Blanch, Blanching or Blanched - Defined with Recipe

Blanching Pan

Blanching Pan © 1869 The Royal Cookery Book

Blanch, Blanching or Blanched—Foods placed to boil in cold water, removed after corning to the boil, poured into a colander and well washed; and in the case of almonds, etc., the skins are then easily removed. To scald. It means to whiten, literally. To blanch almonds is to scald and peel them; to blanch parsley, chives, shallots and herbs is to plunge them a minute in boiling water that they may not go into the sauce raw.

Blanching, in culinary matters, consists in putting any article for a few minutes in warm water. Almonds are blanched by putting them first into boiling, and then into cold water. The skins are then removed very easily.

Blanching Nuts. — Almonds and pistachio nuts are commonly blanched to remove the dark outer skin. To blanch nuts, cover with boiling water, and allow to stand until the skins loosen. Drain off the hot water and plunge the nuts into cold water. The skins can be easily rubbed off.

Do not try to blanch too many nuts at a time. It is difficult to handle more than about two cups in one blanching. If the nuts remain too long in the water they become soaked. If they are removed from the water and stand before having their skins rubbed off, they become dry and the skins stick.

Almonds should always be dried in the oven after being blanched. Pistachio nuts should never be put in the oven after blanching, as the heat spoils their bright green color.

TO BLANCH ALMONDS. Immerse the almonds into boiling water for a few minutes, but don’t let boil, as this would turn tbem yellow. When the skin comes off easily drain them and cool off in cold water, then drain again and remove the skins. Place them in a warm place to dry.


Where production is on a substantial scale, power driven mechanical blanching machines are strongly recommended. This type of equipment brings the almonds in contact with one or two pairs of rubber-laced rolls running at different speeds.

Skins as removed are carried away by suction or air blast and the blanched almonds are discharged onto sorting tables or into suitable containers prior to drying.

As stated in the discussion under grading for size, blanching machines are designed to operate with properly graded almonds, their efficiency otherwise being very low.

It should be clearly understood, however, that a one hundred per cent blanch is never accomplished by any machine and that certain hand work must be done in this connection.

This is accounted for by the fact that a machine capable of handling any volume of almonds cannot be made so mechanically delicate that it will adjust itself to the slight variations of each almond.

Then again, the nature of the operation itself is such that any mechanical device is bound to break or split a certain per cent of the almonds passing chrough.

Such breakage may be attributed to irregular feeding of the rolls, to the rolls being set too close for some almonds, and to broken kernels which always occur in any lot of mechanically shelled almonds.

The desirability of blanching (scalding) peas, snap beans, and lima beans before they are placed in the can or are frozen is now generally accepted. In the case of canned vegetables, the blanching process has commanded the attention of several investigators, yet the changes caused by blanching in the tissues and in their behavior during subsequent operations is far from clear.

Blanching is an effective cleansing operation which removes adhering pieces of foreign plant materials and also reduces the bacterial flora of the vegetables. In several cases blanching also removes certain constituents which are undesirable from the flavor standpoint or prevents the occurrence of detrimental changes in color, odor, and flavor associated with the unblanched material. Finally, the use of blanching has been advocated because the resulting wilting of tissues permits better filling of the cans and also removes most of the intercellular gases.

The extent of blanching which will give the best overall results is at present debatable. The general tendency is towards shorter periods of blanching at higher temperatures as contrasted with longer blanching at comparatively low temperatures. Several studies indicate that in such preliminary thermal treatment of plant material it is important to pass with maximum rapidity through the temperature region where tissue structures are destroyed without a simultaneously complete inactivation of the enzymes present.

The investigations reported in this bulletin had as their first objective establishing the progress of the inactivation during blanching of some enzymes in peas, snap beans, and lima beans. Second, they were designed to show some of the chemical changes which occur in these vegetables during blanching at different temperatures for various lengths of time, and, if possible, to correlate the behavior of enzymes with these chemical changes.

The holding of vegetables in the canning factory after blanching but before processing is often necessary because of uneven deliveries of the raw material, breakdown of machinery, and other unavoidable delays. Therefore, a third objective of this study was to determine the extent of losses and changes in various chemical constituents, especially in ascorbic acid (vitamin C), during such holding and to note the influence of the degree of blanching on these changes.

Although the present study yielded some new and interesting results, it will be seen that most observations support the conclusions drawn from previous investigations. We have felt that it is desirable to record at least some of our experiments in considerable detail because both the number of time-temperature variants and the types of observations made in any given test run are much more extensive than those found in the literature. It was hoped that in addition to supporting the conclusions drawn from the present study, such detailed data will aid future investigators of this topic in the selection of experimental conditions and methods.

The desirability of blanching vegetables before canning is now generally accepted. At the same time it is also clear that the blanching operation will remove or destroy some of the vitamins and nutrients originally present in vegetables. The problem, therefore, is to find blanching conditions which will give the most desirable product from the quality standpoint with the least harm to the vitamins and nutrients.


Peas, snap beans, and lima beans were blanched on a pilot plant scale at various temperatures and for different lengths of time. The progress of the inactivation of the enzymes catalase, peroxidase, and ascorbic acid oxidase was followed and the changes which occurred in several chemical constituents were determined. To clarify the nature of these changes, blanched peas were held at room temperature for several hours and samples removed periodically for analyses.

Of the enzymes studied, catalase was easiest to inactivate, peroxidase was the most stable, while ascorbic acid oxidase showed an intermediate heat lability between catalase and peroxidase. No relationship could be established between the inactivation of any of the enzymes and of various chemical changes.

Dry matter and unit weight changes in peas during blanching indicate a progressive loss of solids and a simultaneously increased imbibition of water. Prolonged blanching increased the removal of solids while the blanching temperature was only of minor importance as exemplified by the changes in sugar content. The degree to which solids were affected by blanching peas varied from large losses of sugar and alcohol-soluble nitrogenous compounds to negligible changes, if any, in alcohol-insoluble solids and crude fiber.

The loss of ascorbic acid from peas during blanching is caused both by the destruction of ascorbic acid by some enzyme system, or systems, in the pea and by leaching. In this study the loss of ascorbic acid in raw or adequately blanched peas during holding for 2 hours at room temperature was negligible. At certain intermediate temperatures (critical temperatures) where the heating was inadequate for enzyme inactivation but sufficient to cause “thermal maceration”, the ascorbic acid losses during holding were rapid and extensive. In the present experiments peas blanched at 160° F for 1 to 7 minutes and at 175° F for 1 minute showed this phenomenon of “thermal maceration”. Where losses were due to leaching as indicated by stable ascorbic acid values during holding, the length of the blanch determined the extent of loss more than did the water temperature.

Extensive losses of ash constituents from peas and snap beans were observed during blanching. Again, the length of the blanching treatment appeared to have a greater influence on the extent of ash removal than did water temperature. Calcium, an important ash constituent, increased with all blanching treatments, undoubtedly by the combination of various constituents of the tissues and the calcium in the wash and blanching waters.

In beans, catalase showed a varying degree of heat resistance, in one case being completely destroyed at 165° F in 6 minutes and in other cases requiring 4 minutes at 180° F for total inactivation. Peroxidase was extremely heat resistant in beans, invariably requiring 4 to 5 minutes of heating at 210° F for complete inactivation. Ascorbic acid oxidase required a temperature of 180° F for 4 to 6 minutes for complete inactivation.

Observations on crude fiber, alcohol-insoluble solids, dry matter, ash, and total nitrogen of snap beans revealed that some of the changes in these constituents were caused by imbibition of water rather than by leaching. Approximately half of the apparent losses of sugars were real. Blanching caused a small fraction of the soluble nitrogen to be converted to the insoluble state. Total calcium is increased by a contribution from the blanching water.

After the tissue has been disrupted by heat (“thermal maceration”), the ascorbic acid of snap beans is especially liable to enzymic oxidation in under-blanched material and to non-enzymic oxidation in adequately blanched beans. These two types of oxidation were well demonstrated by holding the blanched beans at room temperature. No corresponding losses of ascorbic acid occurred in unblanched snap beans.

The less extensive experiments conducted on lima beans indicated a smaller extent of loss of ascorbic acid from this vegetable than from peas or snap beans. Considerable losses occurred in the alcohol-soluble nitrogenous fraction during blanching.

Portions of text from J. C. Moyer, W. B. Robinson, E. H. Stotz, and Z. I. Kertesz, in Bulletin No. 754: Effect of Blanching and Subsequent Holding on Some Chemical Constituents and Enzyme Activities in Peas, Snap Beans, and Lima Beans. GENEVA, N. Y.: NEW YORK STATE AGRICULTURAL EXPERIMENT STATION, CORNELL UNIVERSITY, October 1952.

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