Carbohydrates Fiber Fats Proteins Micronutrients
Vitamins Minerals Bioactive Compounds
Carotenoids Flavonoids Phytosterols References
INTRODUCTION
Nutrient is defined as a substance obtained from food and used in the body to promote growth, mainte- nance, and repair of body tissues, or simply as a sub- stance that provides nourishment.
Broadly speaking, nutrients are classified into two groups, namely macronutrients (also called energy- producing nutrients or energy-yielding nutrients) and micronutrients (which are characterized by their es- sentiality for human health and the low quantities in which they need to be ingested). Energy-producing nutrients include carbohydrates, fats, and proteins.
Micronutrients often refer to vitamins and minerals.
Phytochemicals, also called bioactive compounds, are substances present in foods in low levels that may have a role in health maintenance in humans.
Fruits have proved to be essential for a balanced diet. This is believed to be mainly due to their content of vitamins, fibers, and phytochemicals, the latter be-
ing responsible in part for the antioxidant properties of fruits and foods of fruit origin.
Manufacturing processes are changing the nutri- tional properties of some foods. For instance, partial hydrogenation of vegetable oil results in the forma- tion oftrans-fatty acids, and heat treatment of pro- tein solutions in an alkali environment results in the formation of lysinoalanine. Both of these have been shown to have detrimental health effects. On the other hand, some nutrients and bioactive compounds that are naturally present in fruits may undergo transfor- mations during food processing that neither decrease their nutritional value nor bioactive value but may in- crease it by favoring their absorption and metabolism in the human body.
In general, vitamins, minerals, water, and fibers are considered to be the main nutrients contributed by fruits to a balanced diet, and thus special atten- tion should be addressed to this group of nutrients (Villarino-Rodr´ıguez et al., 2003).
In this chapter, we will present what we consider to be the contribution of fruits to human nutrition in order to understand how the different processing methods used in the food industry may modify their contents, structure, and biological activity in humans.
MACRONUTRIENTS
WaterWater plays two fundamental roles as a nutrient: (1) protective and regulatory, by being a substrate of bio- logical reactions or acting as the matrix or vehicle in which those reactions take place, and (2) an essential role as the temperature and pH regulator in the human body. Water also has a plastic function through the 29
Edited by Y. H. Hui Copyright © 2006 by Blackwell Publishing
30 Part I: Processing Technology maintenance of the cell and tissue integrity. Around
two-thirds of the human body is composed of water, and in general, the higher the metabolic activity of a given tissue, the higher its percentage of water.
Most of the body water is found within three body compartments: (1) intracellular fluid, which contains approximately 70% water, (2) extracellu- lar fluid, which is the interstitial fluid, and (3) blood plasma. These two compartments contain∼27% wa- ter. The body controls the amount of water in each compartment by controlling the ion concentrations in those compartments. Therefore, gains or losses of electrolytes are usually followed by shifts of fluid to restore osmotic equilibrium.
Although a low intake of water has been associated with some chronic diseases, this evidence is insuffi- cient to establish water intake recommendations. In- stead, an adequate intake of water has been set by the Food and Nutrition Board of the Institute of Medicine in the United States, to prevent deleterious effects of dehydration. This adequate intake of total water is 3.7 l for men and 2.7 l for women. Fluids should rep- resent 81% of the total intake, and water contained in foods represent the other 19% (IM, 2004).
The body has three sources of water: (1) ingested water and beverages, including fruit juices, (2) the water content of solid foods, and (3) metabolic wa- ter. Fruits have a high percentage of water that ranges
from 70% to 95% of the eatable part of the fruit (see Table 2.1). For this reason, they are, together with vegetables, a very good source of water in the diet within the solid foods. The content of water in a fruit may be greatly affected by the processing technology, and in fact, some technologies used to increase the shelf life of fruits do so through the reduction of their water content. It is important to bear in mind that the water content of a fruit also changes during matura- tion, therefore the optimum degree of maturation of a fruit for a given processing technology may be dif- ferent than for another processing technology. This will also affect the water content in the final product.
Carbohydrates
Energy is required for all body processes, growth, and physical activity. Carbohydrates are the main source of energy in the human diet. The energy produced from carbohydrate metabolism may be used directly to cover the immediate energy needs or be trans- formed into an energy deposit in the body in the form of fat. Carbohydrates also have a regulatory func- tion, for instance, by selecting the microflora present in the intestines. Fructose has been known to in- crease plasma urate levels due to rapid fructokinase- mediated metabolism to fructose 1-phosphate. This increase in plasma urate levels seems to cause an
Table 2.1. Fruit Composition (Grams per 100 g of Edible Portion)
Fruit Water Carbohydrates Protein Fat Fiber
Apple 86 12.0 0.3 Tr 2.0
Apricot 88 9.5 0.8 Tr 2.1
Avocado 79 5.9 1.5 12 1.8
Banana 75 20.0 1.2 0.3 3.4
Cherry 80 17.0 1.3 0.3 1.2
Grape 82 16.1 0.6 Tr 0.9
Guava 82 15.7 1.1 0.4 5.3
Kiwi fruit 84 9.1 1.0 0.4 2.1
Mango 84 15.0 0.6 0.2 1.0
Melon 92 6.0 0.1 Tr 1.0
Orange 87 10.6 1.0 Tr 1.8
Papaya 89 9.8 0.6 0.1 1.8
Peach 89 9.0 0.6 Tr 1.4
Pear 86 11.5 0.3 Tr 2.1
Pineapple 84 12.0 1.2 Tr 1.2
Plum 84 9.6 0.8 Tr 2.2
Raspberry 86 11.9 1.2 0.6 6.5
Strawberry 91 5.1 0.7 0.3 2.2
Watermelon 93 8.0 1.0 Tr 0.6
Source: Moreiras et al. (2001).
increase in plasma antioxidant capacity in humans (Lotito and Frei, 2004).
In general, the carbohydrates are classified into three groups: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides include pen- toses (arabinose, xylose, and ribose) and hexoses (glucose, fructose, and galactose). Oligosaccha- rides are sucrose, maltose, lactose, raffinose, and stachyose. Polysaccharides include starch (com- posed of amylose and amylopectine, both polymers of glucose), glycogen, and other polysaccharides, which form part of fiber which we will review in the following section.
The recommended dietary allowance (RDA) for carbohydrates is 130 g/day, except in the cases of pregnancy (when it is 175 g/day) and lacta- tion (210 g/day). With respect to the total energy consumed per day, carbohydrates should represent 45–65% (IM, 2002).
After water, carbohydrates are the main compo- nent of fruits and vegetables and represent more than 90% of their dry matter. The main monosaccharides are glucose and fructose. Their concentration may change depending on the degree of maturation of the fruit. The relative abundance of glucose and fructose also changes from one fruit to another (Table 2.2).
For instance, in peaches, plums, and apricots, there is more glucose than fructose and the opposite occurs in the case of apples or pears. Other monosaccharides, such as galactose, arabinose, and xylose, are present in minimal amounts in some fruits, especially orange, lemon, or grapefruit. Fruits such as plums, pears, and cherries also contain the sugar alcohol sorbitol, which
acts as a laxative because of osmotic transfer of water into the bowel.
Sucrose is the most abundant oligosaccharide in fruits; however, there are others such as maltose, melibiose, raffinose, or stachyose that have been described in grapes, and 1-kestose in bananas. Other oligosaccharides are rare in fruits. Starch is present in very low amounts in fruits, since its concentration decreases during maturation. The only exception is banana that may have concentrations of starch higher than 3% (Belitz and Grosch, 1997).
During food processing, carbohydrates are mainly involved in two kinds of reactions: on heating they darken in color or caramelize, and some of them com- bine with proteins to give dark colors known as the browning reaction.
Fiber
Fiber is often referred to as unavailable carbohy- drate. This definition has been a controversy for years. Fiber is a generic term that includes those plant constituents that are resistant to digestion by secre- tions of the human gastrointestinal tract. Therefore, dietary fiber does not have a defined composition, but varies with the type of foodstuff. Perhaps we can say that fiber may not be a carbohydrate and it may be available.
Fiber has mainly a regulatory function in the hu- man body. The role of fiber in human health has been the subject of many studies in the last 30 years. In most of these studies, the results have suggested im- portant roles of fiber in maintaining human health.
Table 2.2. Sugar Contents of Fruits (Grams per 100 g of Edible Portion)
Fruit Fructose Glucose Sucrose Maltose Total Sugar
Apple 5.6 1.8 2.6 – 10.0
Apricot 0.4 1.9 4.4 – 6.7
Avocado 0.1 0.1 – – 0.2
Banana 2.9 2.4 5.9 – 11.3
Cherry 6.1 5.5 – – 11.6
Grapefruit 1.6 1.5 2.3 0.1 5.7
Grape 6.7 6.0 0.0 0.0 12.9
Mango 3.8 0.6 8.2 – 12.7
Orange 2.0 1.8 4.4 – 8.3
Peach 4.0 4.5 0.2 – 8.7
Pear 5.3 4.2 1.2 – 10.7
Plum 3.2 5.1 0.1 0.1 8.6
Strawberry 2.3 2.6 1.3 – 6.2
Watermelon 2.7 0.6 2.8 – 6.2
Source: Belitz and Grosch (1997) and Li et al. (2002).
32 Part I: Processing Technology The role of fiber in human health is mainly protective
against disease, e.g., diseases of the gastrointestinal tract, circulation related diseases and metabolic diseases (Saura-Calixto, 1987).
The major components of dietary fiber are the polysaccharides celluloses, hemicelluloses, pectins, gums, and mucilages. Lignin is the other component that is included in most definitions of fiber but it is not a carbohydrate.
Fiber may be classified as water soluble and in- soluble. Gums, mucilages, some hemicelluloses, and pectins are part of the soluble fiber. Celluloses, hemi- celluloses, and lignins are insoluble fibers. Fruits are good sources of both classes of fibers, especially soluble fiber. Fiber, together with vitamins, is the main nutritional reason for using fruits for a balanced diet.
There are several fiber-associated substances that are found in fruit fiber, which may have some nutri- tional interest. Among them are phytates, saponins, tannins, lectins, and enzyme inhibitors. Saponins, which are mainly present in some tropical fruits, may enhance the binding of bile acids to fiber and reduce cholesterol absorption. Tannins are polyphe- nolic compounds widely distributed in fruits, which can bind proteins and metals and reduce their ab- sorption. Lectins, which are present in bananas and some berries, are glycoproteins that can bind specific sugars and affect the absorption of other nutrients.
The RDA for fiber is 25–30 g/day, depending on age and sex, except in the case of children from 1 to 3 years, in which case it is 19 g/day.
Dietary fiber is present in fruits in amounts that may be as high as 7% of the eatable part of the fruit (see Table 2.1). Within fiber, the most common com- ponents in fruits are celluloses, hemicelluloses, and pectins. Pectins are important in the technological process, since they may be deeply modified and this modification not only has an influence on the nutri- tional value of the final food, but also has an impact on the texture and palatability of the product.
Fats
Fat has three important roles as a nutrient: it is a highly concentrated source of energy, it serves as a carrier for fat-soluble vitamins and there are some fatty acids that are essential nutrients that can only be ingested with fat. Fat also serves as a carrier for some of the bioactive compounds present in fruits such as phytoestrogens and carotenoids that are lypophylic.
Fatty acids are also needed to form cell struc- tures and to act as precursors of prostaglandins. Fatty acids are part of triglycerides, which are the princi- ple form in which fat occurs. Fatty acids may oc- cur naturally with various chain lengths and different numbers of double bonds. They may be saturated (butyric, caproic, caprylic, capric, lauric, palmitic, stearic, and myristic acids), monounsaturated (oleic and palmitoleic acid), and polyunsaturated (linoleic, linolenic, and arachidonic acids) also known as PUFAs. Linoleic and linolenic acids cannot be syn- thesized in the body and are known as essential fatty acids. They are needed to build and repair cell struc- tures, such as the cell wall and, notably, tissues in the central nervous system, and to form the raw material for prostaglandin production. Inflammatory and other chronic diseases are noted for exhibiting a deficiency of polyunsaturated fatty acids in the bloodstream.
Fatty acids that contain double carbon bonds can exist in either of two geometrically isomeric forms:cisand trans.Trans-fatty acids are produced in the hydro- genation process in the food industry and may play a role in atherosclerotic vascular disease (Sardesai, 1998).
In general, fat should represent between 20% and 35% of the total energy consumed per day in order to reduce risk of chronic disease while providing in- takes of essential nutrients. This fat should include 10–14 g/day of linoleic acid and 1.2–1.6 g/day of linolenic acid.
Fat content in fruits is in general very low (see Table 2.1). However, in cherimoya (1%) and avocado (12–16%), the lipid levels are higher. In avocado, the most abundant fatty acids are palmitic, palmitoleic, stearic, oleic, linoleic, and linolenic acids, but the amounts may change a lot with the variety, matu- rity, processing, and storage conditions (Ansorena- Artieda, 2000).
Proteins
The importance of protein in the diet is primarily to act as a source of amino acids, some of which are essential because the human body cannot synthesize them. From the 20 amino acids that are part of the structure of proteins, almost half of them are con- sidered to be essential, including isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryp- tophan, and valine. The RDA for proteins is 34–
56 g/day, depending on age and sex, and in the case of pregnancy and lactation, it is 71 g/day. With respect
to the total energy consumed per day, carbohydrates (proteins) should represent 10–35%.
Proteins are essential structural components of all cells and are needed by the human body to build and repair tissues, for the synthesis of enzymes, hor- mones, and others. They are also involved in the im- mune system, coagulation, etc. Therefore, proteins play both regulatory and plastic roles in the human body.
Proteins are made up of a long chain of amino acids, sometimes modified by the addition of heme, sugars, or phosphates. Proteins have primary, secondary, tertiary, and quaternary structures, all of which may be essential for the protein to be active.
The primary structure of a protein is its amino acid sequence and the disulphide bridges, i.e., all covalent connections in a protein. The secondary structure is the way a small part, spatially near in the linear se- quence of a protein, folds up into␣-helix or-pleated sheets. The tertiary structure is the way the secondary structures fold onto themselves to form a protein or a subunit of a more complex protein. The quaternary structure is the arrangement of polypeptide subunits within complex proteins made up of two or more sub- units, sometimes associated with non-proteic groups.
Food processing may affect these four structures in many ways, thus modifying the activity of the protein and also its nutritional value. Amino acids and pro- teins containing lysine or arginine as their terminal amino acids are also involved in the Maillard reac- tions that have a nutritional and sensory impact on processed foods.
Nitrogenated compounds are present in fruits in low percentages (0.1–1.5%). From a quantitative point of view, fruits are not a good source of pro- teins, however, in general berries are a better source than the rest of the fruits. Cherimoya and avocado also present higher levels of proteins than other fruits (Torija-Isasa and C´amara-Hurtado, 1999).
There are some free amino acids that may be char- acteristic of a certain fruit. This is the case of proline which is characteristic of oranges but cannot be found in strawberries or bananas.