Enzymatic oxidative browning (EOB) occurs when the plant's phenolic compounds react with enzymes in the presence of oxygen. When bruised or cut during preparation, discoloration of some fruits or vegetables may occur. For example, when some varieties of apples, apricots, bananas, cherries, peaches, pears, egg- plant, or potatoes are bruised or cut, the product enzymes are exposed to oxrygen in the atmo- sphere, and the produce is subject to undesirable browning or EOB.
Control measures to prevent EOB may not be easy. For example, there may be more than one substrate existing in a fruit or vegetable, also, oxygen may come from intercellular spaces, not solely surface air, and then the responsible enzyme must be denatured. Damaging enzymes spread in storage, and as mentioned earlier, it is true that "one bad apple spoils the whole bunch!"
One effective control of browning is to avoid contact between the substrate and oxygen. In order to achieve this, food may be covered with a sugar syrup in order to block oxygen, or it may be covered with a film wrap that limits oxygen permeability. Another control is the application of a commercially prepared citric acid powder or ascorbic acid to the cut fruit surface. Lemon juice in a ratio of 3:1 with water may be applied to the surface of the fruit, according to the Pro- duce Marketing Association. In this manner, the vitamin C juice is oxidized instead of the pig- ment, and the acidic pH inhibits enzymatic action.
Pineapple juice, because of its sulfhydryl groups (-SH) acts as an antioxidant, and is an additional, effective means of protection against browning. (As with lemon juice, the Produce Marketing Association recommends dipping cut fruits in pineapple juice [3:1 ratio, pineapple juice to water] for controlling EOB.) Sulfur compounds in the juice interfere with the dark- ening of various foods, such as cut fruit, cut lettuce leaves, and white wine. However, due to health concerns of a small percentage of the population allergic to sulfites, the use of sulfitingagents to prevent browning is restricted in raw products. Other available agents may be used.
Home gardeners usually blanch fruits or vegetables prior to freezing. Blanching destroys the polyphenol oxidase enzyme and enables the product to withstand many months of freezer storage without degradation. Blanching entails the placement of (usually) cut-up fruit or vegeta ble pieces in boiling water for a precise period of time prior to freezing. The exact length of time depends on the volume and texture of the product.
CULINARY ALERT! To control browning,
avoid contact between the substrate and oxy gen-cover susceptible fruit with a sugar syrup or film wrap. Cover or immerse cut fruit in lemon juice, orange juice, pineapple juice, or a commer cial treatment to control browning.
Cooking Effect of food and vegetables:
Cooking has many effects on food-its water retention, color, texture, flavor, and nutritive value to name a few of the effects. When short cooking periods and cooking methods such as steaming are selected, the effect is minimal loss of both flavor and nutritive value. Also, steaming retains the natural color as it does not allow con tact between released internal acids and the food.
Vegetables and fruits may be consumed raw, without cooking, or are made ready for consump tion by methods such as baking, boiling, frying, pressure-cooking, sautéing, steaming, stir-frying, and so forth. Cooking introduces appearance and texture changes, as well as flavor and nutritive value changes, as shown in the following.
Water Retention/Turgor :
Water retention and turgor are changed once a fruit or vegetable is cooked. Once cooked, the cell membranes lose their selective permeability, and unlike the simple movement of water/osmosis thatoccurs in raw produce, the cooked cell membranes allow the additional movement of sugars and some nutrients as well as water. Diffusion occurs as substances move from an area of higher concen tration to an area of lower concentration and the plant cell loses its form, water, and turgor.
Color :
The natural color of raw fruits and vegetables varies, and the color of cooked fruits and vegetables is influenced by a number of factors as previously discussed in this chapter. These factors include the natural plant pigment and pH, age, duration of cooking, use of a pan lid, cooking method employed, and surrounding environment. Blanching serves to inactivate enzymes and expel intercellular air that may negatively affect color.
Cooking in aluminum or cast-iron cookware may negatively discolor cooked products, therefore, instead, the use of stainless steel may be recommended for cooking vegetables or fruits. Another color change accompanies the use of sodium bicarbonate, which yields a brighter green color. However, as earlier mentioned, this usage is not recommended, as vitamin and texture losses occur.
Texture
Without doubt, the texture of a fruit or vegetable changes upon cooking. The texture of the cooked vegetable depends on a number of factors. These factors include pH, age, duration of cooking, and water composition. For example, lengthy cooking in boiling alkaline water drastically softens texture as hemicelluloses break down; cellulose is softened, and pectins degrade. The addition of acid, such as the addition of a tomato to another vegetable recipe, yields a firm cooked vegetable because tissues do not soften, and pec tin precipitates.
Helping to retain texture are calcium ions. These calcium ions are naturally present hardwater or may be added to many canned vegetables in commercial processing. For example, canning tomatoes with the addition of calcium retains the texture of cooked plant tissue forming insoluble salts with pectic substances. For a similar reason, brown sugar or molasses are common additives that are useful in retaining texture as well-e.g., Boston Baked Beans. Of course, the texture is also related to maturity of the plant, which may become tougher and "woody" due to the presence of lignin in older plants.
Flavor
The flavor of cooked vegetables is dependent on factors such as the classification as either Allium or Brassica, and loss of both water-soluble organic acids and sugars from the vacuole. Addi tionally, recipe ingredients including sugar, fat, herbs, and spices vary the flavor of vegetables and may actually encourage a wary person to eat the vegetables!
Nutritive Value
Nutritive value is presented in much more detail later in this chapter. For now, discussion is limited to cooking effect on nutritive value. The nutritive value of cooked fruits and vegetables is influenced by factors such as nutrients naturally present in the food, the type of cooking medium, duration of cooking and added substances. Through diffusion, water-soluble vitamins and sugars in the cell sap are lost from parenchyma cells and may be oxidized. On the other hand, minerals present in plant material are inorganic substances that cannot be destroyed (although they may be discarded in fruit or vegetable trimmings).
Of nutritional benefit in cooking is a short cooking time in a minimal amount of water or steaming the vegetables. Yet, there are times when just the opposite, that is, lengthy cooking, with plentiful water may be desirable to achieve mild taste-foods such as mild tasting cooked onions may benefit from lengthy cooking and plentiful water.
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