Blanching

Enzymes may have undesirable or desirable effects on the quality of fruits and vegetables such as the oxidation of phenolic substances, post-harvest senescence, , starch-sugar conversion, post-harvest demethylation of pectic substances leading to softening of plant tissues during ripening. The major factors useful in controlling enzyme activities are temperature, water activity, pH and the use of chemicals during processing. Blanching is one of the effective ways to control enzyme activity. 

Blanching is a term that usually refers to the very brief cooking of a fresh vegetable prior to freezing. Blanching typically involves vegetables rather than other types of food (like fruit or meat) because vegetables are particularly susceptible to enzyme-triggered changes that can result in lost flavor, lost color, and lost texture during freezing. It is mainly applied to foods before freezing, drying and canning for different purposes. It can be used to loosen fruit or vegetable skins to make peeling easier. Blanching can also be done to remove the skins from certain nuts like peanuts (technically a legume) and almonds. Purposes of blanching are enzyme inactivation, microbial population reduction, tissue gas removal, product shrinkage, Increase peeling efficiency, cleaning, color stabilization and vacuum formation. Blanching time is crucial 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. There are some types of blanching; 

Water Blanching 

For home freezing, the most satisfactory way to heat all vegetables is in boiling water. Use a blancher which has a blanching basket and cover, or fit a wire basket into a large pot with a lid. 

Use one gallon water per pound of prepared vegetables. Put the vegetable in a blanching basket and lower into vigorously boiling water. Place a lid on the blancher. The water should return to boiling within 1 minute, or you are using too much vegetable for the amount of boiling water. Start counting blanching time as soon as the water returns to a boil. Keep heat high for the time given in the directions for the vegetable you are freezing. 

Steam Blanching 

Heating in steam is recommended for a few vegetables. For broccoli, pumpkin, sweet potatoes and winter squash, both steaming and boiling are satisfactory methods. Steam blanching takes about 1½ times longer than water blanching. 

To steam, use a pot with a tight lid and a basket that holds the food at least three inches above the bottom of the pot. Put an inch or two of water in the pot and bring the water to a boil. 

Put the vegetables in the basket in a single layer so that steam reaches all parts quickly. Cover the pot and keep heat high. Start counting steaming time as soon as the lid is on. See steam blanching recommended for the vegetables listed below. 

Microwave Blanching 

Microwave blanching may not be effective, since research shows that some enzymes may not be inactivated. This could result in off-flavors and loss of texture and color. Those choosing to run the risk of low quality vegetables by microwave blanching should be sure to work in small quantities, using the directions for their specific microwave oven. Microwave blanching will not save time or energy. 

Cooling 

As soon as blanching is complete, vegetables should be cooled quickly and thoroughly to stop the cooking process. To cool, plunge the basket of vegetables immediately into a large quantity of cold water, 60ºF or below. Change water frequently or use cold running water or ice water. If ice is used, about one pound of ice for each pound of vegetable is needed. Cooling vegetables should take the same amount of time as blanching. 

Drain vegetables thoroughly after cooling. Extra moisture can cause a loss of quality when vegetables are frozen (Reynolds, 2006, pp. 229-230). 

While boiling any type of fruit or vegetable, it cooks from the outside to in. By putting fruits or vegetables in boiling water for just a minute or two before putting them into ice water allows just the very outer part of the fruit, the part just under the skin, to become soft. Once that happens, it becomes very easy to peel thin skins off of these fruits and vegetables; the skin will easily pull away from the very thin and very soft cooked layer. 

Brightening and fixing color is directly related to the color green in vegetables like broccoli, spinach, kale, green beans, peas, etc. Even though the green of the chlorophyll in those plants is there providing the green color, small air pockets cloud the color. A quick plunge into boiling water bursts some outer cell walls allowing the air to escape and letting the true bright green of the chlorophyll shine through. 

If green vegetables are boiled too long, they turn from a bright, vibrant green to a dull olive green. To prevent this from happening, food that is blanched should be removed to an ice bath when it is still bright green. For tender greens like basil or spinach, this can be as little as 30 seconds. 

For more substantial green vegetables such as peppers, broccoli or green beans, expect to blanch for up to two to three minutes. Blanching not only brightens but it also fixes the color. What this means is that blanched vegetables do not turn brown over time like raw vegetables do. All fruits and vegetables contain enzymes that, if left unchecked, break down the foods into what is ultimately a brown, slimy mess. 

Raising the temperature of the food to over 120°-140°F (depending on the enzyme) essentially denatures or turns off the enzymes so that the food does not brown. 

Blanched vegetables are sweeter and have a pleasing "crisp-tender" texture than the raw vegetables because some of the cell walls will have ruptured, releasing sugars and other flavor compounds. They will be crisp-tender because the outer part of the vegetables will be tender from being cooked while the interior is still crisp and raw-tasting. Crisp-tender is to vegetables what al dente is to pasta. While it takes longer to achieve crisp-tender than it does to blanch to peel, generally speaking, a vegetable will be crisp-tender at about the same time that it is a beautiful bright green color. 

Blanching can turn off enzymatic activity and prevent browning. This is particularly important in freezing fresh fruits and vegetables because enzymes still function at freezer temperatures. The process to blanch foods for freezing is the same as any other blanching. Cell walls of fruits and vegetables will be damaged by ice crystal formation in the freezer. Because of this, once thawed, most fruits and vegetables will become soggy and are best suited to stews and soups (in the case of vegetables) and pies or muffins (in the case of fruits). The faster the food is freezed, the smaller the ice crystals and the less damage to cell walls. Freezing foods in a single layer can minimize damage to the cell walls. (Reluctant Gourmet, 2011, para 1) 

Blanching of vegetables prior to freezing has several advantages, but also a number of disadvantages. The advantages include stabilization of texture, flavor and nutritional quality, destruction of microorganism and wilting of leafy vegetables, which assists in packing. However, since blanching is a heat treatment, changes associated with mild thermal processing can be expected. These include loss of turgor in cells, due to thermal destruction of membrane integrity, and partial degradation of cell wall polymers. A further effect of thermal processing is the degradation of chromophores such as chlorophyll, resulting in color change. Pigment degradation will continue to take place through frozen storage. Blanching can lead to thermally induced degradation of nutrients such as to whether or not the unblanched product has adequate stability to retain structural integrity and quality attributes upon freezing and during frozen storage, and a decision must be reached as to the extent of blanching needed to ensure optimum product quality. Process optimization involves measuring the rate of enzyme destruction, so the blanch time is just long enough to destroy the indicator enzyme. Many processors utilize a heat treatment sufficient to inactivate peroxidase, one of the more stable enzymes present, and not incidentally, one of the enzymes whose activity is relatively easy to measure. Peroxidase leads to a more severe heat treatment than is required for many vegetables and that the enzymes responsible for quality loss, which have been identified, have a lower stability than peroxidase. Therefore, the complete activity loss of peroxidase is not required at the end of the blanching. Generally, a residual activity of peroxidases of about 3-10 % after blanching is recommended. 

Peroxidase 

A peroxidase is one of a number of enzymes that act as catalysts to allow a variety of biological processes to take place. Specifically, they promote the oxidation of various compounds using naturally occurring peroxides, especially hydrogen peroxide (H2O2), which are reduced, forming water. Peroxides are created as byproducts of various biochemical reactions within organisms, but can cause damage as they are oxidizing agents. Peroxidases break these compounds down in to harmless substances by adding hydrogen, obtained from another molecule known as a donor molecule in a reduction-oxidation (redox) reaction in which the peroxide is reduced to form water, and the other molecule is oxidized. There are a large number of these enzymes, and they are found in plants and animals, including humans. 

Like all enzymes, peroxidases are very large, complex molecules with complicated shapes involving multiple folds. They come in a variety of types, some of which can use a wide variety of donor molecules and reduce a wide range of peroxides, and some of which are much more specific. 

The action of Peroxidase is primarily control the level of peroxides, which are generated in almost every reaction in living cells. It constitutes a major activity for the plants to avoid the detrimental effect of free radicals. Beside this one, this enzyme system has gained much attention recently due to its role in modulating the mechanical properties of cell walls during extension. Peroxdase can generate phenolic cross-links connecting biopolymer chains Peroxidases can be found in all parts and organelles of the cell, varying in amount and activity with physiological state. Peroxidases found in plants are iron-containing enzymes. There are many different Peroxidases, and each enzyme differs in structure at the active site by only a small degree. The iron is bound on four sides (horseradish Peroxidases: arginine, phenylalanine, and two histidine) by nitrogen, making up the heme center. In vegetables, Peroxidases are located in soluble form in the cell cytoplasm and in insoluble form as ionically bound and as covalently bound to the cell wall. The bound form can be converted into soluble form by a temperature dependent conversion reaction. All of the POD fractions are present in plant tissues as a combination of various isoenzymes with different thermal stability. The differences in heat resistance of the isoenzymes vary considerably with the vegetable source and origin. (Akyol, 2004, pp. 1-6)

REFERENCES

1. Reynolds S. (2006). So Easy to Preserve. The University of Georgia: Athens.

http://nchfp.uga.edu/how/freeze/blanching.html

2. Reluctant Gourmet (2011). How to Blanch Foods., from http://www.reluctantgourmet.com/blanching.htm

3. Akyol, Ç. (2004). Inactivation of Peroxidase and Lipoxygenase in Green Beans, Peas and Carrots by a Combination of High Hydrostatic Pressure and Mild Heat Treatment. (pp.1-6). Ankara