FOOD ANALYSIS : SOLIDS IN FOOD AND WATER
Everything in a food except water
Fat, protein, lactose, minerals
Only what is dissolved in water
Salts and sugars in solution, in ppm
Total solids is a food value. Total dissolved solids is a water value.
Figure 1: Total solids counts every solid in the food, dissolved or not. Total dissolved solids counts only the solids dissolved in water.
Introduction
Total solids and total dissolved solids sound alike, but they belong to different jobs. Total solids are everything in a food except water, so total solids equal 100 minus moisture: in milk they split into fat and solids-not-fat, and a buyer pays for total solids, not for water. Total dissolved solids are the solids dissolved in water, measured in parts per million, and they matter in water for drinks: a bottling water with total dissolved solids above 500 ppm tastes brackish, and carbonated beverage water has a set tolerance limit.
Quick comparison
Table 1: Total solids compared with total dissolved solids
| Aspect | Total solids | Total dissolved solids |
|---|---|---|
| What it means | Everything in a food except water | Only the solids dissolved in water |
| Where it is used | Foods, above all milk and milk powder | Water for drinks, canning, and ice |
| Unit | Percent of the food weight | Parts per million, ppm |
| Includes | Fat, protein, lactose, minerals, and any suspended solids | Dissolved salts and sugars only: suspended matter is not counted |
| How it is found | Dry a weighed sample in an oven, weigh the residue | Evaporate a measured volume of filtered water, weigh the residue |
| Formula | Total solids = 100 minus moisture percent | Total dissolved solids in ppm = weight of residue in mg per litre of water |
| Milk example | Milk with 12.5 percent total solids and 3.5 percent fat holds 9.0 percent solids-not-fat | Not a milk test: it is a water test |
| Water example | Not a water test | Good bottling water holds less than 500 ppm: above that it tastes brackish |
| Why it matters | Payment, labels, and spotting added water | Taste, clarity, carbonation, and keeping quality of drinks |
Scientific explanation
Definition
Total solids: everything in milk or another food except water, reported as a percentage. Solids-not-fat: total solids minus fat. Total dissolved solids: the solids dissolved in water, reported in parts per million, ppm.
Working principle
Total solids: the water is driven off by drying, and what stays behind is weighed. The dairy handbook gives the total solids test for milk and milk powder: dry a weighed sample in an oven, weigh the residue, and report it as percent total solids. Total dissolved solids: the water sample is filtered to take out suspended matter, a measured volume is evaporated, and the residue left behind is weighed. Only dissolved solids stay in the residue, so the result describes the water itself and not the dirt in it.
Apparatus
Total solids: balance, oven, dishes, and desiccator. Total dissolved solids: filter, evaporating dish, balance, and oven or water bath for evaporation.
Measurement procedure
Total solids: weigh the sample, dry it in an oven, cool it in a desiccator, weigh the residue, and report it as percent total solids. For milk, split the result into fat and solids-not-fat: solids-not-fat is total solids minus fat. Total dissolved solids: filter the water, evaporate a measured volume, dry and weigh the residue, and report the result in ppm.
Factors affecting readings
Total solids: a low solids-not-fat value is one of the signs of added water, because water dilutes the solids without adding any of its own. The sample must come from the same prepared material as the other tests, or the parts will not add up. Total dissolved solids: suspended matter must be filtered out first, or it will be counted as dissolved. High dissolved solids change taste and can spoil carbonation: water with suspended matter will not carbonate readily, and the beverages made from it will rapidly become flat.
Formulas and calculations
Total solids percent = 100 minus moisture percent. Solids-not-fat percent = total solids percent minus fat percent.
Worked example from the FTN Food Analysis notes: a milk with 12.5 percent total solids and 3.5 percent fat holds 9.0 percent solids-not-fat. A second milk tests at 12.8 percent total solids and 3.8 percent fat, so its solids-not-fat is 12.8 minus 3.8 = 9.0 percent. If the usual value for that milk is near 9.0, the milk looks normal: if it were 8.0, added water would be suspected.
Worked example for total dissolved solids, using the Ranganna tolerance values: one litre of bottling water leaves 0.400 g of residue after evaporation. That is 400 mg per litre, which is 400 ppm. It sits below the 500 ppm limit for good bottling water, so the water is acceptable on this measure.
Industrial applications
- Dairies report total solids for milk and milk powder, and buyers pay for total solids, not for water.
- Solids-not-fat is used to spot added water in milk: water dilutes the solids without adding any of its own.
- Drink makers test total dissolved solids in their water, because above 500 ppm the beverage tastes brackish and carbonation suffers.
- Canners and ice makers follow the tolerance limits for their water: the Ranganna table gives 850 ppm for carbonated beverages, 500 ppm for beverages, and 170 to 350 ppm for raw water ice.
Advantages and limitations
Total solids: it is simple, it needs only an oven and a balance, and it gives the value buyers pay for. Its limit is that it says nothing about which solids are present: fat, protein, and minerals are all counted together.
Total dissolved solids: it describes water quality in one number, and it is easy to compare against tolerance limits. Its limit is that it says nothing about which dissolved solids are present, and it leaves out suspended matter entirely, so it must be read with the other water tests.
FAQ
Question: A milk tests at 12.8 percent total solids and 3.8 percent fat. What is its solids-not-fat?
12.8 minus 3.8 = 9.0 percent solids-not-fat. If the usual value for that milk is near 9.0, the milk looks normal.
Question: Why does added water lower solids-not-fat?
Because water dilutes the solids without adding any of its own. A low solids-not-fat value is one of the signs of added water in milk.
Question: What happens when bottling water holds more than 500 ppm of total dissolved solids?
The beverage gets a brackish taste. The Ranganna table gives 500 ppm as the limit for good bottling water.
Question: Is total dissolved solids the same as total solids in milk?
No. Total solids is a food value in percent and counts every solid. Total dissolved solids is a water value in ppm and counts only dissolved solids.
References and further reading
- FTN Food Analysis, Unit 3: Moisture and ash (notes on disk): total solids = 100 minus moisture, milk 12.5 percent total solids, 3.5 percent fat, 9.0 percent solids-not-fat.
- Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, Tata McGraw-Hill (book extract on disk, ranganna.txt): Table 13.4, maximum tolerance limits, total dissolved solids 850 ppm for carbonated beverages, 500 ppm for beverages, 170 to 350 ppm for raw water ice.
- Nielsen, S. S., Food Analysis, Springer: moisture content and total solids (consulted through Google Books).
