Off-flavours due to interactions between

8 Off-flavours due to interactions between

  food components

E. Spinnler, INRA, France

8.1 Introduction

  In a food matrix different elements can generate off-flavours individually. However, some off-flavours can arise from the interaction of compounds in food during formulation or packaging. These compounds may derive from raw materials, additives, flavours or the packaging chosen. They react spon- taneously or are catalysed by enzymes or micro-organisms. Reactions can also be stimulated by the manufacturing processes used.

  This chapter will describe some of the different interactions that may

  be responsible for off-flavours. It begins with the interactions between the food matrix and flavour compounds. The chapter will then consider biological interactions with the food matrix and how these can produce off-flavours.

8.2 Flavour compound volatility in different food matrices

  There is a large variety of interactions between flavour compounds and the food matrix. These interactions depend on the relationships between a number of components including:

  • the solvent (water, fat) • solute concentrations (sugar, salt, etc.) • the macromolecular content of the food matrix (lipids, proteins or poly-

  saccharides for example)

  Off-flavours due to interactions between food components 177 The diversity of chemical structures in flavour compounds will determine

  the degree of retention by the food matrix. A change in the composition of flavour compounds will lead to a change in the interactions between them and the food matrix, producing changes in the flavour perceived. Changes produced by storing the food will also influence these interactions. In some food matrices and storage conditions the changes are negligible or even beneficial. In others the matrix may favour the release of undesirable flavour compounds due to lipid oxidation or other undesired reactions during food ageing.

  In many cases, off-flavours are due to the abnormal concentration in the vapour phase of a normal flavour compound. Analysing the liquidvapour equilibrium provides a means of quantifying the volatility of flavour com- pounds in different food matrices. The laws of thermodynamics mean that the equilibrium between vapour and liquid phases is determined by chem- ical potential. In a closed flask containing a diluted solution at a defined pressure and temperature, at equilibrium between both phases, this poten- tial is minimum. Below its boiling temperature, if a volatile compound has strong affinity for a solvent, the chemical potential of the molecule will be lower in the solvent than in the vapour phase. As an example, a very hydrophobic flavour compound such as gamma-decalactone will be more soluble in oil than in water because the chemical potential in oil is much lower than in water. In the same way, at the same low concentration (e.g.

  1 mgl), the partial pressure of the volatile in an oil solution will be lower than in a water solution.

  Since they are caused by interactions at the molecular level, the molar fraction is commonly used to describe these phenomena. The molar frac- tion is defined as the number of moles of the compound in the total number of moles (solutes and solvent) in a defined volume. In the vapour phase, a volatile compound is characterised by a molar fraction (y i ) and by a partial pressure p i (Fig. 8.1). In a real solution:

  p 0

  i = g ◊ xp () T

  where P 0

  i is the saturated vapour pressure of the compound i, at the tem- perature T, x i is the molar fraction of i in the solvent and g i is the coefficient of activity of the compound i.

  In very dilute solution:

  g i = •

  Experimentally g •

  i is easily calculated as the reverse of the molar fraction

  at the limit of solubility:

  g sat

  i • =1 x

  and

  p i = kx

  178 Taints and off-flavours in food

  P T

  In the vapour phase the flavour compound has a partial pressure P i , and a molar fraction y i

  Flavour compound in solution present at the molar fraction x i , the solvant has a molar fraction x s

  Fig. 8.1 Scheme of a simple equilibrium between a liquid phase and the vapour

  phase.

  where k i is the Henry’s constant with

  p i = yP . T

  A key parameter in the vapourliquid equilibrium is the partition coeffi- cient liquidvapour K i . This coefficient allows us to quantify the affinity of the volatile compound for the food matrix. K i is defined as the ratio between the molar fraction in the vapour to the molar fraction of i in the liquid:

  K i = yx g P 0 () TP T

  Another important parameter, when several flavour compounds are in a mixture, as is generally the case, is the relative volatility of a compound (i) as compared to another ( j). This relative volatility a ij is the ratio of the par- tition coefficient K i to the partition coefficient K j :

  The relative volatility of compound i to water volatility may be expressed as follows:

  If the food matrix changes, this ratio will also change as compounds behave differently in different matrices. This will lead to change in the flavour

  Off-flavours due to interactions between food components 179

  Table 8.1 Example of the behaviour of 5 compounds in water. P i 0 is the saturated vapour pressure, Log(P) is the Logarithm of the

  partition ratio of the compound i between n-octanol and water, g gas water i is the activity coefficient of the compound i at infinite dilution, k i is a

  ratio of concentrations k i =C C ,K i is the ratio of molar fraction between the gas and the liquid, a iw is the relative volatility of the

  compound i to water Compound

  Boiling

  P i 0 (Pa) a Solubility a Log(P)

  (molar mass)

  temperature

  in water

  Butyric acid

  263 b 2.6 b 2.22 2 735

  20 c -1.34

  b at 25°C. c at 15°C. at 20°C.

  180 Taints and off-flavours in food perception of the product. As an example, Table 8.1 shows that the flavour

  compound g-decalactone, which has a boiling point 2.8 times higher than water, is twice as volatile as water in a water-based solution because of its hydrophobicity which leads to a very high value activity coefficient. The structure of the flavour compound will also influence its behaviour. As an example, K i is higher for alcohol, ketone, ester and aldehyde, depending on the carbon chain. In a homologous series, the longer the carbon chain, the higher is the hydrophobic character of the molecule.

8.3 Flavour retention in different food matrices

  In a food matrix lipids have a particularly high retention capacity for flavour compounds. This retention capacity is very important for fat free products. As an example some fat free ‘fromage frais’ develop unexpected and unde- sirable flavours once the fats are removed. There are two main reasons for this change. First, the treatment of the milk to separate the fat may cause oxidation of the trace amount of fat still in the product after treatment. Second, as the product no longer contains much fat, all oxidation volatiles such as (Z) 2-nonenal, (Z) 4-heptenal, 1-Octen-3-one, which are quite hydrophobic and have high g i value, will be released very easily by the high moisture content of the matrix. This process generates undesirable card- board or metallic flavours. Retaining a very small amount of fat (for example 1) may help to avoid this problem without significantly chang- ing the nutritional benefit of the low-fat food product. However, fats can increase the risk of taint by trapping chemicals from the food processing environment. Fats have been found to trap chlorophenols, chloroanisoles from packaging materials, or from wood or paint.

  Other compounds have an important retention capacity. Proteins such as Bovine Serum Albumine, b-lactoglobuline, caseins and soya proteins have been shown to have a retention potential (Fares et al., 1998). Polysac- charides such as starch have also a capacity for flavour retention. A recent study by Lopez da Silva et al. (2002) has shown that the retention of the flavour compounds nonanal, decanal and, to a smaller extent, (E) 2-nonenal and (E,E) 2,4 decadienal is significantly higher in a suspension (2 water) of nongelatinised starch than in water alone. A smaller effect is observed when starch is gelatinised. However, inulin, which is sometimes used as fat replacer, has a much smaller retention effect (Gijs et al., 2000). Emulsifiers also affect flavour release. They do so by changing the rheology of the food matrix and the exchange surface of the interface between lipids and water. It has been shown that 2-stearoyl lactylate, for example, facilitates flavour release in a starch suspension (Lopez da Silva et al., 2002). In contrast, small molecules such as salt or sugars may provoke a salting out effect and emphasise taints.

  Off-flavours due to interactions between food components 181

8.4 Off-flavours caused by reactions between components in the food matrix

  In any food product, each ingredient should not cause off-flavours itself but, in the process of addition to the matrix, should not liberate other com- pounds giving unpleasant flavours. This rule also applies to environmental chemicals which might come into contact with a food product. These may cause taints themselves, but they can react with components in the food matrix to generate off-flavours (Reineccius, 1991).

  A classical example is phenolic derivatives. They can originate from raw materials such as milk, meat, malt, fruits or vegetables. They can also orig- inate from wood or cardboard in contact with the food before or after the packaging (O Connell and Fox, 2001). As an example, chlorophenols are sometimes found in milk products. They are produced in three main ways (de Jong et al., 1994):

  • via wood treatment products, disinfectants or pesticides contaminating

  the food directly • through spontaneous reaction between phenolic compounds present in

  food products and chlorine or other halogenated compounds largely used for sanitation

  • by biological chlorination of phenols from chloride ions through

  haloperoxidase activity exhibited by different fungi. Particular care should be taken in the use of chlorine. Chlorine combines

  easily with aromatic cycles of phenols, which are very common in foodstuffs, to produce chlorophenols. These compounds have a proven role in differ- ent taints and off-flavours. These abnormal phenol compounds have quite high olfactory thresholds. However, they are also very easily methylated by fungi to produce polychloroanisoles with much lower thresholds. They then become responsible for musty off-flavours (Bosset et al., 1994).

  Other ingredients deliberately added to food may cause reactions which then produce off-flavours. Preservative agents can be a source of off- flavours. As an example, it has been shown that sulfites in wine are a stimulating factor for the synthesis of 2-aminoacetophenone responsible for ‘untypical aging off-flavour’ (UTA) (Hoenicke et al., 2002). The same mech- anism could be responsible for stale flavour in beer (Palamand and Grigsby, 1974). Some antioxidants such as gallic acid can promote the decomposi- tion of lipid hydroperoxides to volatile oxidation products. It has been shown that the small size of oil droplets increases the propensity to fat oxi- dation. Jacobsen et al. (2001) have shown that EDTA, for example, is a very efficient antioxidant but that it reduces the size of fat globules in mayon- naise. The appearance of oxidative off-flavours of this kind is an important problem for products such as mayonnaise, especially when enriched with omega-3 and omega-6 polyunsaturated fatty acids. These reactions can also be stimulated by particular food processes. Irradiation, for example,

  182 Taints and off-flavours in food stimulates oxidation reactions. The use of antioxidants such as sesamol has

  been shown to be efficient in reducing off-flavours in pork patties (Chen et al., 1999). However, neither vitamin E, sesamol, rosemary and gallic acid reduced effectively the off-flavours of irradiated turkey sausages (Du and Ahn, 2002).

  For ionisable compounds pH is important because only the molecular form of the compound is volatile. As an example, butyric acid, which is an important compound involved in the flavour intensity of butter may, in certain circumstances, be responsible for a rancid off-flavour. It has been shown that the pH of the butter is a key influence on the behaviour of the butyric acid in milk-fat derived products. The pH of the butyric acid is 4.8. Below this pH most of the compound is in its molecular form and as a con- sequence more volatile. In traditional butters the pH can be around 4.6, and the higher volatility of the butyric acid will lead to butters with higher flavour intensities. Excessive release of butyric acid prompted by some pro- duction techniques causes a rancid off-flavour.

8.5 Bacterial interactions with the food matrix causing off-flavours

  Some raw materials can be precursors of off-flavours in final food products because of certain micro-organisms. The case of fruit juices contaminated by recently identified acidophilic bacteria illustrates this. Alicyclobacillus acidiphilus is a novel thermo-acidophilic omega-alicyclic fatty acid- containing bacterium isolated from acidic beverages such as apple juice and orange juice. It is able to synthesise gaiacol which is widely responsible for off-flavours (Matsubara et al., 2002). In brewing free fatty acids are associ- ated with the formation of aldehydes which produce stale off-flavours. The level of free fatty acids is related to lipase activity (Schwartz et al., 2002). Lipase and protease activities are also stimulating factors for cheese ripen- ing. Protease can be a cause of bitterness (Alkhalaf et al., 1988) and lipase the cause of soapy off-flavours (Kheadr et al., 2002).

  Bitterness is a major off-flavour in a large diversity of foodstuffs. The compounds responsible are very often polyphenols or peptides, for example in dairy products such as cheese (Lemieux and Simard, 1991). In camem- bert cheese, for example, the association of different strains of Penicillium camemberti and Geotrichum candidum can significantly change degrees of bitterness. (Molimard et al., 1994). Penicillium camemberti significantly increases proteolytic activity which produces bitter peptides. In contrast, the use of Geotrichum candidum, which produce high levels of peptidases (carboxy-peptidases and amino-peptidases), results in cheeses perceived as less bitter.

  The availability of substrates can be critical in how quickly bacteria catalyse off-flavours. As an example, mould ripened cheeses made using

  Off-flavours due to interactions between food components 183 stabilised curd technology have sometimes developed a celluloid off-flavour

  (Adda et al., 1989). Penicillium camemberti has been shown to be responsi- ble for the production of the styrene causing this off-flavour (Spinnler et al., 1992). When substrates like lactose or lactate are exhausted the Penicillium attacks proteins and fat. Amino acids such as the phenylalanine are then degraded to form styrene with cinnamic acid as the metabolic intermediate. Substrate uptake is more intense at the cheese surface, where Penicillium grows, than in the inner cheese. If the uptake of Penicillium is quicker than the diffusion of lactate from the inner cheese to the rind, as is the case for ripening temperatures over 15°C, the starving Penicillium starts to break down the other molecules of the medium such as fat or proteins. The reaction is also accelerated when curd is washed to remove a part of the lactose and lactate in order to speed up the ripening reactions.

  The role of bacterial and enzymatic action in generating off-flavours can

  be complex. Tryptophan degradation has been identified as a factor in meaty and faecal off-flavours in cheddar cheese (Ummadi and Weimer, 2001) The process involves the ripening bacteria Brevibacterium linens. However, more than 14 enzyme activities are involved in tryptophan degra- dation by Brevibacterium linens, suggesting that Brevibacterium linens alone is not responsible.

  In wine fermentation, wild yeasts such as Saccharomyces cerevisiae, Rhodotorula sp., Candida sp., Cryptococcus sp, Pichia, Hansenula and espe- cially Brettanomyces are responsible for phenolic off-flavours (POF). These yeasts are able to decarboxilate ferulic acid into Vinyl 4 gaiacol or cinnamic acid into vinyl 4 phenol which cause the off-flavours. This defect is also observed in beer brewing. Yeasts are now selected for their low ability to produce POF, but it is also important to control phenolic acid content during wine production to reduce the risk of yeasts causing this reaction (Shinohara et al., 2000).

8.6 Bacterial interactions with additives causing off-flavours

  Additives may, in some cases, be responsible for off-flavours when in contact with enzymes and micro-organisms. As an example, a fishy off- flavour has been found in coffee cream (Eyer et al., 1990). The intensity of the off-flavour was strongly pH dependent. A dynamic headspace extrac- tion together with analysis by GC-MS identified trimethylamine and ethanol as the causes of the off-flavour. These compounds were caused by bacterial degradation of choline in lecithin during milk treatment. Lecithin from soy or rice is used as an additive in fermented milk products and has also been associated with oxidation off-flavours. The oxidation was attrib- uted to the production of hydrogen peroxide by Lactococcus lactis ssp lactis (Suriyaphan et al., 2001). Hydrogenated soy lecithin or lecithin with a small

  184 Taints and off-flavours in food content of polyunsaturated fatty acids are not sensitive to this oxidation

  process.

  Sorbic acid, which is used to prevent growth of moulds and yeasts, is also involved in off-flavours. Horwood et al. (1981) attributed a kerosene off- flavour found in Feta cheese to the flavour compound 1,3-pentadiene. This compound was produced from the decarboxylation of sorbic acid ((E,E)- 2,4-hexadienoic acid) by bacterial activity.

  Some flavour additives may interact with enzymes and components in the food matrix to produce off-flavours. It has been clearly established for example that milk proteins are able to interact with vanillin or other phenolic compounds to produce off-flavours. Two explanations have been advanced:

  • an interaction of vanillin to cystein residues through a cysteine–

  aldehyde condensation reaction andor Schiff base formation • hydrophobic interactions (Reiners et al., 2000).

  Recently, it was shown that the flavour of vanilla-flavoured ice cream was quite unstable. Storage of the ice cream led to the appearance of a card- board off-flavour. This off-flavour is often attributed to fat autoxidation or packing material. However, an enzyme present in the milk, xanthine oxidase, produces hydrogen peroxide and superoxide radicals from the con- version of vanillin into vanillic acid in the presence of oxygen. These two compounds produce perhydroxyl radicals which react with unsaturated fatty acids to form peroxides which produce the flavour compounds (E)-2- nonenal or heptanal responsible for a cardboard off-flavour (Gassenmeier, 2002, see Fig. 8.2). A sufficient heat treatment of the ice cream premix in denaturating the xanthine oxidase is an efficient way of preventing the appearance of the cardboard off-flavour.