Concentration, evaporation and non-thermal methods
Concentration removes part of the water to make puree, juice concentrate, condensed milk or khoa. Vacuum evaporators boil off water at low temperature, saving colour and flavour.
What concentration does
Concentration removes part of the water from a liquid food, while drying removes most of it. It cuts weight and volume (cheaper storage and transport), is a first step before drying (milk is concentrated before spray drying), makes new products (khoa, jaggery, tomato paste) and helps preservation, because higher solids mean lower water activity. On its own it rarely preserves, so it is combined with sugar (sweetened condensed milk), heat sterilisation (canned evaporated milk), acid or freezing (frozen juice concentrate). Aroma lost during evaporation is often recovered and added back to juice concentrates.
Methods of concentration
| Method | How it works | Examples | Note |
|---|---|---|---|
| Open-pan evaporation | Boiling in open pans or kettles at 100 °C or above | Khoa, basundi, jaggery, small-scale tomato puree | Cheap; browning, cooked flavour, slow |
| Vacuum evaporation | Boiling under reduced pressure, so water boils well below 100 °C | Tomato paste, condensed milk, juice concentrates | Better colour and flavour; costly |
| Multiple-effect evaporation | Vapour from the first vessel heats the next, each at lower pressure | Large dairies, sugar mills, juice plants | Large steam savings |
| Freeze concentration | Water is frozen out as pure ice crystals, which are removed | Fruit juices, coffee extract | Best aroma; costly; limited strength |
| Membrane methods: reverse osmosis (RO), ultrafiltration (UF) | Pressure pushes water (RO), or water and small molecules (UF), through a membrane | Pre-concentrating juice and milk; whey proteins | No heat; cannot reach high strength |
Worked example: concentrating tomato juice
100 kg of tomato juice at 5% total soluble solids (TSS) is concentrated to a puree of 12% TSS. How much puree is obtained and how much water is evaporated? How much paste would you get at 25% TSS? (12% and 25% are example strengths; check your current product standard for legal minimums.)
- Solids in the juice = 100 × 5 ÷ 100 = 5 kg; solids do not change.
- Puree weight = 5 ÷ 0.12 = 41.67 kg (rounded to 2 decimals).
- Water evaporated = 100 − 41.67 = 58.33 kg.
- Paste at 25% TSS = 5 ÷ 0.25 = 20 kg, so 100 − 20 = 80 kg of water is removed.
- Check: 5 ÷ 41.67 = 0.12, i.e. 12%.
About 41.7 kg of puree (58.3 kg of water evaporated); at 25% TSS only 20 kg of paste (80 kg of water removed).
Non-thermal methods of preservation
- High-pressure processing (HPP): packed food is pressed in water at very high pressure; kills growing microbes without heat; used for fresh-tasting juices, dips and ready meals.
- Pulsed electric field (PEF): very short high-voltage pulses punch holes in microbial cell membranes; for juices and milk.
- Ultraviolet (UV) light: treats water, surfaces and clear juices.
- Ultrasound, ozone and cold plasma: newer ways to wash produce and kill surface microbes.
- Membrane filtration (microfiltration): physically removes bacteria from milk and juice.
- Irradiation (Unit V) is also a 'cold' method.
- These keep fresh-like flavour and vitamins, but most do not kill spores, so the products are usually kept chilled.
Concentrates such as tomato puree, juice concentrate and khoa pack nutrients into less weight, which suits hospital and institutional kitchens. Vacuum and membrane methods keep more vitamin C and colour than open-pan boiling.
2 marks: “What is the advantage of vacuum evaporation?” Under reduced pressure water boils well below 100 °C, so colour, flavour and nutrients suffer less heat damage.
5 marks: “Write a note on preservation by concentration and evaporation.” Meaning and aims; methods (open pan, vacuum, multiple effect, freeze concentration, RO and UF); products (puree, paste, condensed milk, juice concentrate, khoa, jaggery); combined with sugar, heat or freezing.
Concentration takes part of the water out; drying takes most of it. Vacuum = low-temperature boiling.