This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Crisping pickles. Calcium chloride is used to make pickles crisp or firm. It is used either in the last soaking water, when the brine is being removed, or added directly to the sweetened vinegar. The proportion used is one pound per barrel of pickles.
The hardening may come from the calcium combining with the cellulose or it may combine with acid of the pickle or melon or be deposited as calcium salts in the food. Some factors other than the ones mentioned may cause the hardening.
The effect of sugar on the firmness of fruit. The effect of sugar of course varies somewhat with the amount used in proportion to the quantity of fruit. Osmosis occurs when a sirup has greater sugar concentration than that of the fruit to which it is added. The vapor pressure of the sirup solution is lower than that of the fruit so that water passes from the fruit to the sirup. This transfer from a region of high concentration to a low one is called osmosis. If a membrane separates the regions of high and low concentrations it is dependent upon the membrane whether all the dissolved constituents of a solution can pass through the membrane. With a semi-permeable membrane only the water can pass through the membrane. Without discussing the possible role of the membrane in osmosis or the osmotic pressure that may be produced, it is sufficient for the purpose here to state that as the permeability of the membrane increases larger and larger ions or molecules dissolved in the water may pass through the membrane. The skin and cell walls of the fruit serve as membranes. These membranes in the different fruits seem to have a greater or a lesser permeability to sugar.
When cooked in a sirup some fruits tend to keep their shape, some become mushy, some shrivel, and others tend to collapse or flatten out. For instance, sweet apples tend to stay whole whether cooked in sirup or in water, whereas sour apples have a greater tendency to break up during cooking. In general, the fruits tend to stay whole better if cooked in a sirup. For purees the fruit is cooked in water and the sugar added later. Cooking seems in most instances to increase the permeability of the fruit membranes to sugar. Some apples, some plums, and most berries can be cooked directly in the sirup without noticeable toughening of the fruit. Peaches, apricots, most apples, and most plums absorb sugar satisfactorily. Lathrop states that the firm texture of the cherry skin and of the peach and blackberry flesh retards penetration of the sugar during cooking. On the other hand, a fruit like Keifer pears is dehydrated if cooked directly in a sirup and becomes shriveled, tough, hard, and rubbery. It is necessary to soften the fruit by cooking and increase its permeability to sugar before the sugar is added. It is of course possible that many changes other than increasing the permeability of the fruit to sugar occur when the fruit is cooked in sirup. Later work has shown that fruit like Keifer pears and quinces can be cooked by adding the sugar directly to them without preliminary cooking. But it is necessary to start with an excess of thin sirup and cook slowly to evaporate the excess liquid, as the sugar penetrates the fruit. So long a time is required that the method is not practical. The fruit becomes more transparent in appearance. It may be that the sugar has some effect on the cellulose.
In fruits like strawberries and seeded sour cherries, and particularly if they are heated rapidly in a sugar sirup, flattening occurs. If a large number of cell walls are broken the fruit becomes mushy. Time is required for sugar to pass through the cell walls of fruits. If osmosis has not progressed far enough, i.e., if the concentration of the sugar within the berry is not as great as in the sirup surrounding it, the berry floats when canned.
In pickling, shriveling can be prevented by heating the pickling material slightly and increasing the concentration of the sugar gradually, allowing about 24 hours or more to elapse before the next addition of sugar. But fermentation is more likely to occur when preserves are treated this way. It is a good practise to add the sugar directly to fruits like strawberries and cherries and let them stand over night. The fruit loses water, shrinks somewhat, and becomes slightly tougher. When heated slowly, and if necessary removed from the heat for a few minutes before boiling commences, there is time for the sugar to penetrate into the interior of the berry, the fruit is plump and will not float when placed in the container. In the commercially vacuum-processed berries, floating in the can is prevented by adding the berries, to which part of the sugar has previously been added, to a hot sirup, but further heating is not continued until the berries have stood for a few minutes. By producing a vacuum, the greater pressure within the berry tends to puff it up; by breaking the vacuum, flattening occurs. In this way the process of osmosis is helped.
Strawberries and the sour cherries develop a strong flavor when boiled for a long time. Hence time must be allowed for the sugar to penetrate into the fruit before the boiling point is reached. Boiling should be rapid, and the quantity of fruit used in a batch small. The popularity of sun preserves for certain fruits is due to osmosis being slow and complete. If the fruit is heated the heating period is very short, so that strong flavors are not developed, less sugar is inverted and caramelized, and the aromatic substances are not lost to such an extent as in long cooking.
Since strawberries usually contain about 90 per cent water, it seems a better practise to add the sugar directly to the berries, for not as long a time is required to cook the berries to the concentration desired. When water is added to the sugar to make a sirup it is necessary to evaporate the water used in the sirup as well as a part of that contained in the fruit. Increasing the proportion of sugar to fruit also lessens the time required to reach a definite concentration of the sirup, so that the preserves do not develop as strong a flavor. When 11/2 pounds of sugar are used with a pound of strawberries only a short time of cooking is required for the sirup to reach a temperature of 103°C, which gives a sirup containing 60 per cent of sugar. See Table 10.