Pigments and Effects of Additional Substances of plant:
Plant pigments enhance the aesthetic value of fruits and vegetables for humans, as well as attract insects and birds, which fosters pollina- tion. These pigments are subject to change with ripening and processing of the raw vegetables or fruits. The four pigments found in plants are chlorophyll, the green pigment; carotenoids, a yellow, red, or orange pigment; and the flavonoids, both anthocyanin, the red, blue, or purple pigment, and anthoxanthin, the white pig- ment. "... a variety of different colors of non- starchy vegetables and fruits, including red, green, yellow, white, purple and orange, as well as tomato-based products and allium vegetables, such as garlic, are recommended daily." (Food Product Design 2012)
High-performance liquid chromatography (HPLC) is generally used for plant pigment anal- ysis. A discussion of the major pigments and a description of how they may change appears in the following material.
Chlorophyll Chlorophyll is perhaps the most well-recognized plant pigment. It is the green pigment found in the cell chloroplast, and it is responsible for photosynthesis (i.e., converting sunlight to chem- ical energy). It is fat-soluble and may appear in vegetable cooking water if the water also contains fat.
Chlorophyll is structurally a porphyrin ring containing magnesium at the center of a ring of four pyrrole groups (Fig. 7.2). Phytol alcohol is esterified to one of the pyrrole groups, and it confers solubility to fat and fat solvents. Methyl alcohol is attached to another pyrrole group.
Initially, as green vegetables are heated, air is removed from in and between the cell, and a bright green color becomes apparent. Then inter- nal organic acids are released and hydrogen displaces magnesium, producing pheophytins. Either magnesium-free pheophytin a, which is a gray-green pigment, or pheophytin b, an olive- green pigment, is formed. These changes to the chlorophyll pigment become more marked with time, so a short cooking time is recommended.
As well, cooking the product uncovered for the first 3 minutes allows the escape of volatile plant acids that would otherwise remain in the cooking water and react to displace magnesium. Using a cover while cooking allows less change of chlorophyll to occur. (This is not true of all vegetable pigments as seen later.) When heated, green-pigmented vegetables that are high in acid content undergo more color change than green vegetables low in acid, and green vegetables show less color change than fruits with their high acid. Even raw green vegetables, such as raw broccoli, change color to the underly- ing yellowish color as the chlorophyll degrades.
CULINARY ALERT! Pigments may change from the natural color due to extended heating and release of the plant's internal organic acids; therefore, minimum cooking is preferred. In addition to the internal organic acids, an external acid environment (i.e., acid added to cooking water) causes the natural green color to change into olive-green pheophytin.
The preceding discussion has been on the effect of acids on pigment color. As opposed to an acid environment, an alkaline environmentalso affects the green pigment. As the profes- sional or home chef knows, the addition of the alkaline material, sodium bicarbonate (baking soda), produces and maintains a desirable green color. The soda reacts with chlorophyll, displacing the phytyl and methyl groups on the molecule, and the green pigment forms a bright- green, water-soluble chlorophyllin.
Nonetheless, although producing a desirable appearance with pH change of added soda, the benefit is accompanied by an unacceptable 1. loss of texture, due to softening of hemicellulose. Sodium bicarbonate also 2. destroys ascorbic acid (vitamin C) and thiamin (vitamin B,). There- fore, due to these texture and nutrient losses the addition of this alkali substance is not recommended.
CULINARY ALERT! Sodium bicarbonate (baking soda) has a positive effect on color. However, it negatively affects texture and nutri- tive value.
In food preparation, the minerals, copper and zinc, may be released in the process of cutting or chopping. Also, some knives, copper bowls, or colanders may produce undesirable color changes in chlorophyll by displacing magnesium.
Regardless of the manner in which chloro- phyll is changed, when the chlorophyll is destroyed, a second underlying carotenoid pig- ment may become apparent. Carotenoids are discussed below.
Carotenoids The carotenoids are red, orange, and yellow fat- soluble pigments in fruits and vegetables, includ- ing carotenes (the hydrocarbon classification) and xanthophylls (the oxygenated class). They are found in chloroplasts along with chlorophyll, where the green pigment dominates, and also in chromoplasts without chlorophyll. The caroten- oid pigment is seen especially in flowers, fruits, including tomatoes, peppers, and citrus fruits, aswell as roots, including carrots and sweet potatoes.
Carotenes are unsaturated hydrocarbons containing many carbon atoms. The conjugated double bonds (i.e., double bonds alternating with single bonds) are responsible for the color; the greater the number of conjugated double bonds, the deeper the color. For example,
• Beta-carotene is naturally orange in color and contains a six-membered ring at each end of the chain (Fig. 7.3) In comparison to beta-carotene.
Alpha-carotene has one less conju- gated double bond and is paler in color.
Lycopene, found in tomatoes and watermelon, has the deepest red- orange color because it has two more double bonds than beta- carotene, and it has two open rings (Fig. 7.3) at each end of the chain.
There exist hundreds of types of carotenes- 40 or more carotenoids are known to be precursors of vitamin A. The most well-known carotene is the aforementioned beta-carotene, cleaved by an enzyme in the intestinal mucosa to yield vitamin A.
Autumn leaves show evidence of destruction of the green chlorophyll pigment, as the carotenes, and "autumn xanthophylls" that existed along with the chlorophyll become visible. Corn contains the xanthophyll cryptoxanthin, and green leaves contain lutein. Paprika also contains xanthophyll pigment.
The carotenoid pigment may undergo autoxi- dation due to the large number of double bonds. This oxidation may result in "off-flavor" and color loss, yielding unsatisfactory products. Antioxidants such as butylated hydroxy anisole (BHA), butylated hydroxy toluene (BHT), or tertiary butylated hydroxy quinone (TBHQ) are frequently added to a wide variety of foods containing fruits and vegetables, herbs, or spices to prevent this detrimental oxidation.
The FDA does not allow health claims for spices. However supportive research into the health benefits of spices fits nicely into two consumer trends: movement toward natural remedies and a growing appetite for spicy foods.
(Hazen 2012) Whereas oxidation causes development of a lighter-color cooked vegetable, caramelization of plant sugar may result in a darker-color cooked vegetable. It is recommended that
Carotene-pigmented vegetables should be either covered during cooking, or cooked quickly, as in stir-frying. Since the pigment is fat soluble, table fat such as butter or margarine should be minimized or omitted in cooking as the pigment may become paler.
The length of cooking time does not negatively affect carotenoid pigments as much as it does for chlorophyll, and changes are not as noticeable. However, upon heating, and in the presence of acid, some molecular isomerization occurs. Spe- cifically, in carotenoids, the predominant trans molecular form, naturally present in plants, is changed to cis configuration in a matter of a few minutes, and the pigment becomes less bright. Unlike the case with chlorophyll pigments, alkali environments do not produce a color change.
Carotenes provide color in food. Food technologists have developed annatto, carrot, paprika, and tomato extracts to provide color in foods. (Pinkish-white flowers of the annatto plant with their small reddish-orange seeds inside offer dye used to color foods such as cheddar cheese.)
In addition to the plant pigments, added herbs and spices also provide carotene coloring and flavor. Albeit in small amounts in foods, they contribute to vitamin A values that appear on nutrition labels. They supply advantageous nutrients such as beta-carotene. This addition offers the same nutrients as a diet of yellow, green, and leafy vegetables, although in signifi- cantly lesser amounts.
Carotene from vegetables or fruits may pre- vent oxidation of body tissues, and development of cancer, although much remains unknown about possible benefits of supplements of this biologically active component of plant material. The Academy of Food and Nutrition advocates foods in the diet as the best source of good nutrition (see "Nutritive Value of Vegetables and Fruits" section rather than supplements).
CULINARY ALERT! Cooking change is minimal for carotenoids.Another group of pigmented compounds consisting of anthocyanins and anthoxanthins are the flavonoids.
Anthocyanin Anthocyanin (Fig. 7.4) is the red, blue-red, blue, or purple pigment in fruits and vegetables such as blueberries, cherries, raspberries, red cabbage, red plums, and rhubarb (not beets; "Betalaines" section). The skins of radishes, red see apples, red potatoes, grapes, and eggplant also contain anthocyanin pigment. It is prevalent in buds and young shoots, and is an underlying pigment of chlorophyll, that becomes apparent as a purplish pigment in autumn leaves when chlorophyll decomposes.
Anthocyanins contain a positively charged oxygen in the central group of the molecule and belong to the flavonoid group of chemicals. Thus they are distinguished from the orange-red found in carotenoids. These pigments are water-soluble and are found in the cell sap of plants. They may be released into the cooking water with soaking or prolonged heat exposure.
In the following is a discussion regarding anthocyanin and pH.
pH and color: Care must be taken when working with the anthocyanin pigments. Mixed fruit juices for a punch drink or fruits incorporated into baked goods with alkaline leavens may produce unde- sirable color. Either the addition of alkali or an alkaline cooking medium produces or turquoise unwelcome violet-blue color. In an acidic environment, the anthocya- nin pigment exhibits a more characteristicred color. A tart, acidic apple is often added to red cabbage while cooking in order to produce a more appealing finished product.
pH and texture: Textural characteristics are also affected by pH. If acids such as lemon juice or vinegar are added to fruits and vegetables (anthocyanin pigment) for better color, it should be after desired softening has occurred because acid prevents softening (see "Cooking Effect" section).
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