2019 ASHS Annual Conference

Titratable acidity provides a simple assessment of the potential organic acid content of fruit values and can be related to the sensory perception of sweet and tart. Determining titratable acidity is slow, requiring neutralization of a known dilution of juice/puree with a titrant to reach a colorimetric or pH endpoint. Automated titrimeters speed up analysis, but are expensive and require ~10 min per sample to generate accurate results. Recently digital acid refractometers (acid meters) have become available for fruits and beverages. A very dilute sample solution (1:50) is placed on the acid meter and gives a reading within seconds. Adaptation for regular use has been slow as acid meter readings need to be compared with a titrimeter for accuracy. An acid refractometer (PAL-BX/acid F5, Atago), equipped with settings for 5 commodities of citrus (1), grape (2), tomato (3), strawberry (4) and blueberry (5) and yields results as % titratable acidity in citric acid equivalents was compared for fruits. A dilute solution of 1:50 made with fruit purees was tested on the acid meter and automated titrimeter (862 Compact Titrosampler, Metrohm). Assorted tomato (dominant organic acid= citric acid) purees from breaker, orange, and ripe stages, yielded a linear relationship with R² of 0.65. There was one outlier reading that persisted despite re-diluting and running the puree. For muscadine grape (‘Cowart’), where citric, malic, and tartaric acid are present, a linear regression with a fit of R² =0.98 was found between the acid meter (blueberry setting) and automated titrimeter. The blueberry setting was best for blackberry varieties with dominant organic acids of isocitric and citric, yielding a linear regression of R²= 0.62. For raspberry (‘Dorman Red’) and black raspberry (‘Jewel’), dominant in citric acid, the citrus setting was best, R² =0.80. Using green, red, ripe, and overripe rabbiteye blueberries, where malic acid dominates, a linear fit with R² =0.95 was found but acid meter values were 30% higher than titrimeter readings. Benefits of the acid meter are low cost (about USD 1000), pocket size portability, ease of use, lack of hazardous chemicals, rapid measurement, and ability to use under rudimentary conditions. Drawbacks of the acid meter include decreased sensitivity, an occasional high or low reading, and a need to know what the dominant organic acid as a means to adjust for titratable acidity. Overall, the acid meter provides a rapid and feasible means for collection of large data sets of titratable acidity.