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Food blanching process
Blanched food manufacturing
The Blanching Process
Author links open overlay panelFrank A.LeeNew York State Agricultural Experiment Station, Cornell University, Geneva, New York
Available online 30 May 2008.
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Blanched food manufacturing
This chapter discusses the process of blanching in the preparation of vegetables for freezing, canning, or dehydration. It describes blanching as a preliminary treatment in the canning process and freezing process. The objective of blanching as a pretreatment of vegetables for canning is the removal of tissue gases; the shrinking of the material so that adequate fills can be contained in the can; and the heating of the material prior to filling so that a vacuum will be obtained after heat processing and boiling. Blanching is necessary as a part of the preparation for freezing preservation to inactivate the enzymes in the tissues and to shrink the material so as to conserve space in packing. The inactivation of the enzymes is very important in this process because no final cook or sterilization is used previous to freezing, and freezing storage, at least at the temperatures commonly employed, does not prevent undesirable deterioration in flavor, odor, and color on the part of the enzymes in the tissues. Many studies have been conducted to determine the effects of steam and water blanching on the vegetables so processed. The relative merits of water-blanching versus steam-blanching have been studied. It seems that steam-blanching is the more effective of the two for the conservation of soluble nutrients. The chapter mentions some of the special techniques for blanching, such as steam pressure and electronic blanching. It presents investigations that have been conducted to determine the causes for the development of off -flavors, colors, and aromas when unblanched and underblanched vegetables are held in frozen storage.
Blanching is a short time thermal process very important to prepare the vegetal raw material. Generally consist of expose the entire product to high temperatures (70-100ºC) for a short period of time. The primary function of this operation is:
- to inactivate or retard bacterial and enzyme action, which could otherwise cause rapid degeneration of quality.
- the expelling of air and gases in the product,
- to soften the product
- to reduce the microbial load
- in some cases, to fix the colour.
Before blanching, the food is preheated. Blanching may be accomplished by direct or indirect heating systems. This may depend on the product. Direct heating is normally made by immersion into hot water, at 80 to 100 °C, or by exposure to live steam. The operation is normally carried out in horizontal chambers. The residence time in the blancher can vary from approximately 1 to 5 minutes depending on the fruit or vegetable being blanched. For some products, direct contact with water is to be avoided so heat-exchangers working with hot water or vapour are applied (European Commission, 2006).
After blanching, the food has to be cooled immediately, using either water or air, to prevent excess of heating.
Food blanching parameters
- Blanching is a high energy demanding operation within the process of fruits and vegetables. Energy consumption in blanching can be 30-40 % of total energy consumed in the process.
- The main blanching technologies are: hot water blanching and steam blanching, both working in a range of temperature near 100º C.
- Blanching is used in most vegetables destined for canning, freezing or drying. Typically, it is carried out using hot water or steam. If the produce is to be frozen, blanching is followed by water or air cooling. Energy consumption depends on, not only the type of blanching device, but also the type of subsequent cooling step.
- In terms of energy consumption, the belt blancher with water cooling has the lowest total consumption. The heat released by the cooling of the product in the cooling zone is used to preheat the vegetables. In this way, less steam is necessary for blanching.
- The electricity consumption of the belt blancher with air cooling, which produces 7 to 30 kWhe/t of frozen product, is significantly higher than that of the belt blancher with water cooling, which produces 2 to 9 kWhe/t of frozen product, or the drum blancher with counter current water cooling, which produces 1 to 2.6 kWhe/t of frozen product (European Commission, 2006)
Vegetable blanching directions and times for food manufacturing
- Food, health and nutrition
- Food safety
- Preserving and preparing
- Vegetable blanching directions and times for home freezer storage
Blanching is a must for most vegetables to be frozen. It slows or stops the enzyme action which can cause loss of flavor, color and texture.
The blanching time is very important and varies with the vegetable and size.
- Underblanching stimulates the activity of enzymes and is worse than no blanching.
- Overblanching causes loss of flavor, color, vitamins and minerals.
- Wash, drain, sort, trim and cut vegetables.
- Use 1 gallon water per pound of prepared vegetables or 2 gallons water per pound leafy greens.
- Put vegetables into blancher (wire basket, coarse mesh bag or perforated metal strainer) and lower into boiling water.
- Or steam blanch: boil 1-2 inches of water in a pot, bring to boil and then put a single layer of vegetables in basket.
- Cover. Start counting blanching time as soon as water returns to a boil.
- Or if steam blanching, start counting immediately.
- Keep heat high for the time given in the directions.
- Cool immediately in ice water or cold water (60 degrees F or below) for the same time used in blanching (except for corn-on-the-cob for which cooling time is twice the time of blanching). Stir vegetables several times during cooling.
- Drain vegetables thoroughly.
- Pack the vegetables either by dry-pack or tray-pack.
- Dry-pack: Pack vegetable tightly into containers or freezer bags. Press out air and seal tightly.
- Tray-pack: Put a single layer of the vegetable on a shallow pan and put the pan into the freezer. As soon as the vegetable is frozen, put them into a freezer bag or container. Press out air and seal tightly.
- Frozen vegetables will maintain high quality for 8 to 12 months at zero degrees F or lower.