth and break the skin.  For each cultivar at each of the four timepoints, 36 berries were measured [N=36; 3 reps x 12 median individual berry subreps]. 

Berry size

Multiple berries were used to measure berry weight.  For each cultivar, at each of the four timepoints, four measurements were done [N=4; 4 reps x 20 berry subreps]. 

Percent healthy fruit

Individual berries were inspected for all signs of postharvest decay.  Visual imperfections including leakiness, shriveling, mold, dents and tears were evaluated. Fruit with any defect was scored as, “unhealthy” and fruit free of defect was scored as, “healthy”. An indication of shelf-life was determined by totaling fruit without visual imperfections and dividing by total berry number and this was expressed as percent healthy fruit. For each cultivar at each of the four timepoints, four measurements were done [N=4; 4 reps x 30 berry subreps/rep]

For all fruit quality traits, measurements were taken for all cultivars at all four time points throughout 30 days in storage.  For simplicity in this article and to review overall trends, results comparing overall blueberry type comparisons over 30 days in storage will be discussed. For this, cultivars within a type were averaged at each timepoint, and the average of a blueberry type (NHB, RE, or SHB) across timepoints were compared using the Tukey HSD test (P≤0.05; SAS v.9.4, SAS Institute, Cary, NC). To quantify the amount of change from beginning to end in cold storage, the percent change of each trait within a blueberry type was also calculated and will be compared below.

Results: How Do These Fruit Quality Traits Change Over Time?

Fruit firmness

In year one, when examining blueberry types overall, RE was the only type that was significantly different from harvest to 30 days after storage with a 26% increase in fruit firmness over time (Figure 1).  For both SHB or NHB types overall, there was no significant difference for firmness over time, indicating that highbush types overall maintained fruit firmness over storage time in the cooler.

In year two, all three types increased in firmness over time in the cooler.  SHB maintained quality in storage the best with the lowest increase in fruit firmness (25%).  RE had an intermediate increase in firmness over time, with a 31% increase over 30 days in storage. NHB lost the most quality for fruit firmness over time with a 33% increase in fruit firmness over time. The results suggest that SHB maintain their quality in terms of fruit firmness the best over time in the cooler, followed by RE and then by NHB. Overall for both years, these results suggest there is variability for fruit firmness maintenance over storage time across years.

Fruit skin strength

In year one, no significant differences were observed for fruit skin strength for all three blueberry types from begin to end in storage (Figure 2).  This indicated that fruit skin strength was not affected by time in cold storage and maintained consistent quality over time regardless of blueberry type.  For year two, all three blueberry types increased in skin strength between harvest and 30 days in storage, with a range of 17-23% increase in fruit skin strength.  Both HB types increased by approximately 17% over the 30 day storage time in the cooler, and RE increased by 23%. Overall for both years, these results suggest that NHB and SHB maintain their fruit skin strength the best over time in the cooler, and that RE fruit skins experience a larger increase in skin toughness over time in the cooler.  These trends suggest that RE don’t maintain their fruit skin strength as consistently over cold storage as do highbush types.

Berry size

No significant change to berry size over time was observed in year one for both NHB and RE types (Figure 3).  SHB had a significant decrease in berry size with a decrease of 12% over 30 days in storage.  In year two, both highbush types had a decrease in berry size over time with SHB decreasing by 12% and NHB decreasing by 9%.  RE fruit maintained consistent berry size over time.  These results suggest that RE fruit do not change size over cold storage time, and that SHB fruit experience the largest decrease in berry size in 30 days of cold storage.

Percent healthy fruit

For all blueberry types across both years, percent healthy fruit decreased over time in cold storage (Figure 3).  In year one, SHB and NHB had the lowest amount of visual fruit decay over time with 25% and 23% (respectively) decrease in healthy from the beginning of cold storage.  RE had the largest amount of fruit decay with a 45% decrease in healthy fruit after 30 days.  In year two, both highbush types had a lower amount of fruit decay over time with SHB and NHB decreasing in healthy fruit by 29% and 34%, respectively.  RE again had the largest percent change in healthy fruit with a 49% reduction by day 30.  These results suggest that highbush types maintain visual appearance and have reduced fruit decay over time, and that RE had the highest amount of fruit decay over storage time.

Lastly, the variation observed across years for all measured fruit quality traits may be attributed to many things.  Differences in quality may be attributed to variations in fruit at harvest maturity (Sams, 1999), harvest timing (Lobos et al., 2014), handling processes (Bower, 2017), flowering time (Suzuki and Kawata, 2001), and amount and composition of fruit wax (bloom) (Lara et al., 2014).  Changes in secondary cell wall structures, and enzymatic changes in the fruit cell wall (Chea et al., 2019) and other environmental factors that affect components of fruit texture may also contribute to overall postharvest keeping quality. It would be useful to determine the differences in these other traits among SHB, NHB, and RE blueberry types to further determine underlying structural differences to determine blueberry types best suited to endure long storage times.

Take Home Points

Results suggest:

1. Fruit firmness: SHB had the highest fruit firmness stability over time in the cooler, with SHB maintaining firmness over storage year one and having the smallest decrease in fruit firmness in year two.
2. Fruit skin strength: NHB and SHB maintain their fruit skin strength the best over time in the cooler, and that RE fruit skins experience a larger increase in skin strength (i.e. skin toughness) over time.  These trends suggest that RE don’t maintain their fruit skin strength as consistently over cold storage as do highbush types.
3. Berry size: RE fruit do not change size due to water loss or other factors over cold storage time, and that SHB fruit experience the largest decrease in berry size in 30 days of cold storage.
4. Percent healthy fruit: Both highbush types maintain visual appearance and have reduced fruit decay over time, and RE had the highest amount of fruit decay over storage time.
5. In a big picture summary: During 30 day postharvest storage, the highest stability observed for texture was by SHB, berry weight was by RE, and visual appearance was by SHB and NHB.  These results do not support the subjective bias that NHB has a higher quality that SHB and RE types.

Literature Cited

Barrett, D.M., J.C. Beaulieu, and R. Shewfelt. 2010. Color, flavor, texture, and nutritional quality of fresh-cut fruits and vegetables: desirable levels, instrumental and sensory measurement, and the effects of processing. Crit. Rev. Food Sci. Nutr. 50(5): 369-389.

Bower, C., 2007. Postharvest handling, storage, and treatment of fresh market berries. FoodSci Technol.168: 261.

Chea, S., D.J. Yu, J. Park, H.D. Oh, S.W. Chung, and H.J. Lee. 2019. Fruit softening correlates with enzymatic and compositional changes in fruit cell wall during ripening in ‘Bluecrop’ highbush blueberries. Sci. Hortic. 245: 163-170.

Lara, I., B. Belge, and L.F. Goulao. 2014. The fruit cuticle as a modulator of postharvest quality. Post Harv Bio Technol. 87: 103-112.

Lobos, G.A., P. Callow, and J.F. Hancock. 2014. The effect of delaying harvest date on fruit quality and storage of late highbush blueberry cultivars (Vaccinium corymbosum L.). Post Harv Bio Technol. 87:133-139.

Sams, C.E..1999. Preharvest factors affecting postharvest texture. Post Harv Bio Technol. 15(3): 249-254..

Suzuki, A. and N. Kawata. 2001. Relationship between anthesis and harvest date in highbush blueberry. J JPN SOC HORTIC SCI. 70:60-62.