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The impact of food processing on antioxidants in vegetable oils, fruits and vegetables
Trends in Food Science & Technology 9 (1998) 336±340
The impact of food processing on antioxidants in vegetable oils, fruits and vegetables M.G. Lindley LinTech, University of Reading Innovation Centre, Whiteknights, P.O. Box 68, Reading, RG6 6BX UK After microbial spoilage, oxidation leading to overt rancidity is the second most important cause of food spoilage. Antioxidants have largely been viewed as tools in the ®ght against oxidation. With increasing awareness of the importance of antioxidants in health maintenance, their retention through food processing and storage has assumed increasing importance. Techniques whereby fats and oils may be processed to retain the highest possible levels of antioxidants are reviewed. Alternative processes and strategies for postharvest storage and handling of fruits and vegetables are discussed. Further research needs that are designed to enhance the antioxidant status of foods are proposed. # 1998 Elsevier Science Ltd. All rights reserved
Dietary antioxidants may be nutrient or non-nutrient. The major nutrient antioxidants are vitamins C and E, with fruits and vegetables being major sources of vitamin C and vitamin E compounds being found in wheatgerm, nuts, green leafy vegetables and vegetable oils. Nonnutrient antioxidants include ¯avonoids (found for example in tea, red wine, onions and apples), polyphenols and terpenes. Antioxidants help to prevent the occurrence of oxidative damage to biological macromolecules caused by reactive oxygen species. Such oxidative damage, if it occurs, may be a signi®cant causative factor in the development of many human
Review diseases. The potential role of antioxidants in preventing or ameliorating relevant disease processes has recently been the subject of detailed critical review [1]. The focus of this paper is to discuss technical approaches to retaining antioxidant levels in foods. As such, it is acknowledged that success in this endeavour does not create functional foods; foods whose antioxidant levels have been retained merely remain more functional than those whose antioxidants have been removed or destroyed during processing, storage and distribution.
Technology
The protection of foods from oxygen is the basic principle upon which antioxidant protective technologies are based. Many of these are drawn from experiences with lipid oxidation. Thus, the contribution of food technology both to food safety and to the maintenance of high nutritional and organoleptic value should not be underestimated. After microbial spoilage, oxidation leading to overt rancidity is the second most important cause of food spoilage. To date, antioxidants have largely been viewed as tools in the ®ght against food oxidation although, with increasing awareness of their importance in health maintenance, retention of antioxidants through food processing and storage has assumed increasing importance. Maintenance of the structural integrity of foods helps to prevent oxidative changes from occurring. In whole food form, contact of these oxygen sensitive components with oxygen is reduced. Thus, processes that prevent exposure of antioxidants to oxygen to the greatest possible extent have become a requirement. Oil re®ning removes those components of edible oils that can have negative eects on taste, stability, appearance or nutritional value. Well controlled re®ning procedures are essential to preserve components with favourable nutritional properties and to prevent chemical changes in the triacylglycerols. Good re®ning practices produce oils with characteristics that are desired, such as bland ¯avour and odour, clarity, low colour and stability to oxidation. Despite scienti®c documentation of the antioxidant eects of selected spices, for example, rosemary, sage, thyme, oregano, ginger, tumeric [2], only (extracts of) rosemary are used commercially. A range of commercial
0924-2244/98/$Ðsee front matter Copyright # 1998 Elsevier Science Ltd. All rights reserved PII: S 0 92 4 - 2 24 4 ( 9 8 ) 0 0 05 0 - 8
M.G. Lindley/Trends in Food Science & Technology 9 (1998) 336±340
products is available and interest in commercial antioxidants based on other spices is increasing. Further investigation of the eectiveness of spices may be limited by the characteristic aroma that spices tend to possess. Consequently, in many commercial products attempts are made to extract the components responsible for the antioxidative eects, while simultaneously reducing extraction of the aromatic essential oils [3]. However, while the use of spice extracts has obvious attractions to food marketeers, their more widespread evaluation and use will require more than the current presumption of safety. The safety of spices and spice extracts when used for this functionality is not really proven and requires evaluation. The addition of antioxidants to foods helps to preserve constituents of the food by preventing auto-oxidation. However, genetic enhancement of antioxidant levels is an attractive alternative to forti®cation and is increasingly feasible. The pathways controlling the biosynthesis of many secondary plant metabolites are well documented. The importance of genetic engineering in crop development programmes has increased in recent years such that techniques to introduce many genes or sets of genes into speci®c plant species of interest, including major crop species, have now been developed. Additionally, advanced breeding programmes permit new traits to be moved into commercial crops in economically viable time-frames. Thus, there is substantial potential for directed genetic manipulation of crops to enhance productivity in a number of important ways, including maximising antioxidant contents (4,5). However, it must be acknowledged that there are important public acceptance issues surrounding the adoption of this technology.
Processing Processing of Fats and Oils
Fats and oils deliver energy and essential fatty acids and are essential for fat soluble vitamin absorption. They may contain natural antioxidants and they provide and carry fat soluble vitamins. Procedures have evolved within the edible fats and oils industry to purify and modify fats and oils in ways which preserve those components with favourable nutritional properties and also prevent chemical changes in triacylglycerols. Harvesting, processing and storage of lipid/antioxidant-containing materials can be viewed as necessitating a series of compromises. For example, whatever level of care taken in handling vegetable oils, from use of stainless steeel equipment, airtight packaging, refrigeration, protection from light and the addition of antioxidants, rancidity and all its associated deleterious changes always occurs sooner or later. The onset of rancidity may be delayed substantially by forti®cation with antioxidants, but it cannot be stopped completely and, once deterioration starts to occur, the process will accelerate.
337
To minimize the potential for oxidation to occur, proper handling of raw materials from their collection is imperative. During each new handling of the raw material, it must be given the requisite protection from elements such as oxygen, heat and variably valent metal ions. Retention of natural antioxidants present in each speci®c raw material will help to extend oil shelf-life considerably [6]. Two main oil re®ning methods are alkaline re®ning and physical re®ning. Both processes employ an initial de-gumming process to remove water, phospholipids and metals. Alkaline re®ning then follows with a neutralisation stage to eliminate fatty acids. In physical re®ning, fatty acids are removed by a steam distillation step. There are limitations to both processes; alkaline re®ning results in relatively low yields and large quantities of liquid euent, while physical re®ning employs higher temperatures. The alkaline re®ning conditions bene®cially remove several impurities, including oxidised components, trace metals and colour. Further removal of impurities, along with removal of a¯atoxins and some pesticides, may occur during the bleaching process. During physical re®ning, volatile components, including o-¯avours, pesticides and polycyclic aromatic hydrocarbons may be removed [7]. Many of the procedures used to help preserve antioxidant levels in foods, particularly physical control procedures, involve adoption of good manufacturing practices. These imply the use of stainless steel equipment, careful de-aeration at less than 100 C before heating to the ®nal stripping temperature, the use of oxygen-free steam, and strict control of trace element contents of feedstocks [7]. However, with the goal of minimizing oil oxidation, it is not merely sucient to ensure high antioxidant contents. For example, studies with virgin olive oils have shown that even with high antioxidant contents, oil stability is decreased if the initial peroxide value is high [8]. This demonstrates the importance of ensuring that all stages in the harvesting and handling of oil-bearing materials utilize stringent control of all factors which could impact on the oxidation status of the oil. In addition, the identity of all those non-nutrient antioxidants which may be important to health is not known. When it is, processes may require adjustment to maximize their retention. Olive oil contains many phenolic compounds whose presence aects stability and ¯avour. However, the complex chemical nature of the phenolics in olive oil has not completely been elucidated and, as yet, no standard method has been proposed for their determination. The system of oil extraction used is critical for total phenolics content. Continuous centrifugal methods produce lower levels of these components than those from oils extracted using other systems. Oil extraction using a 2phase centrifugal decanter yields olive oils with better qualitative characteristics compared with oils produced
338
M.G. Lindley/Trends in Food Science & Technology 9 (1998) 336±340
using conventional 3-phase equipment. The 2-phase process yields oils with higher contents of polyphenols, ortho-diphenols, hydroxytyrosol and tocopherols. Oils prepared using a 2-phase centrifugal decanter had higher oxidative stability. Small dierences in machinery, applied temperatures, duration of contact with water and the volume of water employed may cause signi®cant changes in total polyphenol content [9]. Conventional processing of plant-derived oils and fats involves many dierent raw materials, products and unit operations. The ®rst stage in processing is extraction of the oil. In the case of oilseeds the type of extraction depends on the initial oil content of the raw material. If the oil content is above 20% mechanical pressing is usually employed; for oil contents below 20%, solvent extraction is preferred. In either case, the majority of extractors operate continuously and counter-currently. The ®rst stage in re®ning crude vegetable oils is degumming using water or steam followed by centrifugation. Physical or chemical re®ning procedures may then be employed (see [10], and references therein). Chemical re®ning involves the addition of sodium hydroxide followed by centrifugation. Physical re®ning utilises steam to volatilise free fatty acids. After re®ning, the oil is normally bleached. Bleaching not only removes chlorophyll, but also carotenoids. Thus, this process is a compromize between removal of materials which reduce stability and those, the carotenoids, which provide protection against oxidation. The ®nal stage of oil processing is deodourisation using steam to remove any remaining volatile materials. All of these processes require large amounts of energy, and it is this high cost which has encouraged the development and evaluation of alternative processes such as membrane technology in oil re®ning. Although membrane technology is not yet much used in the vegetable oils industry, newly developed membranes and process engineering developments will facilitate its further evaluation and utilization [11]. The use of membranes that are capable of separating puri®ed oils from the impurities present in oil-rich extracts has received substantial attention. Potential applications of membrane technologies to oil re®ning processes have been reviewed [12]. An ideal membrane for use in the oil-processing industry would combine the required rejection characteristics with a high permeate ¯ux and long-term stability. The opportunties to save on energy costs through the use of membrane separation processes are considerable and constitute a signi®cant driving force for evaluation and up-take of the technology, but there remain many issues related to membrane fouling, cleaning, yields and eciency which require further research and development. Also, the design and operation of industrial-scale membrane reactors for use in oil processing have received little
attention. Experience gained in related industries might prove to be applicable.
Processing of fruits and vegetables
With fruits and vegetables, many changes occur during harvesting, preparation and handling, and many of these changes potentially impact on their antioxidant status. Intact fruits and vegetables obviously are prone to deleterious changes induced by respiratory, metabolic and enzymatic activities, as well as by transpiration, pest and microbial spoilage and temperature-induced injury. Most such changes may impact adversely on the antioxidant status of these products. Identi®cation of appropriate storage/handling conditions for fruits and vegetables is complicated by the fact that there are non-linear relationships between, for example, moisture content and antioxidant content. There appears to be an optimum moisture content, either side of which oxidation can increase quite rapidly. Temperature control, minimizing oxygen contents and protection from light constitute other physical procedures whose eective control can help to ensure maximum retention of antioxidants. Chemical processes aimed at preventing adverse changes in prepared fruits and vegetables have been practiced for many years. Inactivation of polyphenol oxidases is one example where inactivation of degradative enzymes can help to maintain antioxidant status. Addition of nutrient oxidants, particularly vitamin C, is another. Compounds such as benzoates, sorbates, metabisulphite and polyphosphates have been demonstrated as being capable of controlling spoilage and maintaining quality in prepared fruit and vegetables. In addition, preservatives that serve as antioxidants to extend shelflife of fruits and vegetables may also be shown to act through prevention of browning, reduction in discoloration of pigments, protection against ¯avour losses, changes in texture and loss of nutritional quality. Their eciency depends on a range of environmental factors such as pH, water activity (aw), temperature, light, atmosphere and heavy metal content. During the processing of fruits and vegetables, several types of oxidative reactions may occur in which electrons are removed from atoms/molecules leading to the formation of an oxidised form. These reactions cause browning reactions, loss or changes to ¯avour or odour, changes in texture and loss of nutritional value from destruction of vitamins and essential fatty acids. These changes are important in most fruits and vegetables; special problems arise in seed crops and lipid-containing vegetables leading to the development of rancid o-¯avours and oxidation products that may have toxic properties at high levels [13]. Four categories of chemical structures may be used to stabilise fruits and vegetables. They are (i) free radical chain breakers such as tocopherols, (ii) reducing agents
M.G. Lindley/Trends in Food Science & Technology 9 (1998) 336±340
and oxygen scavengers such as ascorbic acid and erythorbic acid, (iii) chelating agents such as citric acid, and (iv) other `secondary' antioxidants such as carotenoids. Of these, the most important compounds used to stabilise fruits and vegetables are reducing agents and certain chelating agents that are not actually antioxidants but are important in preventing oxidative damage. Recent restriction on the use of sulphites has highlighted the need for suitable substitutes and although combinations of ascorbic acid and derivatives with citric acid and other organic acids are quite eective, there remains a need for more eective combinations [14]. Packaging of fresh fruit and vegetables has been practiced for decades to contain and protect from contaminants. An important requirement is to preserve package contents and prevent or retard chemical decomposition, for both fresh and minimally processed fruits and vegetables are living tissues undergoing catabolic metabolism including respiration [15]. In order to select appropriate packaging materials, as much information as possible must be accumulated about the ®nished product, including stage of maturity at harvest, cultivar, chill injury threshold, shelf life duration target, etc. Although some of this information is available, much is unavailable, particularly that relating to the interrelationships between the packaging environment and consequent eects on nutrient and non-nutrient antioxidants [16]. Seemingly obvious routes to maximising antioxidant status through removal of oxygen and packaging in ®lm of low gas permeability actually leads to accumulation of carbon dioxide, ultimately inducing tissue anoxia in an anaerobic environment. This observation demonstrates the diculties inherent in selecting packaging procedures and materials purely on the basis of preserving antioxidants. Quality maintenance and/or improvement through the use of active packaging has received recent attention, with oxygen scavenging techniques being of particular interest. Oxygen absorbent sachets, most commonly containing iron powder or, to a lesser extent, ascorbic acid, may be used to prolong the shelf-life of various foods (17, 18). Alternative approaches include incorporation of oxygen scavenging materials, such as ascorbic acid, into the packaging itself. Vitamin E has also been incorporated into packaging ®lms, from which it may migrate into the food, so eliminating the need to add antioxidants to the food itself [19]. New processing technologies directed towards producing stable foods following minimal treatment are also relevant, not only for the impact these minimal processes will have on product quality, but also on the preservation of antioxidant status. A relevant example may be the application of high electric ®eld pulses for the treatment of fruit juices, a process that has received attention [20].
339
Nutritional/safety implications of processes
Most studies of the impact of processing on fruits and vegetables, in particular, have been concerned with market quality as determined objectively and subjectively by colour, ¯avour and texture measurements. Data on the nutrient content and retention, particularly for minimally processed foods, are generally sparse. Data on ascorbic acid retention may be available. For example, ascorbic acid is retained well in citrus juices for about 4 weeks and only a minimal amount of ascorbic acid is lost when juices are stored in open containers [21]. Data that determine the eects of controlled atmosphere, modi®ed atmosphere and refrigeration on the nutritive value of fruits and vegetables are not generally, as yet, available. Thus, the nutritional implications of such processes, at least insofar as they relate to antioxidant vitamin retention, are incompletely understood [22]. There are more data on the eect of processing conditions on the nutritional content of fats and oils [7]. The processing conditions under which cis-trans isomerisation, especially of linolenic acid, occurs are well understood, and the formation of trans isomers is slow, even under the temperature conditions (for example, 250 C) recommended for industrial deodourisation/ physical re®ning. Data on the formation of polymeric compounds during the re®ning process are also available. In intact and/or formulated foods, another eect of heating is to increase isomerization of carotenoids. For example, supplemental ( -carotene added to breads and crackers before baking showed signi®cant trans to cis isomerization [23], the nutritional signi®cance of which stems from the observation that the cis isomer is less well absorbed than the trans isomer [24].
Process monitoring for functions
Process monitoring for retention of antioxidants throughout processing and storage is needed. To do so, it may be necessary to develop rapid methods to monitor survival of the antioxidants themselves, or to measure secondary eects such as, in the case of oils, peroxide values.
Further research needs
Most food technology needs in this area are driven by nutrition [1]. Food technological procedures have been developed over many years which present high quality fats and oils whose undesirable components (¯avours and contaminants) have been removed and the desirable antioxidants retained. Techniques which lead to quality fruits and vegetables being available for consumers are also well established. In many respects, retention of appropriate texture, desirable ¯avour and appearance correlate well with maintenance of appropriate antioxidant status.
340
M.G. Lindley/Trends in Food Science & Technology 9 (1998) 336±340
However, there are areas where further work is required, the results of which may lead to a requirement for further food technology research into processing in particular. 1.
2.
3. 4.
5.
6.
Generally, retention of maximum levels of antioxidants through application of appropriate food technology processes is an appropriate goal. However, few data exist on the nutritional value of speci®c non-nutrient antioxidants. Assuming such data are generated, there will be a need to adapt processes to maximise their retention. Development of membrane processing techniques for fats and oils which reduce energy consumption, minimise degradation and maximise antioxidant status. Research into the eectiveness of blends of antioxidants may be expected to identify possible synergistic combinations. The potential for spices and extracts to function as eective antioxidants, while delivering no/ minimal ¯avour, needs to be explored further. Following identi®cation of eective, ¯avouracceptable, spice-derived antioxidants, evaluation of their safety will be necessary. Almost all the studies published on minimally processed fruits and vegetables have been market quality studies. Except for irradiation studies, published data on nutrient content and nutrient retention of minimally processed foods are generally sparse and are needed. The potential safety considerations that may result from application of minimal processes also need to be understood.
References
1 Diplock, A.T., Charleux, J.-L., Crozier-Willi, G., Kok, F.J., RiceEvans, C., Roberfroid, M., Stahl, W. and Vina-Ribes, J. (1998) `Functional Food Science and defence against reactive oxidative species' in Brit. J. Nutr. 80, Supplement No. 1, S77±S112 2 Loliger, J., Lambelet, P., Aeschbach, R. and Prior, E. (1996) `Natural antioxidants: from radical mechanisms to food stabilization' in Food Lipids and Health. pp 68±77. Eds. McDonald, R.E. and Min, D.B. Marcel Dekker Inc., NY, NY 3 Madsen, H.L. and Bertelsen, G. (1995) `Spices as Antioxidants' in Trends Fd Sci. Technol. 61, 271±277 4 Timberlake, W.E. (1998) `Agricultural Genomics Comes of Age' in Nature Biotech. 16, 116±117
5 Mullineaux, P.M. and Creisson, G.P. (1996) `Opportunities for the Genetic Manipulation of Antioxidants in Plant Foods' in Biochem. Soc. Trans. 24, 829±835 6 Vinter, H. (1995) The Handling of Fats and Oils in the Food Industry: new views in Food Technol. Internat. Europe, 133±136 7 Korver, O. and Zevenbergen, J.L. (1997) `Nutritional consequences of oil re®ning processes' in Oil re®ning and nutrition, pp 3±6 8 Satue, M.T., Shu-Wen, H. and Frankel, E.N. (1995) `Eect of Natural Antioxidants in Virgin Olive Oil on Oxidative Stability of Re®ned, Bleached and Deodorized Olive Oil' in J. Amer. Oil Chem. Soc. 72, 1131±1137 9 Papadopoulos, G. and Boskou, D. (1991) `Antioxidant eect of natural phenols in olive oil' in J. Amer. Oil Chem. Soc. 68, 669± 671 10 Young, F.V.K. (1994) In The Lipid Handbook, 2nd edition, (Gunstone, F.D. Harwood, J.L., Padley, F., eds) Ch 5.4. Chapman and Hall, London, UK 11 Cuperus, F.P. and Derksen, J.T.P. (1995) `Membranes for the Processing of Vegetable Oils' in Lipid Technol. 7, 101±107 12 Snape, J.B. and Nakajima, M. (1996) `Processing of Agricultural Fats and Oils using Membrane Technology' in J. Food Eng. 30, 1±41 13 Dziezak, J.D. (1986) `Preservative Systems in Foods, Antioxidants and Antimicrobial Agents' in Food Technol. 40, 94±136 14 Wiley, R.C. (1994) 'Preservation Methods for Minimally Processed Refrigerated Fruits and Vegetables' in Minimally Processed Refrigerated Fruits and Vegetables, (Wiley, R.C.) pp.66± 134, Chapman and Hall, New York 15 Institute of Food Technologists, (1990) `Quality of Fruits and VegetablesÐa Scienti®c Status Summary by IFT's Expert Panel on food safety and nutrition' in Food Technol. 44, 99±106 16 Schlimme, D.V. and Rooney, M.L. (1994) `Packaging of Minimally Processed Fruits and Vegetables''in Minimally Processed Refrigerated Fruits and Vegetables, (Wiley, R.C.), pp. 135±182, Chapman and Hall, New York 17 Harima, Y. (1990) `Free Radical Scavenging Packaging' in Food Packaging, 229±252 18 Powers, E.M. and Berkowitz, D. (1990) `Immunochemical Methods for Environmental Analysis' in J. Food Protect. 53, 767±771 19 Nielsen, T. (1997) `Active Packaging' in SIK Report 631 20 Knorr, D., Guelen, M., Grahl, T. and Sitzmann, W. (1994) `Food application of high electric ®eld pulses' in Trends Food Sci. Technol. 5, 71±75 21 Nagy, S. (1980) `Evaluation and Control of Undesirable Flavors in Processed Citrus Juices' in J. Agric. Food Chem. 28, 8±18 22 Matthews, R.H. and McCarthy, M.A. (1994) `Nutritional Quality of Fruits and Vegetables Subject to Minimal Processes' in Minimally Processed Refrigerated Fruits and Vegetables, (Wiley, R.C.), pp.313±326, Chapman and Hall, New York 23 Ranhota, G.S., Gelroth, J., Langemeier, A. and Rogers, D.E. (1995) `Stability and contribution of ( -carotene added to whole wheat bread and crackers' in Cereal Chem. 72, 139±141 24 Gaziano, J.M., Johnson, E.J., Russell, R.M., Manson, J.E., Stampfer, M.J., Ridker, P.M., Frei, B., Henekens, C.H. and Krinsky, N.I. (1995) `Discrimination and absorption or transport of ( -carotene isomers' in Amer. J. Clin. Nutr. 61, 1248±1252
The impact of food processing on antioxidants in vegetable oils, fruits and vegetables M.G. Lindley LinTech, University of Reading Innovation Centre, Whiteknights, P.O. Box 68, Reading, RG6 6BX UK After microbial spoilage, oxidation leading to overt rancidity is the second most important cause of food spoilage. Antioxidants have largely been viewed as tools in the ®ght against oxidation. With increasing awareness of the importance of antioxidants in health maintenance, their retention through food processing and storage has assumed increasing importance. Techniques whereby fats and oils may be processed to retain the highest possible levels of antioxidants are reviewed. Alternative processes and strategies for postharvest storage and handling of fruits and vegetables are discussed. Further research needs that are designed to enhance the antioxidant status of foods are proposed. # 1998 Elsevier Science Ltd. All rights reserved
Dietary antioxidants may be nutrient or non-nutrient. The major nutrient antioxidants are vitamins C and E, with fruits and vegetables being major sources of vitamin C and vitamin E compounds being found in wheatgerm, nuts, green leafy vegetables and vegetable oils. Nonnutrient antioxidants include ¯avonoids (found for example in tea, red wine, onions and apples), polyphenols and terpenes. Antioxidants help to prevent the occurrence of oxidative damage to biological macromolecules caused by reactive oxygen species. Such oxidative damage, if it occurs, may be a signi®cant causative factor in the development of many human
Review diseases. The potential role of antioxidants in preventing or ameliorating relevant disease processes has recently been the subject of detailed critical review [1]. The focus of this paper is to discuss technical approaches to retaining antioxidant levels in foods. As such, it is acknowledged that success in this endeavour does not create functional foods; foods whose antioxidant levels have been retained merely remain more functional than those whose antioxidants have been removed or destroyed during processing, storage and distribution.
Technology
The protection of foods from oxygen is the basic principle upon which antioxidant protective technologies are based. Many of these are drawn from experiences with lipid oxidation. Thus, the contribution of food technology both to food safety and to the maintenance of high nutritional and organoleptic value should not be underestimated. After microbial spoilage, oxidation leading to overt rancidity is the second most important cause of food spoilage. To date, antioxidants have largely been viewed as tools in the ®ght against food oxidation although, with increasing awareness of their importance in health maintenance, retention of antioxidants through food processing and storage has assumed increasing importance. Maintenance of the structural integrity of foods helps to prevent oxidative changes from occurring. In whole food form, contact of these oxygen sensitive components with oxygen is reduced. Thus, processes that prevent exposure of antioxidants to oxygen to the greatest possible extent have become a requirement. Oil re®ning removes those components of edible oils that can have negative eects on taste, stability, appearance or nutritional value. Well controlled re®ning procedures are essential to preserve components with favourable nutritional properties and to prevent chemical changes in the triacylglycerols. Good re®ning practices produce oils with characteristics that are desired, such as bland ¯avour and odour, clarity, low colour and stability to oxidation. Despite scienti®c documentation of the antioxidant eects of selected spices, for example, rosemary, sage, thyme, oregano, ginger, tumeric [2], only (extracts of) rosemary are used commercially. A range of commercial
0924-2244/98/$Ðsee front matter Copyright # 1998 Elsevier Science Ltd. All rights reserved PII: S 0 92 4 - 2 24 4 ( 9 8 ) 0 0 05 0 - 8
M.G. Lindley/Trends in Food Science & Technology 9 (1998) 336±340
products is available and interest in commercial antioxidants based on other spices is increasing. Further investigation of the eectiveness of spices may be limited by the characteristic aroma that spices tend to possess. Consequently, in many commercial products attempts are made to extract the components responsible for the antioxidative eects, while simultaneously reducing extraction of the aromatic essential oils [3]. However, while the use of spice extracts has obvious attractions to food marketeers, their more widespread evaluation and use will require more than the current presumption of safety. The safety of spices and spice extracts when used for this functionality is not really proven and requires evaluation. The addition of antioxidants to foods helps to preserve constituents of the food by preventing auto-oxidation. However, genetic enhancement of antioxidant levels is an attractive alternative to forti®cation and is increasingly feasible. The pathways controlling the biosynthesis of many secondary plant metabolites are well documented. The importance of genetic engineering in crop development programmes has increased in recent years such that techniques to introduce many genes or sets of genes into speci®c plant species of interest, including major crop species, have now been developed. Additionally, advanced breeding programmes permit new traits to be moved into commercial crops in economically viable time-frames. Thus, there is substantial potential for directed genetic manipulation of crops to enhance productivity in a number of important ways, including maximising antioxidant contents (4,5). However, it must be acknowledged that there are important public acceptance issues surrounding the adoption of this technology.
Processing Processing of Fats and Oils
Fats and oils deliver energy and essential fatty acids and are essential for fat soluble vitamin absorption. They may contain natural antioxidants and they provide and carry fat soluble vitamins. Procedures have evolved within the edible fats and oils industry to purify and modify fats and oils in ways which preserve those components with favourable nutritional properties and also prevent chemical changes in triacylglycerols. Harvesting, processing and storage of lipid/antioxidant-containing materials can be viewed as necessitating a series of compromises. For example, whatever level of care taken in handling vegetable oils, from use of stainless steeel equipment, airtight packaging, refrigeration, protection from light and the addition of antioxidants, rancidity and all its associated deleterious changes always occurs sooner or later. The onset of rancidity may be delayed substantially by forti®cation with antioxidants, but it cannot be stopped completely and, once deterioration starts to occur, the process will accelerate.
337
To minimize the potential for oxidation to occur, proper handling of raw materials from their collection is imperative. During each new handling of the raw material, it must be given the requisite protection from elements such as oxygen, heat and variably valent metal ions. Retention of natural antioxidants present in each speci®c raw material will help to extend oil shelf-life considerably [6]. Two main oil re®ning methods are alkaline re®ning and physical re®ning. Both processes employ an initial de-gumming process to remove water, phospholipids and metals. Alkaline re®ning then follows with a neutralisation stage to eliminate fatty acids. In physical re®ning, fatty acids are removed by a steam distillation step. There are limitations to both processes; alkaline re®ning results in relatively low yields and large quantities of liquid euent, while physical re®ning employs higher temperatures. The alkaline re®ning conditions bene®cially remove several impurities, including oxidised components, trace metals and colour. Further removal of impurities, along with removal of a¯atoxins and some pesticides, may occur during the bleaching process. During physical re®ning, volatile components, including o-¯avours, pesticides and polycyclic aromatic hydrocarbons may be removed [7]. Many of the procedures used to help preserve antioxidant levels in foods, particularly physical control procedures, involve adoption of good manufacturing practices. These imply the use of stainless steel equipment, careful de-aeration at less than 100 C before heating to the ®nal stripping temperature, the use of oxygen-free steam, and strict control of trace element contents of feedstocks [7]. However, with the goal of minimizing oil oxidation, it is not merely sucient to ensure high antioxidant contents. For example, studies with virgin olive oils have shown that even with high antioxidant contents, oil stability is decreased if the initial peroxide value is high [8]. This demonstrates the importance of ensuring that all stages in the harvesting and handling of oil-bearing materials utilize stringent control of all factors which could impact on the oxidation status of the oil. In addition, the identity of all those non-nutrient antioxidants which may be important to health is not known. When it is, processes may require adjustment to maximize their retention. Olive oil contains many phenolic compounds whose presence aects stability and ¯avour. However, the complex chemical nature of the phenolics in olive oil has not completely been elucidated and, as yet, no standard method has been proposed for their determination. The system of oil extraction used is critical for total phenolics content. Continuous centrifugal methods produce lower levels of these components than those from oils extracted using other systems. Oil extraction using a 2phase centrifugal decanter yields olive oils with better qualitative characteristics compared with oils produced
338
M.G. Lindley/Trends in Food Science & Technology 9 (1998) 336±340
using conventional 3-phase equipment. The 2-phase process yields oils with higher contents of polyphenols, ortho-diphenols, hydroxytyrosol and tocopherols. Oils prepared using a 2-phase centrifugal decanter had higher oxidative stability. Small dierences in machinery, applied temperatures, duration of contact with water and the volume of water employed may cause signi®cant changes in total polyphenol content [9]. Conventional processing of plant-derived oils and fats involves many dierent raw materials, products and unit operations. The ®rst stage in processing is extraction of the oil. In the case of oilseeds the type of extraction depends on the initial oil content of the raw material. If the oil content is above 20% mechanical pressing is usually employed; for oil contents below 20%, solvent extraction is preferred. In either case, the majority of extractors operate continuously and counter-currently. The ®rst stage in re®ning crude vegetable oils is degumming using water or steam followed by centrifugation. Physical or chemical re®ning procedures may then be employed (see [10], and references therein). Chemical re®ning involves the addition of sodium hydroxide followed by centrifugation. Physical re®ning utilises steam to volatilise free fatty acids. After re®ning, the oil is normally bleached. Bleaching not only removes chlorophyll, but also carotenoids. Thus, this process is a compromize between removal of materials which reduce stability and those, the carotenoids, which provide protection against oxidation. The ®nal stage of oil processing is deodourisation using steam to remove any remaining volatile materials. All of these processes require large amounts of energy, and it is this high cost which has encouraged the development and evaluation of alternative processes such as membrane technology in oil re®ning. Although membrane technology is not yet much used in the vegetable oils industry, newly developed membranes and process engineering developments will facilitate its further evaluation and utilization [11]. The use of membranes that are capable of separating puri®ed oils from the impurities present in oil-rich extracts has received substantial attention. Potential applications of membrane technologies to oil re®ning processes have been reviewed [12]. An ideal membrane for use in the oil-processing industry would combine the required rejection characteristics with a high permeate ¯ux and long-term stability. The opportunties to save on energy costs through the use of membrane separation processes are considerable and constitute a signi®cant driving force for evaluation and up-take of the technology, but there remain many issues related to membrane fouling, cleaning, yields and eciency which require further research and development. Also, the design and operation of industrial-scale membrane reactors for use in oil processing have received little
attention. Experience gained in related industries might prove to be applicable.
Processing of fruits and vegetables
With fruits and vegetables, many changes occur during harvesting, preparation and handling, and many of these changes potentially impact on their antioxidant status. Intact fruits and vegetables obviously are prone to deleterious changes induced by respiratory, metabolic and enzymatic activities, as well as by transpiration, pest and microbial spoilage and temperature-induced injury. Most such changes may impact adversely on the antioxidant status of these products. Identi®cation of appropriate storage/handling conditions for fruits and vegetables is complicated by the fact that there are non-linear relationships between, for example, moisture content and antioxidant content. There appears to be an optimum moisture content, either side of which oxidation can increase quite rapidly. Temperature control, minimizing oxygen contents and protection from light constitute other physical procedures whose eective control can help to ensure maximum retention of antioxidants. Chemical processes aimed at preventing adverse changes in prepared fruits and vegetables have been practiced for many years. Inactivation of polyphenol oxidases is one example where inactivation of degradative enzymes can help to maintain antioxidant status. Addition of nutrient oxidants, particularly vitamin C, is another. Compounds such as benzoates, sorbates, metabisulphite and polyphosphates have been demonstrated as being capable of controlling spoilage and maintaining quality in prepared fruit and vegetables. In addition, preservatives that serve as antioxidants to extend shelflife of fruits and vegetables may also be shown to act through prevention of browning, reduction in discoloration of pigments, protection against ¯avour losses, changes in texture and loss of nutritional quality. Their eciency depends on a range of environmental factors such as pH, water activity (aw), temperature, light, atmosphere and heavy metal content. During the processing of fruits and vegetables, several types of oxidative reactions may occur in which electrons are removed from atoms/molecules leading to the formation of an oxidised form. These reactions cause browning reactions, loss or changes to ¯avour or odour, changes in texture and loss of nutritional value from destruction of vitamins and essential fatty acids. These changes are important in most fruits and vegetables; special problems arise in seed crops and lipid-containing vegetables leading to the development of rancid o-¯avours and oxidation products that may have toxic properties at high levels [13]. Four categories of chemical structures may be used to stabilise fruits and vegetables. They are (i) free radical chain breakers such as tocopherols, (ii) reducing agents
M.G. Lindley/Trends in Food Science & Technology 9 (1998) 336±340
and oxygen scavengers such as ascorbic acid and erythorbic acid, (iii) chelating agents such as citric acid, and (iv) other `secondary' antioxidants such as carotenoids. Of these, the most important compounds used to stabilise fruits and vegetables are reducing agents and certain chelating agents that are not actually antioxidants but are important in preventing oxidative damage. Recent restriction on the use of sulphites has highlighted the need for suitable substitutes and although combinations of ascorbic acid and derivatives with citric acid and other organic acids are quite eective, there remains a need for more eective combinations [14]. Packaging of fresh fruit and vegetables has been practiced for decades to contain and protect from contaminants. An important requirement is to preserve package contents and prevent or retard chemical decomposition, for both fresh and minimally processed fruits and vegetables are living tissues undergoing catabolic metabolism including respiration [15]. In order to select appropriate packaging materials, as much information as possible must be accumulated about the ®nished product, including stage of maturity at harvest, cultivar, chill injury threshold, shelf life duration target, etc. Although some of this information is available, much is unavailable, particularly that relating to the interrelationships between the packaging environment and consequent eects on nutrient and non-nutrient antioxidants [16]. Seemingly obvious routes to maximising antioxidant status through removal of oxygen and packaging in ®lm of low gas permeability actually leads to accumulation of carbon dioxide, ultimately inducing tissue anoxia in an anaerobic environment. This observation demonstrates the diculties inherent in selecting packaging procedures and materials purely on the basis of preserving antioxidants. Quality maintenance and/or improvement through the use of active packaging has received recent attention, with oxygen scavenging techniques being of particular interest. Oxygen absorbent sachets, most commonly containing iron powder or, to a lesser extent, ascorbic acid, may be used to prolong the shelf-life of various foods (17, 18). Alternative approaches include incorporation of oxygen scavenging materials, such as ascorbic acid, into the packaging itself. Vitamin E has also been incorporated into packaging ®lms, from which it may migrate into the food, so eliminating the need to add antioxidants to the food itself [19]. New processing technologies directed towards producing stable foods following minimal treatment are also relevant, not only for the impact these minimal processes will have on product quality, but also on the preservation of antioxidant status. A relevant example may be the application of high electric ®eld pulses for the treatment of fruit juices, a process that has received attention [20].
339
Nutritional/safety implications of processes
Most studies of the impact of processing on fruits and vegetables, in particular, have been concerned with market quality as determined objectively and subjectively by colour, ¯avour and texture measurements. Data on the nutrient content and retention, particularly for minimally processed foods, are generally sparse. Data on ascorbic acid retention may be available. For example, ascorbic acid is retained well in citrus juices for about 4 weeks and only a minimal amount of ascorbic acid is lost when juices are stored in open containers [21]. Data that determine the eects of controlled atmosphere, modi®ed atmosphere and refrigeration on the nutritive value of fruits and vegetables are not generally, as yet, available. Thus, the nutritional implications of such processes, at least insofar as they relate to antioxidant vitamin retention, are incompletely understood [22]. There are more data on the eect of processing conditions on the nutritional content of fats and oils [7]. The processing conditions under which cis-trans isomerisation, especially of linolenic acid, occurs are well understood, and the formation of trans isomers is slow, even under the temperature conditions (for example, 250 C) recommended for industrial deodourisation/ physical re®ning. Data on the formation of polymeric compounds during the re®ning process are also available. In intact and/or formulated foods, another eect of heating is to increase isomerization of carotenoids. For example, supplemental ( -carotene added to breads and crackers before baking showed signi®cant trans to cis isomerization [23], the nutritional signi®cance of which stems from the observation that the cis isomer is less well absorbed than the trans isomer [24].
Process monitoring for functions
Process monitoring for retention of antioxidants throughout processing and storage is needed. To do so, it may be necessary to develop rapid methods to monitor survival of the antioxidants themselves, or to measure secondary eects such as, in the case of oils, peroxide values.
Further research needs
Most food technology needs in this area are driven by nutrition [1]. Food technological procedures have been developed over many years which present high quality fats and oils whose undesirable components (¯avours and contaminants) have been removed and the desirable antioxidants retained. Techniques which lead to quality fruits and vegetables being available for consumers are also well established. In many respects, retention of appropriate texture, desirable ¯avour and appearance correlate well with maintenance of appropriate antioxidant status.
340
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However, there are areas where further work is required, the results of which may lead to a requirement for further food technology research into processing in particular. 1.
2.
3. 4.
5.
6.
Generally, retention of maximum levels of antioxidants through application of appropriate food technology processes is an appropriate goal. However, few data exist on the nutritional value of speci®c non-nutrient antioxidants. Assuming such data are generated, there will be a need to adapt processes to maximise their retention. Development of membrane processing techniques for fats and oils which reduce energy consumption, minimise degradation and maximise antioxidant status. Research into the eectiveness of blends of antioxidants may be expected to identify possible synergistic combinations. The potential for spices and extracts to function as eective antioxidants, while delivering no/ minimal ¯avour, needs to be explored further. Following identi®cation of eective, ¯avouracceptable, spice-derived antioxidants, evaluation of their safety will be necessary. Almost all the studies published on minimally processed fruits and vegetables have been market quality studies. Except for irradiation studies, published data on nutrient content and nutrient retention of minimally processed foods are generally sparse and are needed. The potential safety considerations that may result from application of minimal processes also need to be understood.
References
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