Chemical Composition of Plant Material :
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carbohydrate : Carbohydrate constitutes the largest percentage of the dry weight of plant material. It is the basicmolecule formed during photosynthesis when water (H2O) and carbon dioxide (CO2) combine to yield carbohydrate (CHO) and oxygen (O2).
Carbohydrate is present in both the simple and complex forms. For example, simple carbohydrates are the monosaccharides-including glucose and fructose; and disaccharides, such as sucrose, that may increase during the fruit ripening process. Complex carbohydrates or polysaccharides are synthesized from simple carbohydrates and include cellulose and starch.
Various complex carbohydrates and the effect of heat on those carbohydrates are discussed in the following .
Starch: is the storage component of carbo- hydrate located in roots, tubers, stems, and seeds of plants. When subjected to heat and water, starch absorbs water and gelatinizes Vegetables vary in their starch content. Some vegetables such as potatoes are starchy, some moderate, and others such as parsley are less starchy. Starch is digestible as the bonds between the glucose units are a-1,4.
Cellulose: is water-insoluble fiber that provides structure to plant cell walls. The molecular bonds between glucose units are B-1,4; therefore, cellulose remains indi- gestible to humans, although it may be softened in cooking.
Hemicellulose :fiber provides structure in cell walls, and the majority is insoluble. It is softened when heated in an alkaline environment, such as, if baking soda is added to cooking water for the purpose of green color retention.
Pectic substances: (Chap. 5) are the firm, intercellular "cement" between cell walls, the gel-forming polysaccharide of plant tissue, and are hydrolyzed by cooking. Large insoluble forms of pectin become soluble pectin with ripening of the plant material.
In addition to carbohydrates, there is a noncarbohydrate fiber material present in the complex vascular and supporting tissue. It is lignin and is found in older vegetables. Lignin remains unchanged by heat and may exhibit an unacceptable "woody" texture.
Protein: Protein makes up less than 1 % of the composi- tion of a fruit, and protein is low in most vegetables. Protein is most prevalent in legumes-peas and beans-yet, even then, it is an incomplete protein as it lacks the essential amino acid, methionine. Protein that is present as enzymes may be extracted from plants and used in other foods. Examples include the proteolytic enzymes that contribute the beneficial tenderizing effects to meats such as papain (derived from papaya), ficin (obtained from figs), and bromelain (extracted from pineapple).
Fat : composes approximately 5 % of the dry weight of roots, stems, and leaves of vegetables. It makes up less than 1 % of the dry weight of a fruit, except for fruits such as avocados and olives that contain 16 % and 14 % fat, respec- tively. Fat is instrumental in development during the early growth of a plant.
Vitamins : The vitamins present in vegetables and fruit are primarily carotene (a vitamin A precursor) and vitamin C. Beta-carotene, is present in dark orange fruits, vegetables, and as an underlying pigment in green vegetables. Vitamin B, (thiamin) is also present. Fruits supply more than 90 % of the water-soluble vitamin C and a major percentage of the fat soluble vitamin A in a diet.
Water-soluble vitamin losses may occur upon soaking when vitamins leach out, and also in heating. Losses occur primarily in heating. In addition to soaking and heating, enzymatic action may negatively affect the nutritive quality of fruits and vegetables. Specifically, the enzymes, ascorbic acid oxidase and thiaminase, can cause nutritional changes in vitamins C and B1, respectively, during storage. Therefore, retention of these vitamins is controlled by deactivating the enzymes in blanching prior to freezing.
CULINARY ALERT! It is interesting to note that vitamins A and C, so plentiful in fruits and vegetables, are both listed on Nutrient Facts labels as vitamins that Americans lack, therefore increase consumption of fruits and vegetables.
Minerals : Minerals are more prolific in vegetables than in fruits and are notably calcium, magnesium, and iron. Calcium ions are added to some canned vegetables in order to promote firmness and lessen softening of pectic substances. Yet, since the oxalic acid in spinach and the phytates in peas bind calcium, decreasing its bioavailability, cal- cium is not added to these canned vegetables.
Water : Water is found in and between plant cell walls. Some of its functions in the plant are to transport nutrients, to promote chemical reactions, and to provide plants with a crisp texture if cell membranes are intact.
Water constitutes a small percentage (10 %) of seeds and is a substantially larger percent of leaves. It makes up 80-90 % of a plant, as is evidenced by the drastic size reduction of a mea- sure of vegetables that is subject to dehydration.
CULINARY ALERT! Think about how the volume of plant material changes significantly when a food dehydrator is used to remove water from food.
Phytochemicals (More in Appendices: Phytochemicals are plant chemicals. They are non-nutrient materials that may be especially significant in preventing disease and controlling cancer. These chemicals are the focus of much research concerning their importance to human health.
The list is long of the many examples of such plant chemicals. It includes the beta-carotene of carotenoid pigments, the flavonoid group of pigments, as well as the sulfur-containing allyl sulfide and sulforaphane. Additionally, dithio- Ithiones, indoles, and isothiocyanates in crucifer- ous ("cross-shaped blossom," cabbage family) vegetables, isoflavones, phytosterols, protease inhibitors, saponins in legumes, and limonene and the phenols of citrus fruit are among the plant chemicals that may be effective in disease prevention.
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