USDA determines grades of strawberry fruits by size and defaces (USDA AMS). However, common parameters for defining fruit quality of greenhouse strawberry are 1) sweetness, 2) acidity, 3) firmness, 4) color, 5) shape, and 6) size. These attributes are influenced primarily by cultivar (genotype). Environmental conditions of both aerial and root zones have also significant influence on fruit quality. While more research is needed in this area, the following is our current understanding of factors affecting these quality attributes of strawberry fruit.
Flavor density, acidity, and sweetness
A widely used measurement to evaluate sugar content is Brix. Brix is an accurate measurement (unit: %) when sucrose is the only solute in water. Fruit juice typically contains other soluble solids (glucose, fructose and organic acids) that affect the Brix measurement. Therefore, Brix is only an indicator of overall flavor density and is often referred to as ‘soluble solid content’ instead of sugar content or sweetness. The higher the Brix, the denser the fruit flavor. UC Davis Postharvest Technology Center recommends acceptable Brix for strawberries in US market is 7 or higher. However, in our opinion, Brix 7 is too low and we should aim to achieve 9 for greenhouse strawberry market (considering high consumer expectation for fruit quality). In our previous studies, Brix of ‘Albion’, ‘Nyoho’, and ‘Portola’ fruit recorded were 8.9 ± 0.2, 10.6 ± 0.1, and 5.8 ± 0.2 during the production seasons of 2014-2015 (‘Albion’ and ‘Nyoho’) and 2013-2014 (‘Portola’) in Arizona. While ‘Portola’ fruit was very fragrant and yield was outstanding (30% more production than ‘Albion’), the low Brix may be problematic to successfully introduce it in a potential premium market of greenhouse strawberry.
Acidity is an important attribute of strawberry fruit. A typical measurement is total titratable acidity (TA), which is expressed as citric acid equivalent acidity for strawberry. UC Davis Postharvest Technology Center recommends desirable TA for strawberries in US market is 8 g/L or lower. In our previous studies, TA of ‘Albion’, ‘Nyoho’, and ‘Portola’ fruit recorded were 10.1 ± 0.2, 10.4 ± 0.1, and 8.6 ± 0.2 g/L during the production seasons of 2014-2015 (‘Albion’ and ‘Nyoho’) and 2013-2014 (‘Portola’) in Arizona. Acidity is highly affected by temperature and the impact of nighttime temperature is especially significant. For example, ‘Albion’ fruit recorded during 2017-2018 Ohio season was at a lower TA, 8.7± 0.2 g/L, while Brix was at a similar level (9.4± 0.2) as those recorded in Arizona.
The ratio between Brix (soluble solid content) and TA (acidity) seems to represent the overall sense of sweetness and therefore, favorable taste of strawberry fruit. When the ratio of Brix and TA exceeds 1.0, strawberry fruit seems to be perceived as sweet. Growing environmental conditions affect the ratio within the potential ranges determined by cultivars. As mentioned earlier, ‘Albion’ fruit grown in Ohio had lower TA than those in Arizona, resulting in higher Brix/TA ratio (9.4/8.7 = ~1.1) in Ohio than in Arizona (8.9/10.1 = ~0.9) and noticeably greater sense of sweetness and improved flavor of the fruit.
Color and size
Red pigments in strawberry are anthocyanins and therefore fruit grown under UV-deficient light environment tends to develop lighter color within the range determined by cultivar. Greenhouse glazing has varied transmission of UV light and the highest transmission of UV-B (the most biologically effective UV light) is a UV-transmitting ETFE film, followed by glass. Polycarbonate or UV-rated polyethylene film transmits very little UV light and therefore they are not recommended for strawberry production.
Size of flower and therefore fruit are largely affected by temperature, within the ranges determined by cultivars. The lower the daily average temperature, the larger the flower and fruit become. Fruit on the primary truss is the largest fruit and fruit size becomes smaller for the secondary and tertiary trusses. Size is also highly correlated with number of fruit developed per truss. Therefore, cultivars that produce many flowers on trusses are generally characterized by small fruit size (e.g., ‘Nyoho’ and ‘Elsanta’). US market tends to prefer large fruit (15-20 grams or more). In our previous experiments, ‘Albion’, ‘Nyoho’, and ‘Portola’ average fruit size recorded was 21.6 ± 0.2, 11.7 ± 0.1 and 24.4 ± 1.2 grams during the production seasons of 2014-2015 (Albion and Nyoho) and 2013-2014 (Portola) in Arizona.