What are the composition of fruits
The cellular structure and pigments of fruits are similar to those of vegetables. Organic acids, pectic substances, and phenolic compounds are also found in some vegetables, but have more relevance to fruits.
Natural sugars such as fructose, glucose, and sucrose are the major contributors to sweetness in fruits, while the tart flavor component is partially due to organic acids located in the cell sap. Acidity varies with the maturity of the plant, usually decreasing as the fruit ripens. The organic acids found in fruits are either volatile or nonvolatile. Volatile acids vaporize during heating, while nonvolatile acids do not, but they can leach out when fruit is cooked in water.
The common organic acids in fruit include citric acid in citrus fruits and tomatoes; malic acid in apples, apricots, cherries, peaches, pears, and strawberries; tartaric acid in grapes; oxalic acid in rhubarb; and benzoic acid in cranberries. Cranberry juice in addition has a unique blend of the organic acids quinic and malic acid, with the ratio of the two so constant that it is used by juice processors to determine cranberry juice authenticity and to calculate the percentage of cranberry juice content in juice drinks. The oxalic acid in rhubarb can combine with calcium in the intestine to form calcium oxalate, an insoluble complex that cannot be absorbed. It may also combine with other minerals to form similar compounds.
Acids cause most fruits to have a pH value below 5.0. The tartness of fruits is related in part to their acidic content. For instance, limes, lemons, and cranberries are very tart fruits, with the lowest pH values (around 2.0). The least acidic fruits are more bland and sweet in flavor, and those with a pH above 4.5 most often serve as vegetables.
There are three groups of pectic substances: protopectin, pectin (pectinic acids), and pectic acid. Pectin, a general term describing this group of polysaccharides found in fruits, acts as a cementing substance between cell walls and is partially responsible for the plant’s firmness and structure. It is used commercially to contribute to the gelling of fruit preserves. The string-like pectin molecules bond to each other under the right conditions to form a net-like solid structure that is able to trap water and form a gel. Pectin is found between the plant cells and within the cell walls, but not in the juice, so commercial sources of pectin include the pulp (pomace) remaining after apples are pressed for juice, and the spongy albedo of citrus fruits.
Pectin is also used in a number of other foods as an emulsifier, stabilizer, thickener, and texturizer. Frozen foods benefit from pectin’s ability to improve texture by controlling ice crystal size, preventing loss of syrup during thawing, and improving overall shape. Fruit pieces in yogurt are evenly distributed with the aid of pectin, and diet soft drinks impart more body if pectin is added.
While fruit is immature, prior to the formation of pectin, its pectic substance is protopectin, a large, insoluble molecule. As the fruit ripens, enzymes convert protopectin to the more water-soluble pectin. Enzymes play a key role in the softening of fruits, with the largest influence derived from those enzymes that break down pectin. Ripening mechanisms trigger the pectinase enzymes, which break down the pectic substances as the fruit ripens, and the degree of fruit softening is related to how many pectic substances were degraded. The stage of ripeness affects pectin concentration, and it is the pectin extracted from ripe fruits that is used to gel jams and jellies.
As fruit continues to ripen and becomes overripe, all the pectin gradually turns to pectic acid. Since neither protopectin nor pectic acid can contribute to gelling, only fruit at the height of ripeness should be used for making fruit spreads without added pectin. Heating also converts pectin to pectic acids by hydrolyzing the chemical bonds holding the molecules together, causing the texture of the fruit to become soft and eventually mushy.
Another role that pectic substances play in the food industry is their contribution to cloudiness in fruit juices. When juice is extracted from fruits, pectic substances can sometimes cause it to cloud. Although this is desirable in orange juice, where it contributes to body, other juices, such as apple juice, are often more appealing to consumers if they are clear. One way to remove the cloudiness is through a clarification process in which enzymes such as pectinases are added to the juice to break down the pectin compounds responsible for juice cloudiness. Juice processors can also add enzymes to certain juices to increase juice extraction. Enzymes such as cellulase and hemicellulase break down the cellulose and hemicellulose in cell walls, releasing more juice.
Another group of compounds found in fruits, phenolic compounds, are responsible for the browning and bruising that often occurs in ripening fruit. These compounds, also known as tannins, are found predominantly in unripe fruits, giving them a bitter taste and leaving an astringent feeling in the mouth. Fruits containing phenolic compounds include apples, apricots, avocados, bananas, cherries, dates, grapes, nectarines, papayas, peaches, persimmons, pears, and strawberries. All of these foods turn brown from enzymatic browning, which occurs in the presence of three substances: phenolic compounds, found within the cells; polyphenoloxidase enzymes (enzymes that oxidize phenolic compounds), also known as phenolase, catecholase, and tyrosinase; and oxygen, which enters the cells when the food is cut or bruised. The polyphenoloxidase enzymes turn the color of the phenolic compounds from clear to brown. These brown compounds, called melanins, though unappetizing, are safe to consume.