Flavour can be described as a critical property of food which contributes to the overall sensory perception that is the utmost important factor for all consumers .Flavours are vital for survival as they can indicate nutritional value or danger (Labuda, 2009). Food flavours can be natural as well as artificial (used to enhanced weak flavours, or replace natural flavour that are lost during food processing), desirable and non desirable (e.g. rotten eggs). Flavours are organic compounds that undergo various biochemical, and chemical processes and are compounds that belong to different chemical classes, such as alcohols, aldehydes, amines, esters, lactones, trepenes, etc. Many volatiles are produced by plants and food during stages of flowering, ripening or maturation but only few are responsible for giving food its unique flavour that helps human recognise appropriate foods and avoid poor or dangerous food choices first by its odour. Many sensory systems (smell, taste, sight, feeling, and sound) are involved in the flavour recognition along with receptive properties of different receptors present in nose and oral cavity. Flavours can be divided into two groups namely, primary and secondary aroma compounds that undergo many biochemical and chemical processes to produce flavour which will be discusses further in this study using apple, cheddar cheese, and meat(beef) as main examples.
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Apple Flavour formation :
Flavour components are stored in exocarp and mesocarp of a fruit which develops during ripening (Lopez et.al, 2006), (Dixon et.al, 2000). Apple fruit produces many volatile compounds (approximately 300) many of which are esters that are responsible for the apple aroma (78-92% of total volatiles), alcohols (6-16% of total volatiles), aldehydes, ketones and ethers, which are present in various amounts in different cultivars (Christensen et.al, n.d.), (Dixon et.al ,2000), many of these volatiles give the green note in apples when unripe and only few contribute to give the fruit its unique flavour (e.g. sweet rosy apple), although these flavours can be influenced by many biochemical and environmental factors that may improve or effect fruits flavour and quality (Dixon et.al, 2000).
Volatiles are very important for aroma, and flavours which are synthesised from amino acids, lipids and carbohydrates. Many biosynthetic processes are involved in the volatile synthesis such as the β- oxidation and lipoxygenase which give rise to aroma compounds; aldehydes, ketones, alcohols, lactones, and esters from lipids (fig 1.1). β- Oxidation produces volatiles in intact fruits while lipoxygenase is activated when the fruit tissue is disrupted. Β-oxidation of fatty acids is a primary biosynthetic process that provides alcohols and acyl co-enzymes (coA) for ester formation which give pleasant aromas. Process of flavour formation in apples commences by the degradation of fatty acids which are generated from lipids (triglycerides, phospholipids or glycolipids) present in the cell membrane. These lipids are primarily composed of linoleic and linolenic acids that may be due to the breakdown of chloroplast. Alcohols with an even carbon number (butanol and hexanol) were the most important for flavour development (Christensen et.al, n.d.), (Dixon et.al, 2000).
Fig: 1.1. Chemical reaction during apple flavour formation (Lopez et.al, 2006)
According to many experimental studies on apple flavour , it is found that β- oxidation is active in ripe apples and the formation of most esters depend on availability of C6-C8 acids and alcohols (C6 and C9 are specifically flavour metabolites observed is most apples). In a study of Gala apple fragrances it was found that butyl acetate, hexyl acetate, hexyl propanoate, butyl 2-methylbutanoate, and hexyl 2-methylbutanoate were responsible for the apple-like, fruity aroma and methyl 2-methylbutanoate, ethyl 2-methylbutanoate and propyl 2-methylbutanoate gave sweet and berry-like odours (Christensen et.al, n.d.). Fatty acid acyl-CoA derivatives are converted to short chain acyl-CoAs by losing two carbons in every step of oxidation cycle, requiring flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD), and free CoA. Acyl CoAs are reduced by acyl CoA reductase to aldehyde that in turn is reduced by alcohol dehydrogenase (ADH) to alcohol for use by alcohol acyl CoA transferase (AAT) to produce esters (Dixon et.al , 2000).
Meat (beef) flavour formation
Meat is composed of proteins, fats, water, carbohydrates and inorganic compounds (Spanier et.al, 1994). Surface of meat (phospholipids of beef muscle membrane) is exposed directly to molecular oxygen that leads to the production of off-flavours (undesirable) or deterioration of beef flavour (Imafidon et.al, 1994). Many factors such as pH, temperature, water content etc can contribute to food spoilage. If the meat is exposed to oxygen or high energy it emits electrons that produce ions and free radicals( produced when water splits into two atoms).Energised electrons target water molecules (gamma rays) to give out free radicals (oxygen ions, OH ions, etc ). Interaction with these free radicals may lead to formation of hydrogen peroxide (Brewer, 2009). Secondary products produced are hydroperoxide ( which are odourless and tasteless( their breakdown products are smelly, having low threshold and are catalysed by heavy metals and haem compounds (hydroperoxide lyases). Lipid peroxidation is one of the primary mechanisms of quality deterioration in foods and especially in meat products( Kanner, 1994). Raw beef contain many iron containing proteins (catalase and peroxidise) which becomes highly active in high temperature producing free radicals from hydrogen peroxide (fig.1.2). In beef meat, heme iron has been thought to be a pro-oxidant that may influence meat flavour due to its reaction to produce radicals that can promote lipid oxidation (which provided presence of oxygen) in biological systems So, as the temperature increases, peroxidatation activity also increases (e.g. If temperature is 140 C, then linoleic acid peroxidise increases to 96 µmole/min). Volatiles that are responsible for flavour and odour in beef meat are; acids, alcohols, unsaturated aldehydes, aromatic compounds, esters, ethers, furans, hydrocarbons, ketones, lactones, pyrazines, pyridines, pyrroles, sulphides, thiazoles, and oxazoles ( sweet,sour, salty,bitter, and umami) (Calkins et.al, n.d.).
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Meat is packed with numerous amounts of flavour compounds (mainly sulphur containing compounds, derived from cystine and ribose), most of which are distorted during cooking and storage. Meat also undergoes Maillard reaction (non-enzymatic browning) that produces meaty like flavours contain sulphur. These sulphurous and cabonyl compounds are major contributors to meat (beef) flavour. Hydrophilic residues (peptide subclass) are associated with desirable flavour, while hydrophobic residues are associated with undesirable or off flavours. It has been found that bis(2-methyl-3-furyl) disulfide, 2-furfuryl 2-methyl-3-furyl disulfide, bis(2-furfuryl) disulfide, dimethylfuryl 2-methyl-3-furyl disulfide, 2-methyl-3- furyl 2-methyl-3-thienyl disulfide are responsible for the specific flavour in beef (Calkins et.al, n.d.), ((Brewer, 2009). Most of the meat flavour develops during cooking which is influenced by lipid peroxidation (haemoglobin, myoglobin and cytochrome) that are depended on the composition of phospholipids, concentration of metal ions, polyunsaturated fatty acids, oxygen, salt and other oxidants (Calkins et.al, n.d.).
The ability of unsaturated fatty acids, especially those with more than two double bonds, to rapidly oxidise, is important in regulating the shelf life of meat (rancidity and colour deterioration). However, this tendency to oxidise is important in flavour development during cooking (Wood et.al, 2004). Linoleic and arachidonic fatty acids oxidizes to produce 9-hydroperoxide and 11-hydroperoxide, forming 2, 4 decadienal, 2-nonenal, 1-octen-3-one, 2, 4-nonadienal, and 2-octenal through b-scission with 2-nonenal , 2, 4-decadienal (can produce undesirable flavours in beef at higher concentrations), However trans-4,5-epoxy-(E)-2-decenal, 1-octen-3-one, 2,4 decadienal, 2,4,7-tridecatrienal, and hexanal having are the most intense aroma compounds. Through oxidation of beta-carotene, intense aromatic beta-ionone is formed; 12-methyltridecanal with odour threshold of 0.1 µg/kg (derived from glycerophospholipids) is found in great amounts in beef. Oxidation is controlled by the amount of antioxidants compounds present in meat.
R ion+O2ïƒ RO2 ion (Fast reaction)
RO2 ion+ RHïƒ ROOH+ R ion (Slow reaction)
RO ion + RHïƒ ROH+R ion
ROOH ïƒ RO ion +OH ion
2ROOH ïƒ RO2 ion + RO ion +H20
6, 7 and 8 are stable products. too much hydroperoxide will spoil food
Î‡H +O2ïƒ Î‡HO2 (This is a very reactive reaction, only occurs under acidic condition and accelerates oxidation)
Î‡ HO2ïƒŸïƒ H + Î‡O2
2Î‡HO2ïƒ H2O2 + O2 Hydrogen peroxide is an intermediate which is not very reactive, and produces hydroxyl ions.
Fig.1.2 formation of hydrogen peroxide
Cheddar cheese flavour formation( milk ïƒ fresh cheddar curd ïƒ cheddar cheese)
Cheddar Cheese contains high amount of fat ( round 30.5% or more) which is a important characteristic for its unique texture and flavour formation. Cheese ripening is a slow process which involves microbiological, biochemical and chemical reactions ( Singh et.al, 2003) Cheddar cheese contain variety of volatile flavour compounds which originate from degradation of milk element ( lactose, citrate, lipids, proteins or caseins), but only few of these flavour volatiles formed by glycolysis, lipoxygenase, lipolysiss and proteolysis directly contribute to specific cheese flavour (e.g. acids, alcohols, esters, aldehydes, ketones, sulphur-containing compounds, phenolics, etc) although most of which are responsible for off-flavours in cheese. Lactic acid and bacteria (LAB) are the major source of proteolytic enzymes that widely effect ripening in cheddar cheese (Wit et.al, 2005). According to many research findings it is known that the desirable or undesirability of cheese is highly influenced by treatment of milk, ripening temperature, age of cheese ( how long it's left to ripen) and PH (approximately 5.0-5.2 have an effect on cheese texture and flavour (Singh et.al, 2003), (Rehman et.al , 2000)
Cheddar cheese undergoes oxidation of lactate (amount of lactate present are high in cheddar cheese), producing acetate and carbon dioxide responsible for flavour formation, although this oxidation activity is depended on oxygen availability. Milk lipase activity is more active than the cheddar cheese lipases, although if lipase in cheese ripening is left for too long than it may cause rancidity in cheese. Lipolysis ( breakdown of lipids) is the principal biochemical pathway, which involves lipase enzyme to produces short chain free fatty acids (FFA) (conatin even numbered FFA,C4-C12, with low perception threshold, giving cheddar its characteristic flavour note), methyl, ethyl, higher esters, methyl ketones, pentan-3-one, diacetyl, acetoin, -lactonnes, -lactones, aromatic hydrocarbons,short and long chain alcholos (ethanol and butan-2-ol are the major ones) aldehydes, amines, amides, phenols ,and sulphur compounds that contribute to cheddar cheese flavour formation ( desirable compounds ) ( Singh et.al, 2003), (Wit et.al, 2005),(Urbach, 1993), (Collins et.al, 2003). Esters produced by lactic acid bacteria gives cheddar its flavour, but sometimes give off-flavour and Lactones have been considered important for flavour in cheddar cheese which posses strong overall cheese aroma .For cheddar flavour formation casein is broken down into small peptides and amino acids (alpha and beta casein (Wit et.al, 2005).Unstable sulphur compound in cheddar , 4-methyl-mercaptopentan-2-one is responsible for odour in cheddar cheese (Urbach, 1993).
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General pathway for the metabolism of milk triglycerides and fatty acids
Lipid oxidation involves the reaction of molecular oxygen with unsaturated fatty acids through free radical mechanism, Lipoxygenase catalyses hydroperoxidation of polyunsaturated fatty acids and esters containing a 1,4- pentadiene system.this enzyme initiates lipid oxidation by which hydroperoxide are formed and then subsequently degraded to form a variety of secondary products leading to off-flavour).
Development of flavour in various foods such as fruits, vegetables, meat etc undergoes many biochemical and chemical pathways to produce the most desirable flavour compounds that give the food its unique aroma. These pathways may be influenced by many environmental factors such as, temperature, pH, harvest period, oxygen, carbon dioxide, animal, antioxidants, animal diet; etc .For example off odour flavour of meat can be described as rotten, roasty, burnt, barbequed ,fishy, sulphur, bloody which occur due to the free radial formation which is dependent on temperature, oxygen exposure etc. Cooked meat is highly susceptible to oxidation because heat denatures antioxidant-forming compounds, causing lipid membrane damage (exposed to the environment). Cheddar cheese relating to FFA is based on three approaches; determination of levels of FFA that correlate with flavour development ,removal of FFA from cheese to determine flavour alterations and manufacture of reduced fat cheddar cheese.