Blanching is a cooking process in which fruits/vegetables are immersed in hot water, removed after a short time and put into iced water to halt the cooking process.
Blanching helps to remove peel of fruits, modify their texture and to inactivate certain enzymes. Blanching is also used as pre-heat treatment before drying, freezing or canning of fruits and vegetables.
Some possible undesirable effects of blanching are damage to tissue cells, loss of colour, leaching of water soluble and heat sensitive nutrients etc due to heat treatment. To avoid this, low temperature process may be followed but it needs longer process time than a high temperature condition.
For large samples, hot steam has been considered an efficient heat source for the blanching. Blanching sometimes also refers to preserve color, flavor, and nutritional value of fruits/vegetables.
Usually for almost all vegetables to be frozen, blanching is essential step. In this case, blanching stops some enzyme actions which otherwise may cause loss of colour, flavor and texture of such vegetables.
Blanching also cleans the fruit/vegetable surface and so dirt and microorganisms are removed thus decreasing microbial load. This will helps in brighten the colour and to retard the vitamins loss.
Example: Green beans and carrots when blanched for optimum time turn them into vivid green and orange colour respectively.
The process of blanching is used in home kitchen and in food industry too as a pretreatment.
As discussed above, the sole purpose of blanching is to inactivate enzymes that otherwise cause browning, off flavor and textural changes. Some of enzymes that cause deterioration are lipoxygenase, polyphenoloxidase, polygalacturonase etc.
So it would correct to say that blanching preserves the flavors in fruit/vegetables by inactivating such enzymes responsible for off flavor development. Lipoxygenase is common enzyme responsible for off-flavor development in many vegetables.
As blanching removed trapped air inside fruit/vegetable tissues (e.g. pears), it results in better texture and reduces oxidation of the product. This inter cellular gas expelling also results in better color retention.
Blanching of some nuts (e.g. Almonds and pistachio) helps softening and easy removal of outer skin. Some more uses of blanching mentioned in literature are for enhancing drying rate and product quality, removing pesticide residues, increasing extraction of bio-active compounds, decreasing microbial load, surface cleaning etc.
In general, blanching is done using either a hot water or by saturated steam as the heating medium. Fruits/vegetables are heated to short time in both these methods and then immersed into cold water to halt the cooking process.
At commercial level, fruits/vegetables to be blanched moves on a continuous conveyor belt which moves through preheating, holding and cooling. Following are different methods of blanching.
- Hot water blanching
- Steam blanching
- Blanching with steam and water system
- Pipe blancher
- Fluidized bed blanching
- Individual quick blanching
- Vacuum steam blanching
- In-can blanching
- Microwave blanching
- Hot gas blanching
Nutritional quality of blanched product
Though blanching process is relatively mild and its effect on food product is relatively small as compared to other thermal treatments, still blanching may reduce quality attributes of food products to some extent.
The effect of blanching on nutrients in fruit and vegetables depends on several factors. Ratio of surface area to volume for individual fruit/vegetable pieces is one such factor. This factor also responsible for colour and flavour retention in selected fruits/vegetables.
Most pigments in fruits and vegetables are temperature sensitive.Texture of fruits and vegetables also affected by blanching process. Blanching process at high temperature and short time would result in retention of quality attributes of fruits/vegetables to be blanched.
Individual quick blanching (IQB) is suitable in this case. In this process individual pieces has maximum exposure to heating medium during a rapid heating process. More information on how blanching affects micro nutrients, vitamins, minerals etc can be read here.