FOOD PROCESSING AND PRESERVATION: THERMAL PROCESSING
Blanching in four steps
Hot water or steam
1 to 5 minutes, enzymes switched off
↓
Fast cooling
Cold water stops the cooking at once
↓
Check the enzyme
Peroxidase test: still active means too short
↓
Freeze or process
Blanching alone does not preserve the food
Figure 1: Blanching is a short preparation step before freezing, drying, or canning. Boiling is a longer cook in water at 100 °C. The four boxes are steps in order, so the arrows show a real sequence.
Introduction
Blanching and boiling both put food in hot water, but they are different jobs. Blanching is a short dip in hot water, or a short burst of steam, usually 1 to 5 minutes, followed by fast cooling in cold water. Its job is to switch off enzymes before freezing, drying, or canning. Boiling is cooking food in water at 100 °C until it is cooked through, which kills growing microbes but not spores, and softens the food. Blanching is a preparation step, not a preservation method on its own; boiling is a cooking method that also makes food safer to eat at once.
Quick comparison
Table 1: Blanching compared with boiling
| Aspect | Blanching | Boiling |
|---|---|---|
| Purpose | Switch off enzymes before freezing, drying, or canning | Cook food through and kill growing microbes |
| Time | Usually 1 to 5 minutes | Minutes to over an hour, until cooked |
| Temperature | Hot water or steam, then fast cooling in cold water | Water at 100 °C, held at a rolling boil |
| Cooling after | Fast cooling in cold water is part of the method | Food is served hot, or cooled for later use |
| Enzymes | Switched off: that is the main job | Switched off, but that is not the main aim |
| Microbes | Cuts the surface microbe load, but does not sterilize the food | Kills growing cells, but not bacterial spores |
| Texture | Slightly softened, colour kept bright | Fully softened, often much softer |
| Vitamin loss | Small, if timed right | Larger: vitamins break down and leak into the cooking water |
| Typical foods | Green beans, peas, and other vegetables before freezing | Potatoes, beans, pasta, rice, and vegetables cooked for the table |
Scientific explanation
Definition
Blanching is a short hot water or steam dip, usually 1 to 5 minutes, given to vegetables and fruit before freezing, drying, or canning. Boiling is cooking food in water at 100 °C, the temperature at which water boils at normal pressure.
Working principle
Blanching works on enzymes. Its jobs are to switch off the enzymes that would cause browning and off-flavours in frozen storage, to soften the tissue slightly, to clean the surface, and to drive out air from the spaces inside the food. It also wilts leaves so they pack well and cuts the surface microbe load. Blanching is judged by enzyme tests, not by taste: if the enzyme peroxidase is still active, the blanch was too short, and the frozen vegetable will go stale in storage even though it looks fine on the day. If the blanch is too long, the vegetable loses colour, flavour, and vitamins. The fast cooling after blanching matters as much as the heating: slow cooling lets the food sit warm, which is exactly where microbes grow fastest. Boiling works on microbes and texture. Heat kills by unfolding proteins, leaking membranes, and damaging DNA, so boiling kills growing cells. But a bacterial spore survives boiling water, which is why boiling alone cannot sterilize low-acid food.
Apparatus
Blanching uses a blancher: a vessel of hot water or a steam chamber, with a basket that lifts the food out on time, and a cold-water bath for fast cooling. Boiling uses a pot of water on a heater, with the food held at a rolling boil until cooked. In a factory, blanching is done in continuous belt blanchers, where the food moves through hot water or steam on a belt and then through cold water.
Measurement procedure
Blanching is checked with the peroxidase test: a sample is tested for the enzyme, and if peroxidase is still active, the blanch was too short. Time and temperature are recorded for each batch, and the cooling step is timed too. Boiling is checked by temperature and time: the water must be at a full boil, and the food must be held there long enough to cook through to the centre. The cold spot, usually the centre of the thickest piece, decides whether the food is cooked.
Factors affecting readings
Piece size changes both methods: thin pieces blanch and boil fast, thick pieces need longer, and their centres lag behind. Under-blanching leaves enzymes alive, and the food spoils in store. Over-blanching cooks the vegetable, loses vitamins into the water, and softens the texture. For boiling, vitamin C and the B vitamins are the most heat-sensitive: they break down in hot water and leak into cooking water, which is why steaming keeps more of them than boiling, and why a short hot treatment loses less than a long mild one. A vegetable boiled for an hour loses much and gains little over a shorter cook.
Formulas and calculations
Neither blanching nor boiling has a standard formula given in the books for this comparison, so this section states that plainly instead of inventing one. The numbers that do govern both methods are the times and temperatures: blanching is usually 1 to 5 minutes in hot water or steam, and boiling is at 100 °C. The peroxidase test, not a formula, is the check that a blanch was long enough.
Industrial applications
- Blanching: vegetable freezing plants, where peas, beans, and carrots are blanched before blast freezing; fruit and vegetable drying lines; canneries, where vegetables are blanched before filling.
- Boiling: cooking of potatoes, pasta, rice, and vegetables for the table and for ready meals; boiling of jam and syrups; the boiling-water bath used to process acid foods such as fruit, pickles, and tomato products in canning.
Advantages and limitations
Blanching, advantages: switches enzymes off, so frozen food keeps its colour and flavour; short, so vitamin loss is small; cleans the surface and drives out air. Limitations: does not preserve food on its own; must be timed by the peroxidase test; under-blanching leaves enzymes alive and over-blanching cooks the food.
Boiling, advantages: simple and cheap; kills growing microbes; softens food and makes it easy to eat. Limitations: does not kill spores; vitamins leak into the cooking water; long boiling dulls colour, softens texture, and loses flavour.
FAQ
Does blanching preserve food? No. Blanching alone does not preserve food. It switches enzymes off before freezing, drying, or canning, and the later step does the preserving.
How do you know a blanch was long enough? By the peroxidase test. If the enzyme peroxidase is still active, the blanch was too short, and the frozen vegetable will go stale in storage.
Why must cooling be fast after blanching? So the food does not sit warm, where microbes grow fastest, and so it does not overcook.
Can boiling sterilize food? No. Boiling water at 100 °C kills growing bacteria but not bacterial spores, so low-acid foods need pressure heating at 115 °C to 121 °C to be sterilized.
Why does steaming keep more vitamins than boiling? Because vitamin C and the B vitamins break down in hot water and leak into cooking water, while steaming uses less water and shorter contact, so less is lost.
References and further reading
FTN subjects: Principles of Food Preservation, Unit 2 (Preservation by Heat); Principles of Food Processing, Unit 2 (Heat Transfer and Thermal Processing). Books used: Fellows, P. J., Food Processing Technology: Principles and Practice; Frazier, W. C., and Westhoff, D. C., Food Microbiology; Rahman, M. S., Handbook of Food Preservation.
