Part 2 Processing and nutritional quality
Part 2 Processing and nutritional quality
8 The nutritional enhancement of plant foods D. Lindsay, CEBAS-CSIC, Spain 8.1 Introduction The question of why it is necessary to improve the nutritional value of plant foods is one that at first hand might seem difficult to justify. What evidence is there that this is a problem? In the developed world there are no overt signs of maIlnutri tion even amongst strict vegans. The reasons for this are that many processed plant foods are fortified with essential nutrients Fortification is utilised to replace nutrients lost in the heat processing of foods and through oxidation. Few vege tartans are dependent on a single plant source to provide their basic nutritional In addition, vegetarians frequently consume vitamins as supplements and the growth in this industry has been rapid. The fact that people are resorting to the consumption of vitamins as supplements is a reflection of their belief that more of a good thing will result in an improvement in their health. This is a very dubious argument. Nonetheless, it is important to recognise that the recom- mended intakes of nutrients, that have been determined by expert groups of nutri- tionists,are based on the evidence that a specific intake level for a nutrient is required to ensure healthy growth and development. They do not reflect the growing body of evidence that suggests different, and often higher, intakes of these same nutrients are required to optimise health and lead to an active fe through the prevention of chronic degenerative diseases associated with ageing. The critical issue is to determine what intakes are required to optimise health rather than to compromise it
8 The nutritional enhancement of plant foods D. Lindsay, CEBAS-CSIC, Spain 8.1 Introduction The question of why it is necessary to improve the nutritional value of plant foods is one that at first hand might seem difficult to justify. What evidence is there that this is a problem? In the developed world there are no overt signs of malnutrition even amongst strict vegans. The reasons for this are that many processed plant foods are fortified with essential nutrients. Fortification is utilised to replace nutrients lost in the heat processing of foods and through oxidation. Few vegetarians are dependent on a single plant source to provide their basic nutritional needs. In addition, vegetarians frequently consume vitamins as supplements and the growth in this industry has been rapid. The fact that people are resorting to the consumption of vitamins as supplements is a reflection of their belief that more of a good thing will result in an improvement in their health. This is a very dubious argument. Nonetheless, it is important to recognise that the recommended intakes of nutrients, that have been determined by expert groups of nutritionists, are based on the evidence that a specific intake level for a nutrient is required to ensure healthy growth and development. They do not reflect the growing body of evidence that suggests different, and often higher, intakes of these same nutrients are required to optimise health and lead to an active life through the prevention of chronic degenerative diseases associated with ageing.1,2,3 The critical issue is to determine what intakes are required to optimise health rather than to compromise it
196 The nutrition handbook for food processors 8.2 The nutritional importance of plants Plants are the staple food for the vast majority of the worlds population. It is known that many staple plant foods are deficient in essential nutrients and, con- sequently, malnutrition is widespread. It has been estimated that over 100 million children worldwide are vitamin a deficient and improving the vitamin A content of their food could prevent as many as two million deaths annually in young chil- dren. This is apart from the deficiencies in iodine intake, resulting in goitre, and in iron-deficient anaemia which are estimated to affect millions in the develop- ing world. There is also an important need to improve the amino acid content of legume proteins that are deficient in essential sulphur amino acids. Nutritional deficiencies can lead to a reduction in immune responsiveness, rather than a spe cific attributable disorder, making it difficult to establish clearly how many people are suffering from malnutrition In the developed world all public health authorities are urging consumers to onsume more plant-based foods as part of a healthy diet. There is a significant body of evidence to suggest that the traditional Mediterranean diet, rich in plant foods, reduces the risk of many age-related diseases. Epidemiological studies show a strong and consistent inverse relationship between fruit and vegetable intake and the risk of cardiovascular diseases and some cancers. An explosion of interest in trying to define what are the factors in fruit and vegetables which might be responsible for these observations has not yet led to a clear set of expla nations although many theories abound. Plants contain 17 mineral nutrients, 13 vitamins and numerous phytochem cals that have been shown to have potentially beneficial effects on health espe cially against the initiation or progression of degenerative diseases. Almost all human nutrients can be obtained from plant foods, the exceptions are vitamins B12 and D. However, the adequacy of a plant diet in delivering a health benefit from a specific component will depend on the amount ingested and its bioavail- ability. Many beneficial plant compounds that are associated with the plant cell wall are not easily bioavailable. Any way in which overall levels can be increased will help overcome this difficulty. 8.3 Strategies for nutritional enhancement There is no single approach to the improvement of the nutritional quality of plant foods since this is affected by a wide variety of factors. Amongst these are The application of traditional breeding methods to select for varieties with an ncreased level of the bioactive compound a reduction in the content of antinutritional factors The use of genetic manipulation to introduce new traits in plants Improvements in handling, storage and food processing technologies Each of these approaches has a role to play but genetic manipulation prov mechanism for the improvement of nutritional quality that overcomes the pro- blem of the aDsence of a specific biochemical pathway in a staple crop
8.2 The nutritional importance of plants Plants are the staple food for the vast majority of the world’s population. It is known that many staple plant foods are deficient in essential nutrients and, consequently, malnutrition is widespread. It has been estimated that over 100 million children worldwide are vitamin A deficient and improving the vitamin A content of their food could prevent as many as two million deaths annually in young children.4 This is apart from the deficiencies in iodine intake, resulting in goitre, and in iron-deficient anaemia which are estimated to affect millions in the developing world. There is also an important need to improve the amino acid content of legume proteins that are deficient in essential sulphur amino acids. Nutritional deficiencies can lead to a reduction in immune responsiveness, rather than a specific attributable disorder, making it difficult to establish clearly how many people are suffering from malnutrition.5 In the developed world all public health authorities are urging consumers to consume more plant-based foods as part of a healthy diet. There is a significant body of evidence to suggest that the traditional Mediterranean diet, rich in plant foods, reduces the risk of many age-related diseases. Epidemiological studies show a strong and consistent inverse relationship between fruit and vegetable intake and the risk of cardiovascular diseases and some cancers. 2 An explosion of interest in trying to define what are the factors in fruit and vegetables which might be responsible for these observations has not yet led to a clear set of explanations although many theories abound. Plants contain 17 mineral nutrients, 13 vitamins and numerous phytochemicals that have been shown to have potentially beneficial effects on health especially against the initiation or progression of degenerative diseases. Almost all human nutrients can be obtained from plant foods, the exceptions are vitamins B12 and D. However, the adequacy of a plant diet in delivering a health benefit from a specific component will depend on the amount ingested and its bioavailability. Many beneficial plant compounds that are associated with the plant cell wall are not easily bioavailable. Any way in which overall levels can be increased will help overcome this difficulty. 8.3 Strategies for nutritional enhancement There is no single approach to the improvement of the nutritional quality of plant foods since this is affected by a wide variety of factors. Amongst these are: • The application of traditional breeding methods to select for varieties with an increased level of the bioactive compound. • A reduction in the content of antinutritional factors. • The use of genetic manipulation to introduce new traits in plants. • Improvements in handling, storage and food processing technologies. Each of these approaches has a role to play but genetic manipulation provides a mechanism for the improvement of nutritional quality that overcomes the problem of the absence of a specific biochemical pathway in a staple crop. 196 The nutrition handbook for food processors
The nutritional enhancement of plant foods 197 8.3.1 Application of traditional breeding methods Plant varieties have not been selected to date on the basis of nutritional qualities but there are wide natural variations that can be found in the gene pool of crop plants. Examples of where significant variations in the nutrient content of geno- ypes have been documented include a 2-fold variation in calcium concentration in beans 6 4-fold variation in B-carotene concentrations in brocco 4-fold variation in folates in beetroot. .2-3-fold variation in iron and zinc levels in maize 9 In the case of the pro-vitamin A carotenoids, plants provide highly variable amounts depending on their colour. Varieties of sweet potato may contain levels varying from 0. 13 mg to 11.3 dry weight B-carotene. Similar variations in levels can be found in carrots and cassava. In the case of the tomato, genes have been identified that are associated with high and low lycopene content. Incorpo- ration of genes that increase lycopene content and/or elimination of genes that decrease the lycopene content can be achieved by pedigree selection and back cross programmes. Such techniques have produced hybrids with a three-or four- fold increase in content of lycopene in tomato fruits. 8.3.2 Reduction in antinutritional factors The interest in reducing antinutritional factors in plants has been predominantly focused around improving the nutritional value of feedstuffs Phytates are present in many plant seeds and limit phosphorus uptake as well as other elements. The potential for introducing a phytase gene into feedstuffs has been explored. However, there are other strategies that seem to be of greater overall value human nutrition. Thioredoxin is thought to be an activator of the germination process in seeds. It is able to activate proteins to degradation by proteolysis and results in improved digestibility. It also has the potential advantage of being ole to reduce allergenicity, presumably because of its capacity to break disul phide bonds by the action of the reduced thiol groups in the molecule and ensure the tertiary structure of the protein is accessible to degradation by proteases. The insertion of the wheat thioredoxin gene into barley has produced a transgenic plant where thioredoxin accounts for 7%o of the total protein content in the barley and is a good source of sulphur amino acids 8.3.3 The application of genetic manipulation Genetic engineering is being applied to enhance levels of functional compounds in food crops. Indeed, for some purposes it will be the only approach feasible especially where there are widespread deficiency diseases and the population is dependent on staple crops which are not sources of the nutrient required. There are many examples where the technology has been applied with success although tional bene roducts which have yet reached the marketing stage where nutri there are have been the main focus
8.3.1 Application of ‘traditional’ breeding methods Plant varieties have not been selected to date on the basis of nutritional qualities but there are wide natural variations that can be found in the gene pool of crop plants. Examples of where significant variations in the nutrient content of genotypes have been documented include a: • 2-fold variation in calcium concentration in beans.6 • 4-fold variation in b-carotene concentrations in broccoli.7 • 4-fold variation in folates in beetroot.8 • 2–3-fold variation in iron and zinc levels in maize.9 In the case of the pro-vitamin A carotenoids, plants provide highly variable amounts depending on their colour. Varieties of sweet potato may contain levels varying from 0.13 mg to 11.3 mg g-1 dry weight b-carotene.10 Similar variations in levels can be found in carrots and cassava. In the case of the tomato, genes have been identified that are associated with high and low lycopene content. Incorporation of genes that increase lycopene content and/or elimination of genes that decrease the lycopene content can be achieved by pedigree selection and backcross programmes. Such techniques have produced hybrids with a three- or fourfold increase in content of lycopene in tomato fruits.11 8.3.2 Reduction in antinutritional factors The interest in reducing antinutritional factors in plants has been predominantly focused around improving the nutritional value of feedstuffs. Phytates are present in many plant seeds and limit phosphorus uptake as well as other elements. The potential for introducing a phytase gene into feedstuffs has been explored.12 However, there are other strategies that seem to be of greater overall value in human nutrition. Thioredoxin is thought to be an activator of the germination process in seeds.13 It is able to activate proteins to degradation by proteolysis and results in improved digestibility.14 It also has the potential advantage of being able to reduce allergenicity, presumably because of its capacity to break disulphide bonds by the action of the reduced thiol groups in the molecule and ensure the tertiary structure of the protein is accessible to degradation by proteases.14 The insertion of the wheat thioredoxin gene into barley has produced a transgenic plant where thioredoxin accounts for 7% of the total protein content in the barley and is a good source of sulphur amino acids.15 8.3.3 The application of genetic manipulation Genetic engineering is being applied to enhance levels of functional compounds in food crops. Indeed, for some purposes it will be the only approach feasible especially where there are widespread deficiency diseases and the population is dependent on staple crops which are not sources of the nutrient required. There are many examples where the technology has been applied with success although there are no products which have yet reached the marketing stage where nutritional benefits have been the main focus. The nutritional enhancement of plant foods 197
198 The nutrition handbook for food processors Potential strategies for the enhancement of specific metabolites could target 1. Over-expression of enzymes that control the final steps in the biosynthesis f a metabolite 2. Over-expression of rate-limiting enzymes 3. Silencing of genes whose expression causes the metabolite to be degraded 4. Increased expression of genes that are not subject to metabolic feedback 5. Increasing the number of plastids in a plant. 6. Increasing metabolic flux into the pathway of interest. 7. Expression in storage organs using site-specific promoter e The strategy that has had the greatest success at present is the first one,espe- ally in conjunction with the last strategy. In practice, if a substantial Increas in the concentration of a metabolite is required, the use of specific promoters directing the synthesis to a particular organelle normally used for storage pur poses, or where the plant normally synthesises the metabolite, is essential. Failure to use these approaches could cause toxicity in the plant by interfering with the production or function of other essential metabolites. However, this strategy pre supposes the metabolite of interest is the final one in a particular pathway y. Few strategies have yet been applied where multiple gene insertions are nec essary to produce the metabolite, although these are progressing rapidly, and none where plastid numbers have been increased. However, the accumulation of sequence data of both chromosomal DNA and expressed sequence tags of plants and other species is providing rapid advances in knowledge of the genetic make up and functions of several plants, and it is expected that these other possibili ties will soon be feasible 8.4 The priorities for nutritional enhancement 8.4.1 For the developed world Although it is known that the distribution and processing of food can lead to a significant loss in nutritional quality, there are few instances where present evi- dence suggests there is a need to change current practices. There is very little evi- dence for nutritional deficiencies In those cases where public health authoritie have thought there is a potential problem, food supplementation with nutrients i a commonly adopted policy. The use of nutritional supplements is widespread Whilst the focus of current interest is on the need to consider nutrients and other phytochemicals as protective against the development of disease in later life, the levels of intake that may be necessary to optimise protection are far from resolved at the present time The only plant-derived food product on the market where nutritional health benefits are claimed (as opposed to implied) is the enrichment of margarines with plant sterol and stanol esters for the reduction in plasma cholesterol levels(Fig
Potential strategies for the enhancement of specific metabolites could target on: 1. Over-expression of enzymes that control the final steps in the biosynthesis of a metabolite. 2. Over-expression of rate-limiting enzymes. 3. Silencing of genes whose expression causes the metabolite to be degraded. 4. Increased expression of genes that are not subject to metabolic feedback control. 5. Increasing the number of plastids in a plant. 6. Increasing metabolic flux into the pathway of interest. 7. Expression in storage organs using site-specific promoters. The strategy that has had the greatest success at present is the first one, especially in conjunction with the last strategy. In practice, if a substantial increase in the concentration of a metabolite is required, the use of specific promoters directing the synthesis to a particular organelle normally used for storage purposes, or where the plant normally synthesises the metabolite, is essential. Failure to use these approaches could cause toxicity in the plant by interfering with the production or function of other essential metabolites. However, this strategy presupposes the metabolite of interest is the final one in a particular pathway. Few strategies have yet been applied where multiple gene insertions are necessary to produce the metabolite, although these are progressing rapidly, and none where plastid numbers have been increased. However, the accumulation of sequence data of both chromosomal DNA and expressed sequence tags of plants and other species is providing rapid advances in knowledge of the genetic makeup and functions of several plants, and it is expected that these other possibilities will soon be feasible. 8.4 The priorities for nutritional enhancement 8.4.1 For the developed world Although it is known that the distribution and processing of food can lead to a significant loss in nutritional quality, there are few instances where present evidence suggests there is a need to change current practices. There is very little evidence for nutritional deficiencies. In those cases where public health authorities have thought there is a potential problem, food supplementation with nutrients is a commonly adopted policy. The use of nutritional supplements is widespread. Whilst the focus of current interest is on the need to consider nutrients and other phytochemicals as protective against the development of disease in later life, the levels of intake that may be necessary to optimise protection are far from resolved at the present time. The only plant-derived food product on the market where nutritional health benefits are claimed (as opposed to implied) is the enrichment of margarines with plant sterol and stanol esters for the reduction in plasma cholesterol levels (Fig. 198 The nutrition handbook for food processors