200 The nutrition handbook for food processors The ideal situation would be for sufficient sterols to be present in our diets to ensure that plasma cholesterol levels are kept reasonably low without the need to buy a specific functional food, and that they would be in a fat-soluble form for effective uptake. The evidence favours, in increasing order of preference, the 1. Plant sterol esters with low campesterol contents. 2. Sterol esters from tall oil(derived from pine wood) which have a high stanol content than edible oils 3. Plant stanol esters A vegetable oil rich in plant stanols, especially in sitostanol esterified with polyunsaturated fatty acids, would also have the benefit of being less susceptible to oxidation at frying temperatures than the sterols. The potential health benefits of this class of bioactive compounds are unlikely to be met by the use of classi cal plant breeding methods but genetic engineering could make these targets feasible 8.4.2 For the developing world The world-wide deficiency of vitamin A is being tackled both through conven- tional plant breeding and by genetic manipulation. However, the use of conven- tional plant breeding to deliver adequate intakes is dependent on the availability of carotenoid-rich staple foods. Often these are available for very restricted times of the year in some societies. In those countries where rice is a dietary staple the problem is particularly severe and the deficiency is likely to be corrected only by he introduction of rice that has been genetically manipulated to produce p- carotene. However, yellow rice is produced and this may give rise to problems of acceptability to consumers used to white rice Manipulation of the carotenoid pathway in rice The nature of the challenges faced in manipulating plant secondary metabolites is well illustrated through the attempts that have been made to produce carotenoids in rice plants. A simplified version of the pathways leading to the synthesis of the carotenoids principally found in food plants is shown in Immature rice endosperm is capable of synthesising the early pathway inter- mediate geranylgeranyl diphosphate(GP). Four plant genes corresponding to the enzymes phytoene synthase (psy)(1), phytoene desaturase(2), zeta carotene desaturase (3)and lycopene cyclase(crt)(4)are required. Enzyme(1)was obtained from the daffodil(narcissus pseudonarcissus),(2) from a bacterium Erwinia uredovora-which is capable of achieving steps(2)and (3)from the single enzyme, and (4) from the daffodil The genes need to be expressed in a tissue-specific manner through the inser tion of specific promoters. This has been achieved in rice through the use of the daffodil psy gene. In rice the daffodil psy cDNA insertion is under the controlThe ideal situation would be for sufficient sterols to be present in our diets to ensure that plasma cholesterol levels are kept reasonably low without the need to buy a specific functional food, and that they would be in a fat-soluble form for effective uptake. The evidence favours, in increasing order of preference, the use of: 1. Plant sterol esters with low campesterol contents. 2. Sterol esters from tall oil (derived from pine wood) which have a higher stanol content than edible oils. 3. Plant stanol esters. A vegetable oil rich in plant stanols, especially in sitostanol esterified with polyunsaturated fatty acids, would also have the benefit of being less susceptible to oxidation at frying temperatures than the sterols. The potential health benefits of this class of bioactive compounds are unlikely to be met by the use of classi￾cal plant breeding methods but genetic engineering could make these targets feasible. 8.4.2 For the developing world The world-wide deficiency of vitamin A is being tackled both through conven￾tional plant breeding and by genetic manipulation. However, the use of conven￾tional plant breeding to deliver adequate intakes is dependent on the availability of carotenoid-rich staple foods. Often these are available for very restricted times of the year in some societies. In those countries where rice is a dietary staple the problem is particularly severe and the deficiency is likely to be corrected only by the introduction of rice that has been genetically manipulated to produce b￾carotene. However, yellow rice is produced and this may give rise to problems of acceptability to consumers used to white rice. Manipulation of the carotenoid pathway in rice The nature of the challenges faced in manipulating plant secondary metabolites is well illustrated through the attempts that have been made to produce carotenoids in rice plants. A simplified version of the pathways leading to the synthesis of the carotenoids principally found in food plants is shown in Figure 8.2. Immature rice endosperm is capable of synthesising the early pathway inter￾mediate geranylgeranyl diphosphate (GP). Four plant genes corresponding to the enzymes phytoene synthase (psy) (1), phytoene desaturase (2), zeta carotene desaturase (3) and lycopene cyclase (crt) (4) are required. Enzyme (1) was obtained from the daffodil (narcissus pseudonarcissus), (2) from a bacterium Erwinia uredovora – which is capable of achieving steps (2) and (3) from the single enzyme, and (4) from the daffodil. The genes need to be expressed in a tissue-specific manner through the inser￾tion of specific promoters. This has been achieved in rice through the use of the daffodil psy gene.19 In rice the daffodil psy cDNA insertion is under the control 200 The nutrition handbook for food processors