Factors Influencing Measures of °Brix (Soluble Solids), an Indicator of Potential Crop Quality

°Brix readings continue to interest and confuse farmers and others. Collecting a reading is far easier than making decisions based on it. In fact, it takes just moments to obtain a °Brix (soluble solids) reading in the field, packing shed, or elsewhere; the major steps include collecting a small drop of plant sap or juice and placing it on a properly maintained and used refractometer, a handheld instrument that fits easily in your pocket. A reading typically can be in hand in less than two minutes. However, making proper use of the °Brix value requires effort and experience for reasons outlined below.

The sugar sucrose is perhaps the most prevalent soluble solid in plant juice. Therefore, many vegetable-based °Brix (refractometer) readings are set primarily by the number of sucrose molecules in the sap or juice used as the sample (unless the sample is contaminated). Within a crop, these sucrose levels are, in turn, influenced by:
1. Variety;
2. Plant population/density;
3. Irrigation or soil moisture status;
4. Nutrient management or soil fertility status;
5. The plant part sampled (e.g., root, stem, leaf, fruit) and exact portion of it;
6. The age (maturity, position) of the plant part sampled;
7. Time of day of sampling;
8. Temperature and light conditions;
9. Post-harvest conditions; and
10. Other factors.

Not surprisingly, experienced refractometer users understand that it is essential:

1. To use a standardized, consistent approach involving sampling the same plant part (and portion) at the same development stage at the same time of day, etc. That way, comparisons based on other factors are more reliable.
2. To obtain and record many values (the process is nearly free minus small initial investments). Much like measures of blood sugar, cholesterol, heart rate, etc, the worth of one °Brix reading in decision-making is often based on comparing it to readings collected previously and the conditions under which they were collected.

We have measured °Brix levels in vegetable crops grown on Ohio farms and at OSU research stations for nearly twenty years using protocols explained in fact sheets at https://u.osu.edu/vegprolab/research-areas/product-quality-2/ and taking factors listed above into account. The data below were collected in July-November 2011 by Dr. Natalie Bumgarner (then a graduate student at The OSU and now with Cooperative Extension at the University of Tennessee). Note the variation within and across crops.

Contact Matt Kleinhenz (330.263.3810; kleinhenz.1@osu.edu) for more information.

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