Non-microbiological factors affecting quality and safety H M. Brown and M.N. Hall, Campden and Chorleywood Food Research association 9.1 Introduction As the chilled foods market has expanded and become more competitive, so have the demands for diversity, quality and longer shelf-life. Meeting these demands in a responsible, safe and cost-effective manner requires the application of an understanding of the factors that affect product safety and quality. Many problems can be avoided by applying this knowledge to a formalised HACCP approach to identify critical control points relating to quality as well as safety and to make realistic predictions of shelf-life. Considering these issues early in the product development process offers the best chance of providing a product that meets the consumer's expectations and delivers the desired market opportunities to the company. Food is probably the most hemically complex substance that most people encounter. There are over half million naturally occurring compounds in fresh plant food and more are formed as a result of processing, cooking and storage. They are responsible for the appearance, flavour, texture and nutritional value of the food(quality ) and for its physiological effects when consumed(safety) Non-microbiological factors that affect quality and safety of chilled foods be broadly divided into chemical, biochemical and physico-chemical factors Each of these is dependent on properties of the food(e.g. pH, water activity )and the conditions in which the food is held(e.g. temperature, gaseous atmosphere) Attention to the selection of raw materials in order to achieve high quality is paramount, since subsequent processing cannot compensate for poor-quality raw materials, particularly for chilled foods in which the perception of "freshness is one of the most important criteria for its purchase
9.1 Introduction As the chilled foods market has expanded and become more competitive, so have the demands for diversity, quality and longer shelf-life. Meeting these demands in a responsible, safe and cost-effective manner requires the application of an understanding of the factors that affect product safety and quality. Many problems can be avoided by applying this knowledge to a formalised HACCP approach to identify critical control points relating to quality as well as safety and to make realistic predictions of shelf-life. Considering these issues early in the product development process offers the best chance of providing a product that meets the consumer’s expectations and delivers the desired market opportunities to the company. Food is probably the most chemically complex substance that most people encounter. There are over half a million naturally occurring compounds in fresh plant food and more are formed as a result of processing, cooking and storage. They are responsible for the appearance, flavour, texture and nutritional value of the food (quality), and for its physiological effects when consumed (safety). Non-microbiological factors that affect quality and safety of chilled foods can be broadly divided into chemical, biochemical and physico-chemical factors. Each of these is dependent on properties of the food (e.g. pH, water activity) and the conditions in which the food is held (e.g. temperature, gaseous atmosphere). Attention to the selection of raw materials in order to achieve high quality is paramount, since subsequent processing cannot compensate for poor-quality raw materials, particularly for chilled foods in which the perception of ‘freshness’ is one of the most important criteria for its purchase. 9 Non-microbiological factors affecting quality and safety H. M. Brown and M. N. Hall, Campden and Chorleywood Food Research Association
226 Chilled foods he effects of chemical, biochemical and physio-chemical factors mutually exclusive but these categories provide a convenient framework for iscussion. The effects of these factors are not always detrimental and in some instances they are essential for the development of the desired characteristics of a product. In this chapter, some of the characteristics of chemical, biochemical and physico-chemical reactions are described, along with examples that are of significance to chilled food 9.2 Characteristics of chemical reactions Chemical reactions will proceed if reactants are available, if they are in a suitable form and if the activation energy threshold of the reaction is exceeded The presence of inorganic catalysts reduces the activation energy threshold and causes reactions to proceed that would otherwise not have done so. The reaction rate is dependent on the concentration of the reactants and on the temperature Increases in temperature speed up the random movement of reactant molecules increasing the probability of their coming into contact. A general assumption is that for every 10C rise in temperature the rate of reaction doubles 9.3 Chemical reactions of significance in chilled foods 9.3.1 Lipid oxidation Lipid oxidation is one of the major causes of deterioration in the quality of meat and meat products. Cooked meats and poultry rapidly develop a characteristic oxidized flavour, termedwarmed-over' flavour (WoF) by Tims and Watts (1958). The flavour is best described as that associated with reheated meat and has been described as such by sensory assessors during free profiling of precooked meat, reheated after chill storage( Churchill et al. 1988, Lyon 1987) Further descriptors have been defined for WOF in pork(Byrne et al. 1999a)and chicken meat (Byrne et al. 1999b) and have resulted in the development of sensory vocabularies containing 16 and 18 terms respectively. In cooked meats held at chill storage temperatures, this stale, oxidized flavour becomes apparent within a short time(48 hours) which contrasts with the slower onset of rancidity during frozen storage(weeks)(Pearson and Gray 1983). Although WoF has generally been recognized as affecting only cooked meat, there is evidence that it develops just as rapidly in raw meat that has been ground and exposed to the air (Greene 1969, Sato and Hegarty 1971) and in restructured fresh meat products as a consequence of disruption of the tissue membranes and exposure to oxygen( Gray and Pearson 1987). Nevertheless, the significance of the levelopment of this flavour to food processors has increased with the advent and f markets for cooked chilled ready meals such as TV dinners airline catering, and fast food outlets. The consumer expectation in these situations is for ' freshly prepared flavours. The continued development and
The effects of chemical, biochemical and physio-chemical factors are rarely mutually exclusive but these categories provide a convenient framework for discussion. The effects of these factors are not always detrimental and in some instances they are essential for the development of the desired characteristics of a product. In this chapter, some of the characteristics of chemical, biochemical and physico-chemical reactions are described, along with examples that are of significance to chilled foods. 9.2 Characteristics of chemical reactions Chemical reactions will proceed if reactants are available, if they are in a suitable form and if the activation energy threshold of the reaction is exceeded. The presence of inorganic catalysts reduces the activation energy threshold and causes reactions to proceed that would otherwise not have done so. The reaction rate is dependent on the concentration of the reactants and on the temperature. Increases in temperature speed up the random movement of reactant molecules, increasing the probability of their coming into contact. A general assumption is that for every 10ºC rise in temperature the rate of reaction doubles. 9.3 Chemical reactions of significance in chilled foods 9.3.1 Lipid oxidation Lipid oxidation is one of the major causes of deterioration in the quality of meat and meat products. Cooked meats and poultry rapidly develop a characteristic oxidized flavour, termed ‘warmed-over’ flavour (WOF) by Tims and Watts (1958). The flavour is best described as that associated with reheated meat and has been described as such by sensory assessors during free profiling of precooked meat, reheated after chill storage (Churchill et al. 1988, Lyon 1987). Further descriptors have been defined for WOF in pork (Byrne et al. 1999a) and chicken meat (Byrne et al. 1999b) and have resulted in the development of sensory vocabularies containing 16 and 18 terms respectively. In cooked meats held at chill storage temperatures, this stale, oxidized flavour becomes apparent within a short time (48 hours) which contrasts with the slower onset of rancidity during frozen storage (weeks) (Pearson and Gray 1983). Although WOF has generally been recognized as affecting only cooked meat, there is evidence that it develops just as rapidly in raw meat that has been ground and exposed to the air (Greene 1969, Sato and Hegarty 1971) and in restructured fresh meat products as a consequence of disruption of the tissue membranes and exposure to oxygen (Gray and Pearson 1987). Nevertheless, the significance of the development of this flavour to food processors has increased with the advent and expansion of markets for cooked chilled ready meals such as ‘TV dinners’, airline catering, and fast food outlets. The consumer expectation in these situations is for ‘freshly prepared’ flavours. The continued development and 226 Chilled foods
Non-microbiological factors affecting quality and safety 227 success of fast food facilities and precooked chilled meals will depend to some extent on the ability of processors to overcome the development of WoF id oxidation has long been considered to be the primary cause of woF supported by studies correlating increases in WoF determined sensorily (Love 1988) with measurements of the thiobarbituric acid (TBa)number(an indicator of lipid oxidation)(Igene et al. 1979, Igene et al. 1985, Smith et al. 1987), and identification of the volatile compounds extracted from the headspace above meat samples(St Angelo et al. 1987, Ang and Lyon 1990, Churchill et al. 1990) As with other examples of oxidative rancidity, the process of lipid oxidation results in the formation of many different compounds, some of which are more significant than others to the undesirable odour and flavour associated with rancidity. This gives rise to a less than perfect relationship between measured chemical markers and sensory assessment of rancidity The reactivity of food lipids is influenced by the degree of unsaturation of constituent fatty acids, their availability and the presence of activators or nhibitors. The composition of fats in meat reflects a number of factors including the diet of the animal and the type of fat. Lipids are most abundant as either storage depot(adipose) fats or in cell membranes as phospholipids During cooking, the unsaturated phospholipids, as opposed to the storage triglycerides, are rendered more susceptible to oxidation by disruption and dehydration of cell membranes. The higher degree of unsaturation of fatty acid in the phospholipids contributes to their more rapid rate of oxidation(Igene et al. 1981). The role of phospholipids in the formation of WoF (Igene and Pearson 1979)and TBa reactive substances( Roozen 1987, Pikul and Kummerow 1991) has been demonstrated Autoxidation of lipids is generally accepted to involve a free radical chain reaction(Fig. 9.1), which is initiated when a labile hydrogen atom is abstracted from a site on the lipid(RH with the production of lipid radicals (R) (initiation). Reaction with oxygen yields peroxyl radicals(ROO") and this followed by abstraction of another hydrogen from a lipid molecule. A hydroperoxide(rooh) and another free radical(R") which is capable of perpetuating the chain reaction, are formed (propagation). Decomposition of the hydroperoxides involves further free radical mechanisms and the formation of non-radical products including volatile aroma compounds Despite much research effort, the mechanism of initiation leading to the formation of the lipid(alkyl or allyl) radical(r") in meat is still an area of confusion and debate. The involvement of iron has been established(minotti and Aust 1987), but beyond this various mechanisms have been suggested but not supported by conclusive evidence(Ashgar et al. 1988) The rate of formation of free radicals is increased by the presence of metal atalysts. In the case of warmed-over flavour development in cooked meats, both free ferrous ions and haemoproteins, including metmyoglobin in the presence of hydrogen peroxide(Asghar et al. 1988)have been shown to have a prooxidant effect. The availability of free iron is known to increase as a result of cooking(Igene et al. 1979)as haemoproteins are broken down and release free
success of fast food facilities and precooked chilled meals will depend to some extent on the ability of processors to overcome the development of WOF. Lipid oxidation has long been considered to be the primary cause of WOF, supported by studies correlating increases in WOF determined sensorily (Love 1988) with measurements of the thiobarbituric acid (TBA) number (an indicator of lipid oxidation) (Igene et al. 1979, Igene et al. 1985, Smith et al. 1987), and identification of the volatile compounds extracted from the headspace above meat samples (St Angelo et al. 1987, Ang and Lyon 1990, Churchill et al. 1990). As with other examples of oxidative rancidity, the process of lipid oxidation results in the formation of many different compounds, some of which are more significant than others to the undesirable odour and flavour associated with rancidity. This gives rise to a less than perfect relationship between measured chemical markers and sensory assessment of rancidity. The reactivity of food lipids is influenced by the degree of unsaturation of constituent fatty acids, their availability and the presence of activators or inhibitors. The composition of fats in meat reflects a number of factors, including the diet of the animal and the type of fat. Lipids are most abundant as either storage depot (adipose) fats or in cell membranes as phospholipids. During cooking, the unsaturated phospholipids, as opposed to the storage triglycerides, are rendered more susceptible to oxidation by disruption and dehydration of cell membranes. The higher degree of unsaturation of fatty acids in the phospholipids contributes to their more rapid rate of oxidation (Igene et al. 1981). The role of phospholipids in the formation of WOF (Igene and Pearson 1979) and TBA reactive substances (Roozen 1987, Pikul and Kummerow 1991) has been demonstrated. Autoxidation of lipids is generally accepted to involve a free radical chain reaction (Fig. 9.1), which is initiated when a labile hydrogen atom is abstracted from a site on the lipid (RH) with the production of lipid radicals (R• ) (initiation). Reaction with oxygen yields peroxyl radicals (ROO• ) and this is followed by abstraction of another hydrogen from a lipid molecule. A hydroperoxide (ROOH) and another free radical (R• ) which is capable of perpetuating the chain reaction, are formed (propagation). Decomposition of the hydroperoxides involves further free radical mechanisms and the formation of non-radical products including volatile aroma compounds. Despite much research effort, the mechanism of initiation leading to the formation of the lipid (alkyl or allyl) radical (R• ) in meat is still an area of confusion and debate. The involvement of iron has been established (Minotti and Aust 1987), but beyond this various mechanisms have been suggested but not supported by conclusive evidence (Ashgar et al. 1988). The rate of formation of free radicals is increased by the presence of metal catalysts. In the case of warmed-over flavour development in cooked meats, both free ferrous ions and haemoproteins, including metmyoglobin in the presence of hydrogen peroxide (Asghar et al. 1988) have been shown to have a prooxidant effect. The availability of free iron is known to increase as a result of cooking (Igene et al. 1979) as haemoproteins are broken down and release free Non-microbiological factors affecting quality and safety 227
228 Chilled foods Initiation RH→R R+O ROO ROo+rh- ROoH+R Fig 9. 1. Free radical chain reaction. iron. The amount released is dependent on the rate of heating and the final temperature, and therefore on the method of heating. Slow heating releases more free iron than fast heating- roasting or braising of meat releases more than does microwave heating(Shricker and Miller 1983) Procedures for the prevention of woF were reviewed by Pearson and Gray (1983). The method used is very often restricted by the requirements of the final product Phenolic antioxidants such as BhT and BHA are of little value in intact meat cuts(Watts 1961), whereas they may be more suited to comminuted meat products since a more even distribution of the antioxidant can be achieved Overheating or retorting of meat to produce compounds that have antioxidant activity(Maillard Reaction products) may be suitable for canned products but tends to result in a product with characteristics contrary to the fresh perception that is a necessary part of many chilled foods. Alternatively, these compound can be added to meat, but they are then restricted by the same considerations that apply to artificial antioxidants. Reduction of woF has also been achieved by use of vitamin E. Kerry et al. (1999)demonstrated that addition of alpha-tocopherol to cooked pig meat reduced lipid oxidation and WOF. Difficulties in achieving adequate distribution of the antioxidant in the meat could be overcome by the incorporation of vitamin E supplements to the feed of the animals. Addition of alpha-tocopherol acetate to the diet of rabbits(Lopez-Bote et al. 1997) and broiler chicks(ONeill et al. 1998) has been shown to be reflected by an increase in the muscle tissue and result in reduced woF development. Investigations of natural antioxidants present in vegetables have shown some benefits for using extracts from green peppers, onions and potato peelings(Pratt and Watts 1964) and herbs and spices, particularly rosemary, sage, marjoram(Hermann et al. 981) and clove (ayathilakan et al. 1997. Reports of the effectiveness of rosemary oleoresin as an antioxidant in precooked meats are conflicting although Murphy et al.(1998)found rosemary oleoresin and sodium tripolyphosphate to be effective in the prevention of woF in precooked roast beef slices. Precooked pork balls processed with rosemary stored at 4C for 48 hours did not develop oxidized flavours as the controls did(Korczack et al 1988), whereas restructured beef steaks processed with oleoresin rosemary stored at refrigerated temperatures showed no significant improvement in comparison to the controls(Stoick et al. 1991) The addition of nitrite between 50 and 200 ppm is an effective inhibitor of the development of WoF (Sato and Hegarty 1971, Cho and rhee 1997). Nitrite and
iron. The amount released is dependent on the rate of heating and the final temperature, and therefore on the method of heating. Slow heating releases more free iron than fast heating – roasting or braising of meat releases more than does microwave heating (Shricker and Miller 1983). Procedures for the prevention of WOF were reviewed by Pearson and Gray (1983). The method used is very often restricted by the requirements of the final product. Phenolic antioxidants such as BHT and BHA are of little value in intact meat cuts (Watts 1961), whereas they may be more suited to comminuted meat products since a more even distribution of the antioxidant can be achieved. Overheating or retorting of meat to produce compounds that have antioxidant activity (Maillard Reaction products) may be suitable for canned products but tends to result in a product with characteristics contrary to the ‘fresh’ perception that is a necessary part of many chilled foods. Alternatively, these compounds can be added to meat, but they are then restricted by the same considerations that apply to artificial antioxidants. Reduction of WOF has also been achieved by use of vitamin E. Kerry et al. (1999) demonstrated that addition of alpha-tocopherol to cooked pig meat reduced lipid oxidation and WOF. Difficulties in achieving adequate distribution of the antioxidant in the meat could be overcome by the incorporation of vitamin E supplements to the feed of the animals. Addition of alpha-tocopherol acetate to the diet of rabbits (Lopez-Bote et al. 1997) and broiler chicks (O’Neill et al. 1998) has been shown to be reflected by an increase in the muscle tissue and result in reduced WOF development. Investigations of natural antioxidants present in vegetables have shown some benefits for using extracts from green peppers, onions and potato peelings (Pratt and Watts 1964) and herbs and spices, particularly rosemary, sage, marjoram (Hermann et al. 1981) and clove (Jayathilakan et al. 1997). Reports of the effectiveness of rosemary oleoresin as an antioxidant in precooked meats are conflicting although Murphy et al. (1998) found rosemary oleoresin and sodium tripolyphosphate to be effective in the prevention of WOF in precooked roast beef slices. Precooked pork balls processed with rosemary stored at 4ºC for 48 hours did not develop oxidized flavours as the controls did (Korczack et al. 1988), whereas restructured beef steaks processed with oleoresin rosemary stored at refrigerated temperatures showed no significant improvement in comparison to the controls (Stoick et al. 1991). The addition of nitrite between 50 and 200 ppm is an effective inhibitor of the development of WOF (Sato and Hegarty 1971, Cho and Rhee 1997). Nitrite and Fig. 9.1. Free radical chain reaction. 228 Chilled foods
Non-microbiological factors affecting quality and safety 229 H3C H3C Pr C= cH Pr -NO Fig. 9. 2. Nitrosylmyoglobin. Myoglobin with the nitrite ligand haemoproteins form nitrosylmyochrome and nitrosylhaemochrome complexes in which the iron is stabilised by the linking of nitric oxide to the porphyrin ring (Fig. 9.2); however, the pink coloration of the meat may be undesirable; causes of pinking in uncured cooked meat is further considered later in this chapter. The effectiveness of pyrophosphate, tripolyphosphate and hexametaphosphate, which chelate metal ions, particularly prooxidative ferrous ions, was demon strated by Tims and Watts(1958)in pork. It has since been verified for ground beef (Sato and Hegarty 1971), for restructured beef steaks(Mann et al. 1989) and for battered and breaded chicken(Brotsky 1976). Phosphates in combination with ascorbic acid may exert a synergistic effect, such that cooked ground pork was protected against lipid oxidation for up to 35 days at 4 C( Shahidi et al. 1986) An alternative approach is to protect the meat from oxidation. This can be achieved by creating an oxygen barrier, using a sauce or a gravy that can be in place at the time of cooking and during subsequent storage. This principle has been demonstrated by comparing the shelf-life of frozen meat to that of the same meats cooked without gravy coverings(Dalhoff and Jul 1965). Cooked pork covered with gravy could be stored at -18C for more than 100 weeks, whereas pork stored without gravy was unacceptable after 22 weeks Modified-atmosphere packaging to reduce Wof has been applied to precooked turkey and pork and pork products. Although those stored in nitrogen and carbon dioxide atmospheres were less oxidized than those in air, packaging was the most effective(Nolan et al. 1989 and Juncher et al ( 1998). Shaw(1997)reviewed the potential benefits of the use of MAP for cook chill ready meals. Protection against oxidation at the time of cooking is also beneficial. Cooking and subsequent storage of chicken breasts in a nitrogen
haemoproteins form nitrosylmyochrome and nitrosylhaemochrome complexes in which the iron is stabilised by the linking of nitric oxide to the porphyrin ring (Fig. 9.2); however, the pink coloration of the meat may be undesirable; causes of pinking in uncured cooked meat is further considered later in this chapter. The effectiveness of pyrophosphate, tripolyphosphate and hexametaphosphate, which chelate metal ions, particularly prooxidative ferrous ions, was demonstrated by Tims and Watts (1958) in pork. It has since been verified for ground beef (Sato and Hegarty 1971), for restructured beef steaks (Mann et al. 1989), and for battered and breaded chicken (Brotsky 1976). Phosphates in combination with ascorbic acid may exert a synergistic effect, such that cooked ground pork was protected against lipid oxidation for up to 35 days at 4ºC (Shahidi et al. 1986). An alternative approach is to protect the meat from oxidation. This can be achieved by creating an oxygen barrier, using a sauce or a gravy that can be in place at the time of cooking and during subsequent storage. This principle has been demonstrated by comparing the shelf-life of frozen meat to that of the same meats cooked without gravy coverings (Dalhoff and Jul 1965). Cooked pork covered with gravy could be stored at 18ºC for more than 100 weeks, whereas pork stored without gravy was unacceptable after 22 weeks. Modified-atmosphere packaging to reduce WOF has been applied to precooked turkey and pork and pork products. Although those stored in nitrogen and carbon dioxide atmospheres were less ‘oxidized’ than those in air, vacuum packaging was the most effective (Nolan et al. 1989 and Juncher et al (1998). Shaw (1997) reviewed the potential benefits of the use of MAP for cookchill ready meals. Protection against oxidation at the time of cooking is also beneficial. Cooking and subsequent storage of chicken breasts in a nitrogen Fig. 9.2. Nitrosylmyoglobin. Myoglobin with the nitrite ligand. Non-microbiological factors affecting quality and safety 229