This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Cellulose forms the structural, fiber, or woody part of the plant. Other substances such as pectic substances may occur in combination with cellulose. In the young plants or the new growth of older plants the cell walls are at first yielding. As they mature, they grow more resistant and may change physically and chemically, their function becoming more specialized with the occurring changes. In tree trunks some cell walls become woody or lignified. Doree and Barton-Wright state that the stone cells distributed throughout the flesh and particularly near the core of pears are lignified cellulose.
Cellulose is a carbohydrate. It is found in plants in several forms. There is no unanimity of opinion regarding its definition, although the cotton fiber is always taken as a standard cellulose product.
Hemicellulose, wood cellulose, and gelatinized cellulose are names given to different forms of cellulose. Probably each group includes several products. Schorger defines hemicellulose as "A polysaccharide soluble in dilute alkalies and convertible into simple sugars by heating with dilute acids at atmospheric pressure." He adds that a hemicellulose in the natural state should be insoluble in boiling water.
Effects of alkalies, acids, and calcium on the structural or woody part of fruits and vegetables. Ammonia and sodium bicarbonate added to the water in which vegetables are boiled, or ammonia or ammonium carbonate added to water in which vegetables are steamed, causes them to soften in a shorter time than if these substances had not been added. With longer cooking the vegetable becomes mushy and disintegrates. The disintegration begins with the surface layer of the food, and its depth depends on the length of time of cooking and the size of the piece of food.
If vinegar is added to the water in which vegetables are cooked the effect is opposite from that of sodium bicarbonate: a firmer and more solid texture is obtained and the vegetable requires longer to cook. In cooking vegetables like cabbage and spinach the amount of calcium and magnesium salts found in the water seems to have little effect upon the texture or the length of time for cooking.
The pulp and white part of the rind of watermelon as well as the pulp of muskmelon or cantaloupe, and perhaps other foods, can be made very firm and brittle or woody if soaked in saturated calcium hydroxide (lime) water. The longer they are soaked the harder and firmer they become if the water is kept saturated with lime. Five to six hours' soaking is usually sufficient to produce enough firmness to prevent shriveling when cooked in sirups for preserves or pickle. The melon pulp may be cooked in water until transparent and tender, then put in lime water to harden, and finally cooked in a sirup. Or cooking first in only water does not prevent hardening of the pulp by calcium. The few times that cucumbers have been soaked in lime water either slight or no hardening occurred. But other calcium compounds such as calcium chloride do produce hardening in pickles.
Pickles. Cucumber pickles are seldom made in class work; but why brined pickles spoil is a perennial question. Rahn has offered a solution of the difficulty. An abstract of his work follows.
The inside of the cucumber is free of bacteria, but the brine into which the cucumbers are placed contains several thousand bacteria per cubic centimeter. There are many different kinds of bacteria present in the beginning but with the strength of brine maintained most of them die. However, a few kinds that thrive in the brine survive. Fresh brine contains no food for bacteria. But with placing of the pickles in brine, or the formation of brine by adding dry salt to cucumbers, small amounts of sugar and other bacterial food are dissolved from the cucumbers as they shrink. Hence bacteria that can tolerate salt grow and decompose the sugar. The resulting decomposition products of the sugar are acid and gas. The gas produces froth. The acid, probably mostly lactic, is very important, for the keeping of the pickles depends upon it. The concentration of the acid is rather high when frothing ceases, varying from 0.6 to 1.2 per cent. The acid is a fairly good disinfectant and practically all bacteria in the brine are killed.
In addition to bacteria, yeasts are present in the brine; but because they need oxygen they grow principally on the surface. The scum yeast feed on the acid of the brine and decompose it, thus decreasing the quantity of acid in the tank or container. If scum grows long enough, all the acid is destroyed and then the pickles become slippery, soft, and mushy. It is better not to stir the pickles too much after they are in the brine, for the acid tends to stay at the bottom of the container and the yeasts grow at the surface.
There are two periods when the acid is low, and these stages are the periods of greatest danger in fermenting pickles. The first stage is before fermentation has started and no or little acid has been formed; the second is after the acid has been used up by scum yeast, for spoilage only occurs when the acidity of the brine is sufficiently low. The slipperiness and softness are caused by so-called potato bacteria, which resist the salt well but do not tolerate acid. Because oxygen is necessary for their growth and the bacteria only grow at the surface, the pickles protruding from the brine become soft. These potato bacteria can cause great loss and damage before a concentration of acid great enough to delay their growth is formed. A single potato bacterium, if all conditions are favorable, produces from 10 to 100 million new bacteria in 24 hours.
Hence, to prevent spoilage all pickles should be kept under the brine. Some acid or vinegar can be added with the brine to prevent spoilage. It has been interesting to see that this has been advocated for commercial practise since Rahn's work was published. Other methods of preventing growth of the yeast are to keep the container in the sunlight or irradiate with ultra-violet light, both of which are not practical in the home. After fermentation is complete, that is, after frothing has ceased, oil or paraffin may be used over the surface of the brine. Since the oil is rather difficult to wash from the pickles, paraffin is preferable.