As my experiments with homemade black bean burger patties (so far documented here and here) have helped to demonstrate, it can be difficult to come up with substitutes for the various substances of animal origin that give texture and consistency to foodstuffs. Collagen, gelatine, egg protein, egg lecithin, milkfat--all of these play roles in cooking that are hard to replace. The mouth feel and juiciness of a good beef hamburger patty, for instance, depend on the presence of collagen to keep the patty from falling apart and the presence of saturated fats that are solid at room temperature but melt upon cooking. Without some way of emulating the properties of these substances a black bean patty will tend to be dry and mushy.
In thinking about how to improve upon the recipe, it's worth examining what sort of options there are for the cook at home to imitate the properties of food constituents found only in animal products. Commercial food chemists have an easier time of it. They can freely use all manner of clever techniques and semi-synthetic derivatives of natural compounds (e.g. alkylated celluloses as thickeners, sucrose esters as fat substitutes) largely unavailable to the ordinary consumer, although a few of the substances long used in food chemistry are now to an extent available for home cooking thanks to the ludicrously-named fad of "molecular gastronomy". If all you've got to work from is what you can get at the local megamart, however, your options are limited. Still, some useful ingredients can be purchased over the counter, including...
1. Starches. An increasing number of pure starches are sold in grocery stores. Corn starch is the most common; potato and arrowroot starches are also fairly easy to find, as well as tapioca (although you're more likely to find that in the form of "pearls" rather than powder.) If starchy flours are included (e.g. the flours of wheat, rice, and barley) the range is even wider. Whatever the source of the starch, its use in food chemistry is the same: when a suspension of starch in water is cooked, the polysaccharides released from the starch granules "gelatinize" to form a colloidal dispersion of great viscosity even at high temperatures. Despite the name, however, this process does not form a true gel. That is to say, even a very thick suspension of cooked starch will remain fluid and yield to even the slightest pressure, unlike a true gel such as gelatine which is elastic and returns to its original shape. There is one other property of starch that is problematical for some cooking: being purely a simple carbohydrate, starch is not conducive to a "low-carb" diet if such is desired.
2. Vegetable gums. These substances are polysaccharides but of a much wider range of compositions than the starches, depending on the source of the gum. Therefore not all vegetable gums have the same properties in cooking although some generalizations can be made. A huge range of gums are used in food chemistry but only a very few are likely to be found in stores. Chief among these are guar gum or guar flour, milled from a certain type of bean, and xanthan gum, which though classified with the vegetable gums is really the product of a bacterial culture. Both of these act somewhat like starch in producing suspensions of very high viscosity, but not true gels, when cooked in water, although mixtures of these gums with certain substances are capable of producing true gels similar to gelatine. I've tried to use guar gum in cooking a couple of times and found that it imparted a nasty, stringy texture when used as a substitute for starch, but that stringiness might be of use in other applications. Unfortunately guar gum has disappeared from the shops around here. Xanthan gum I have never tried to cook with.
3. Vegetable mucilages. This is an even less clearly defined group of polysaccharides, sometimes classified with the vegetable gums although they have somewhat different properties. A number of them are available to the cook but never in pure form, as are the starches and gums. Most commonly available are psyllium husks, either as whole husks or in powder; filé powder, the ground leaves of the sassafras tree, used for thickening gumbo; and okra, which so far as I know is available only as the whole seed pods and is also used in gumbo. Certain edible seeds also contain mucilages, particularly yellow mustard seeds, the mucilage of which is responsible for the ability of mustard to stabilize dispersions of oil in water. More exotic are the seeds of chia, which I have seen called for in certain recipes. (Also it's used in "chia pets" because the mucilage exuded by the seeds when wet causes the seeds to stick easily to the cheesy terra-cotta figurine.)
The behavior of these mucilages in water is hard to describe. The mucilage in psyllium husks, for example, swells enormously in water; the undissolved part of the husks, even in quite large pieces, never sink to the bottom. Yet the suspension is quite fluid and pourable. A concentrated enough suspension can be drawn out into weak ropes or strings. The thermal behavior of these suspensions I do not know. I do know that psyllium powder has been used in various recipes but possibly only as a potent source of "soluble" fiber.
4. Seaweed derivatives. A number of very interesting polysaccharides derive from seaweeds, most notably agar from certain red algae. This when heated in water produces a strong, thermally reversible gel as does gelatine, although unlike gelatine agar gel is stable at room temperature whereas gelatine gel must be refrigerated to keep it solid. Agar is reasonably easy to find in Asian markets but, at least from what I've seen, is usually admixed with sugars for the use in desserts resembling Jell-O, while unflavored, pure agar is harder to obtain. It's also not very cheap. I have never used agar myself in cooking although I've used it often in microbiological experiments. (Even in scientific use the high cost of agar has prompted searches for replacements.) The other two well-known seaweed gums, carageenan from "Irish moss" and alginic acid (or rather its salts) from various brown seaweeds, also form thermally reversible gels under the correct conditions are heavily used in the food industry but not to be found at the usual markets. It may be possible to use more readily available kelp powder as a source of alginic acid. Its strong fishy smell and taste would limit its general use however.
5. Pectin. This polysaccharide, an essential component in the cell walls of all plants and especially in certain fruits, also forms thermally reversible gels but only under rather specific conditions of low pH and a high concentration of dissolved sugar. The gels formed are weak, though strong enough to hold their shape at low temperatures. Under conditions in which pectin does not form a gel it nevertheless still contributes to the viscosity and tackiness of aqueous suspensions. One difficulty in the use of pectin in cooking is that, in the form usually available in grocery stores, it is mixed with sugars and a weak acid such as citric acid or fumaric acid to insure its gelation when used for making jams and jellies. If pure pectin is available at supermarkets I've never seen it.
6. Gluten. This remarkable protein, found chiefly in wheat and to a lesser extent in rye and barley, is responsible for the elasticity and tenacity of bread doughs. For those who are metabolically unable to digest gluten, considerable art and trickery must be put forth in order to fake the ability of gluten to hold bread together. Gluten is insoluble in water but, when mixed with water and subjected to mechanical stress, tends to organize itself into a network of elastic strands that undergoes thermally irreversible "setting" with sufficient heat, as in baking. These properties of gluten have also prompted its use in food chemistry as a binder such as in vegetarian meat substitutes. One great advantage of gluten is that it is available in a largely pure form as so-called "gluten flour"; lacking this specialty flour it may still be prepared straightforwardly, if tediously, from ordinary wheat flour.
6. Soy products. Soybeans and its derived products such as tofu contain a large array of substances whose food chemical properties, to greater and lesser degrees, resemble the behavior of various animal products. For example soybeans contain various phospholipids known collectively as lecithins, similar to those found in egg yolks. The lecithins are excellent emulsifying agents for stabilizing oil-water suspensions, as in numerous sauces and dressings conventionally made with eggs, suggesting the use of soy to emulate this behavior. Moreover, soy proteins possess to some extent the ability of ovalbumin, the protein of egg white, of thermosetting to an irreversible gel. (From what little I've picked up in my reading, however, soy proteins are considerably inferior to egg protein in this regard.) Soy products are widely available in the form of tofu, soy flour, "textured vegetable protein", and other foodstuffs.
7. Other vegetable proteins. Here I am entering uncertain territory. I have learned that "vegetable albumins", proteins related to ovalbumin and to the serum albumins found in blood, can be found in numerous plant materials, though (so far as I've been able to discover so far) never in great quantity and always in combination with other types of protein. Legumes contain such albumins; one of these is soy, already mentioned. Other legumes said to be rich in vegetable albumins are pea, black-eyed pea, broad beans, and lentils. Whether a preparation of these proteins can be made which thermosets in the same way as egg white is unknown to me.
There is considerable scope for research and experimentation here, which I intend to document here at intimidating length. I am particularly interested in investigating the properties and uses of the vegetable mucilages and the vegetable albumins, since this seems to be ground far less explored.
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