SOP: Determination of Total Soluble Solids by Brix Hydrometer
GIFSQ — Global Institute for Food Safety & Quality
Principle
Total soluble solids are estimated from the specific gravity of the juice, which is measured with hydrometers. The Brix (Balling) hydrometer reads per cent sucrose by weight directly; fruit juices contain more sugar than any other soluble constituent, so the reading is a useful guide to soluble solids or sugar content. Soluble solids other than sucrose do not affect specific gravity to the same extent as sucrose, so Brix found by hydrometer differs from Brix found by refractometer.
Apparatus required
- Brix (Balling) hydrometer set, each covering a 10 °Brix range (10 to 20, 20 to 30, 30 to 40, 40 to 50, 50 to 60 °Brix), graduated in 1/10 ° divisions — always use the instrument whose range covers the sample
- Hydrometer jar or cylinder
- Thermometer
Procedure
Bring the sample to, or as close as possible to, the standard temperature marked on the hydrometer stem (usually 20 °C / 68 °F), and note the actual temperature
↓
Pour the well-mixed, filtered sample into the hydrometer jar, free from air bubbles
↓
Lower the hydrometer gently into the liquid and let it settle without touching the sides
↓
Read the Brix value at the level of the liquid surface at eye level
↓
If the test was not made at the calibration temperature, apply the temperature correction from the book’s Table 28-3 (at temperatures above the standard, the observed reading is too low; at temperatures below, too high)
Calculation
Soluble solids, °Brix = observed hydrometer reading ± temperature correction
Related conversions (the book’s tables):
- Rational Baumé scale (continental): the book prints the scale as “Sp. gr. at 45° C (compared to water at 15° C = 1)” with the 144.3 constant — carried exactly as printed in the available copy (the “45” is very likely an OCR misread of 15; check the printed book before critical use).
- American Baumé (Manufacturing Chemists’ Association, for liquids heavier than water at 15.55 °C / 60 °F): Baumé reading = 145 − (145 / sp. gr.), as printed; the book’s reverse expression (“P- St. = 445 — Baume reading”) is garbled in the available copy and is not reproduced here — check the printed book.
- Twaddle: 1 °Tw corresponds to 1.005 specific gravity (each degree Twaddle = 5 units increase in the third decimal of specific gravity), from which °Tw = (sp. gr. − 1) × 200.
Precautions
- The Baume and Twaddle scale formulas as printed are carried with the book’s own garble notes above; verify against the printed book before critical work.
- Always make the test at the temperature marked on the instrument, or apply the tabulated correction.
- Hydrometers used repeatedly in hot syrups lose accuracy — check them frequently against a more accurate instrument; keep duplicate or triplicate sets of accurate hydrometers.
- Hydrometers with long stems and widely spaced divisions are read more easily than short-stem ones.
- For single-strength juices a hydrometer or a refractometer may be used; the two readings differ because hydrometer and refractometer respond differently to non-sucrose solids.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.