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Fatma A.M. Hassan, Mona A.M. Abd El- Gawad, A.K. Enab. Flavour compounds in cheese (review). International
Journal of Academic Research Part A; 2012; 4(5), 169-181. DOI: 10.7813/2075-4124.2012/4-5/A.20
FLAVOUR COMPOUNDS IN CHEESE (REVIEW)
Fatma A.M. Hassan, Mona A.M. Abd El- Gawad, A.K. Enab
Dairy Science Departement of National Research Center. Dokki, Giza (EGYPT)
Cheese flavour development is a complex process in which enzymes -from milk, starter cultures, rennet and
secondary flora- are involved in the degradation of milk proteins, fat and carbohydrates. Variations in non-starter
lactic acid bacteria (NSLAB) and derived compounds depend on cheese variety, processing and ripening
conditions. Starter has an important role during ripening process this may be due to (1) the production of lactic acid,
which together with the rennet cause the curd forming, act as a preservative and contribute to the acid flavour of
cheeses, (2) metabolism of citric acid, which is widely regarded being essential for flavour production, (3)
breakdown of the protein, (4) some contribution to the breakdown of the diglycerides formed from the milk
triglycerides by the lipoprotein lipase from the milk (5) the breakdown of hippuric acid to benzoic acid. Also,
enzymes have an important role during ripening. These enzymes include Lipases and Proteinases. Chymosin is the
major proteinases in traditional animal rennets. The general pathways for the formation of volatile and nonvolatile
compounds are well characterized for most cheese varieties, and detailed knowledge is available on the production
of the primary products of lipolysis; free fatty acids; (FFA), glycolysis; (lactate and products of citrate metabolism)
and proteolysis; free amino acids; (FAA) in certain cheese varieties (e.g., Gouda and Cheddar). However, much
work remains to be done in order to understand the mechanisms by which these primary products are converted to
volatile flavour compounds.
Key words: cheese flavour, flavour compounds, lipolysis, proteolysis, metabolism of lactate.
Cheese is a biochemically dynamic product undergoes significant changes during its ripening period.
Freshly-made curds of various cheeses have bland and largely similar flavours. During the ripening period, flavour
compounds are produced which are characteristic for each variety (McSweeney and Sousa 2000,
Cheese flavour development is a complex process in which enzymes from milk, starter cultures, rennet and
secondary flora are involved in the degradation of milk proteins, fat and carbohydrates. Variations in on non-starter
lactic acid bacteria (NSLAB) and derived compounds depend on cheese variety, processing and ripening condition
(Novikova and Ciproviča 2009).
The flavor of fresh cheese, which is ready to be eaten immediately after manufacture, is the result of the
action of starter bacteria and is due largely to diacetyl and possibly acetaldehyde. The flavour of matured cheese is
the result of the interaction of starter bacteria; enzymes from the milk; from the rennet and accompanying lipases,
and secondary flora (Urbach 1997).
Starter bacteria have a dual role in the production of cheese: acid production during manufacture and flavour
development during ripening. Most rennet cheeses are ripened before consumption to achieve desirable
organoleptic qualities. Ripening involves a series of complex biochemical processes, which can be grouped broadly
into proteolysis, lipolysis and lactose/ lactate metabolism. The extent and type of ripening depend on storage time
and temperature, cheese composition (especially moisture and salt levels) and the type and activities of enzymes
and microorganisms present (Farkye and Fox 1990).
Cheese flavor is one of the most important criteria determining consumer choice and acceptance. Cheddar
cheese flavor varies widely with source, age, and fat content. However, aged Cheddar cheese flavor is
characterized by sulfur, brothy, and nutty flavors (Urbach, 1997, Drake et al, 2001, Avsar et al., 2004). The role of
sulfur compounds in Cheddar cheese flavor (Milo and Reineccius 1997) and their formation from sulfur containing
amino acids by bacterial activity (Urbach, 1995, Weimer et al., 1999) or Strecker degradation (Griffith and
Hammond 1989) have been investigated extensively and reviewed (Weimer et al., 1999). Unlike sulfur flavor,
knowledge on the nutty flavor of Cheddar cheese is scarce. First of all, defining the sensory term “nutty” appeared
to be a difficult task, as the aroma quality in all nuts are not exactly the same (Clark and Nursten, 1977). Drake et
al. (2001) developed a defined sensory language for Cheddar cheese flavor. Nutty flavor was defined as the
“(nonspecific) nut-like aromatic associated with different nuts.” Lightly toasted unsalted nuts, unsalted wheat thins,
or roasted peanut oil extract were used as references for nutty flavor. It is not clear whether nutty flavor is a product
of a single compound or a combined effect of several compounds. Also, nutty character and the volatile source of
nutty flavor may vary with different types of cheese (Clark and Nursten 1977, Avsar et al. 2004).
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The content of free amino acids (FAA) increases during ripening of cheese. Amino acids are the final
products of casein breakdown performed by rennet, plasmin & lactic acid bacteria (LAB) and sometimes also other
microorganisms in co-operation. The composition of free amino acids in cheese, however, only partly reflects the
composition of casein, one explanation is that not all parts of casein are broken down equally and which parts are
hydrolyzed depends on the cheese variety (Sousa et al., 2001, McSweeney 2004).
2. DEFINITION OF FLAVOUR
Flavour is the sensation produced by a material taken in the mouth, perceived principally by the senses of
taste and smell, and also by the general pain, tactile, and temperature receptors in the mouth. Flavor also denotes
the sum of the characteristics of the material which produces that sensation. Flavor is one of the three main
sensory properties which are decisive in the selection, acceptance, and ingestion of a food.
Little is known about the nature of the aroma compounds, but it is clear that the breakdown products of
lactose and citric acid (lactic acid, diacetyl, CO2, etc.), of paracasein (peptides and amino acids), and of lipids (free
fatty acids) are essential for the flavour. A correct balance must exist between the various flavour substances.
Lactic acid causes the refreshing acid taste, which is particularly noticeable in young cheese. An excess of lactic
acid renders the cheese sour. Indirectly, lactic acid exerts influence on the texture of cheese. Large change in
flavour developed during maturation. Numerous secondary products formed during the fermentation of lactose and
the subsequent partial transformation of lactic acid affect aroma and taste (e.g. aldehydes, ketones, alcohols,
esters, organic acids, CO2). Proteolysis is also essential in flavour formation. Paracasein is tasteless, but many
degradation products are not; for example, peptides may be bitter and many amino acids have specific tastes,
sweet, bitter or broth like, in particular. Short peptides and amino acids contribute- at least- to the basic flavour of
cheese. (Carbonell et al. 2002, Vitova et al. 2006).
Several important flavour compounds of different types of cheeses are shown in Table 1. Since not all
products were analysed by GC-O, not all flavour components may be called key-flavours. The flavour compounds
are categorised by the metabolic pathway/substrate they are most likely derived from, as will be discussed as an
indication the following references are given: Limburger (Urbach 1993), Gruye`re (Rychlik and Bosset 2001a,
Rychlik and Bosset 2001b), Gorgonzola (Moio et al., 2000), Mozzarella (Moio et al., 1993), Parmigiano (Bosset
and Gauch 1993, Qian and Reineccius 2002), Grana Padano (Moio and Addeo 1998), Maho´ n, Fontina,
Comte´, Beaufort and Appenzeller (Bosset and Gauch 1993).
3. ROLE OF STARTER DURING RIPENING
Information to verify the function of starter on flavour production of cheese dose not completely describes
the mechanisms for production of the full flavour of mature cheese or if the lactic bacteria a major direct role in
forming the flavour.
The functions of the starter bacteria are : (1) the production of lactic acid, which together with the rennet
causes the curd forming, act as a preservative and contributeto the acid flavour of cheeses, (2) metabolism of citric
acid, which is widely regarded being essential for flavour production, (3) breakdown of the protein (in conjunction
with the rennet and enzymes from milk), (4) some contribution to the breakdown of the diglycerides formed from the
milk triglycerides by the lipoprotein lipase from the milk (5) the breakdown of hippuric acid to benzoic acid, (Sieber
et al. 1990) which acts as a natural preservative. Traditionally, mixed or undefined strain starter cultures were used
which composed a number of strains of Lactococcus lactis subsp. cremoris or the closely related lactococcus lactis
sub sp. lactis. Species of lactic acid bacteria which were able to metabolise citrate were sometimes present in the
mixed strain starter cultures (Cogan and Hill 1995).
Table 1. Examples of important flavour components in some types of cheese (Smit et al. 2005).
Metabolism Gouda Cheddar Camembert Swiss – type
Amino acid 3- Methylbutanal
Sugar Diacelyl Propionic acid
2.3- Butanedione Propionic acid
Fat Butyric acid
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Rest and combined
Phenylethyl acetate Ethyl butyrate
References (Neeter and De Jong
(Curioni and Bosst
(Curioni and Bosset
However some negative aspects of these cultures were occurred, resulting in variation in the rate and level
of acid production and they could cause undesirable open texture in the cheese. A defined strain system was
developed to overcome these problems (Lawrence and pearce 1972). However, the strains are selected primarily
on their ability to produce acid and little attention has been focused on their flavour-generating capacity during
ripening. The complete role of starters in the development of Cheddar flavour has not been fully elucidated (Crow
et al.1993, Quintans et al. 2008) However, due to the high starter cell numbers reached during cheese
manufacture, it is to be expected that starter strains and their enzymes play an important role in flavour
development. Coryne bacterium variabile is part of the complex microflora on the surface of smear-ripened cheeses
and contributes to the development of flavor and textural properties during cheese ripening. Still little is known
about the metabolic processes and microbial interactions during the production of smear-ripened cheeses
(Schröder et al. 2011).
4. ROLE OF ENZYMES DURING RIPENING
Enzymatic processes are responsible for the production of a considerable number of compounds which; as a
result of their presence, concentration and proportions; are often characteristic of particular cheese types (Sablé
and Cottenceau 1999). The influence of the native milk flora on the flavour and texture of raw milk cheese is still
not well known. Cheese made from raw milk tends to develop a stronger and more specific flavor and generally
ripens more quickly than cheese made from pasteurized milk. Changes in cheesemilk caused by pasteurization
include denaturation of indigenous enzymes, slight denaturation of whey proteins and their interaction with caseins
and the destruction of thermolabile member of the indigenous microflora were discussed by (Bachman et al. 1996,
Quintans et al. 2008 ).
Lipases in cheese originate from 6 sources: the milk, rennet preparation (rennet paste), starter, adjunct
starter, non starter bacteria and- if used extensive lipases. The origin of lipases in varieties characterized by
exogenous lipolysis is usually from the coagulant or from the adjunct starter (mould ripened cheeses) (McSweeney
and Sousa, 2000, Mc Sweeney 2004).
The coagulants used to clot milk and crud preparation of selected proteinases which often possess a
considerable proteolytic activity. Chymosin is the major proteinase in traditional animal rennets (88- 94 % milk
clotting activity, MCA), with the remainder pepsin (Rothe et al. 1977). The principal role of chymosin (or other
coagulants) in cheese making is to specifically hydrolyze the Phe105 - Met 106 bond of the micelles-stabilizing
protein, κ-casein, during the coagulation of milk. Most of coagulant activity added to the milk is lost in the whey, but
about 6% is retained in the curd depending on factors including coagulant type, cooking temperature and pH at
drainage; residual coagulant contributes to proteolysis in many varieties (Creamer et al. 1985). In high cooked
cheese (e.g., Emmental), chymosin in denatured extensively and makes relatively little contribution to ripening
(McSweeney and Sousa, 2000, Mc Sweeney 2004).
5. BIOCHEMICAL PATHWAYS DURING CHEESE RIPENING
1-Metabolism of lactose, lactate and citrate:
Lactose metabolism to lactate is essential to the production of all cheese varieties. Depending on starter
type, lactose is metabolized by the glycolytic (most starter bacteria) or phosphoketolase (leuconostoc spp.)
pathways (Cogan and Hill 1993). The principal products of lactose metabolism are L-or D-lactate or a racemic
mixture of both, although some strains, e.g., Leuconostoc spp., produce other products, e.g., ethanol (Vedamuthu
1994, Quintans et al., 2008). Certain starter bacteria (e.g., Streptococcus thermophilus) are unable to metabolize
the galactose moiety of lactose and must grow with galactose –Positive (Gal+) microorganisms (e.g., Gal+
lactobacilli), or galactose will accumulate in the curd. Lactate contributes to the flavour of acid-curd cheeses and
probably contributes to the flavour of ripened cheese varieties, particularly early in maturation. Acidification of the
cheese has a major indirect effect on flavour, since it determines the buffering capacity of the cheese and thus the
growth of various microorganisms during ripening and the activity of the enzymes involved in cheese ripening.
Depending on variety, lactate may also be further metabolized by a number of pathways to various compounds
which contribute to cheese flavour (Fig.1).
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Fig. 1. Pathways by which lactate is metabolized in cheese during ripening. 1) racemization,
2) metabolism by Propionibacterium freudenreichii in Swiss cheese, 3) oxidative metabolism of lactate,
4) conversion to formate, ethanol and acetate and 5) anaerobic metabolism of lactate to butyrate and H2,
which leads to late gas blowing (Reprinted from Cheese: Chemistry, Physics and Microbiology, Vol 1 (3rd edition)
Fox P F, McSweeney P L H, Cogan T M & Guinee T P (eds). McSweeney P L H & Fox P F. Metabolism of residual
lactose and of lactate and citrate, pp 361–371, Copyright 2004, with permission from Elsevier).
The production of D-lactate during ripening is probably greater in cheeses made from raw milk (Steffen et
al. 1980). Racemization of lactate has little impact on flavour but may have undersirable nutritional consequences,
particularly for infant. The solubility of Ca-D-lactate is less than that of Ca-L-lactate, and Ca-D-lactate may
crystallize in cheese forming white specks, particularly on cut surfaces (Fox et al. 1990).
Lactate can be oxidized in vitro to acetate and CO2 by components of the non-starter lactic bacteria
(NSLAB) present in hard cheeses (Fox, et al. 1995b) as shown in (Fig.2).
Acetate, an important flavour compound in many cheeses, in addition to be formed from lactose by lactic
acid bacteria (LAB) may also be formed as a result of citrate and lactate metabolism, or as a product of the
catabolism of amino acids.
In case of swiss-type cheeses propionibacterium SP., metaboliz L- lactate to propionate, acetate and CO2.
The carbon dioxide produced is essential for eye development; & propionate and to a lesser extent, acetate
contribute to the flavour of these cheeses. Fermentation of lactose and lactate in swiss-type cheeses has been
described by (Steffen et al., 1987).
In Camembert and Brie (surface mould-ripened cheese) the metabolism of lactate is most important
(Karahadia and Lindsay 1987). The mesophilic starter bacteria produce lactic acid in the curd (about 1%) which is
quickly metabolizes by secondary microorganisms. The yeasts and moulds rapidly metabolize lactate to CO2 and
H2O,and the pH of the cheese surface increases when the lactate has been exhausted, P.camemberti metabolizes
amino acids released from the casein with the production of NH3 (Gripon, 1993).
Milk contains about 8mmol/L-1 citrate, most of which is lost in the whey during cheese making, because
about 94% of the citrate is in soluble phase of the milk. Nevertheless, the low concentration of citrate in cheese
curd (10 mmol/kg) is a great importance since it may be metabolized to a number of volatile flavour compounds by
certain mesophilic starters (citrate positive, cit +, lactococci and leuconostoc sp.) by pathways summarized in (Fig.
Citrate metabolism has been reviewed by several researchers (Cogan and Hill 1993, Fox et al. 1990,
Hugenholtz 1993). Cit+ microorganisms do not utilize citrate as an energy source, but rather it is co-metabolized
with lactose or some other sugar.
The principal flavour compounds produced on metabolism of citrate are acetate, diacetyl, acetoin and 2,3-
butanediol. Diacetyl is usually produced only in small amount (1-10 µg/ ml in milk), but acetoin is generally
produced in much higher quantities (10-50 fold higher than diacetyl concentrations). Acetate is produced from
citrate in equimolar concentrations. Production of 2, 3- butanediol by starters has not been studies in detail, despite
its importance, the exact reactions which result in the formation of diacetyl remain unclear. Diacetyl could be
produced directly from acetaldehyde-thiamine pyrophosphate (TPP) and acetyl- COA by enzymic action, but
diacetyl synthase has never been identified clearly in LAB.
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Fig. 2. Metabolism of lactate by lactococci (modified from Fox P.F., Singh T.K., McSweeney P.L H.,
Biogenesis of flavour compounds in cheese, in: Malin E.L., Tunick M.H. (Eds.), Chemistry of
Structure/Function Relationships in Cheese, Plenum Press, New York, pp. 59–98,
Copyright 2004, with permission from Elsevier).
Acetoin is produced from α-acetolactate by the action of acetolactate decarboxylase. Products of citrate
metabolism produced by pure cultures of Cit+ lactococci and leuconostoc sp. differ: the former produce diacetyl,
acetoin and CO2 in addition to lactate, but the latter produce large amounts of lactate and acetate.
Acetate is produced from acetylphosphate with the concomitant production of 1 mol ATP, resulting in faster
growth of the microorganisms. In mixed cultures, leuconostoc sp. produced diacetyl and acetoin; perhaps; because
their ability to take up lactose is greatly reduced below pH 5.5.
Citrate metabolism is of particular importance is responsible for eye formation. Diacetyl is an important
aroma compound in a number of varieties; including Dutch- type cheese, Quarg and Cottage cheese. Diacety can
be converted to acetoin and 2, 3- butanediol and 2 butanone, which are also important flavour compounds is some
cheese varieties (Dimos et al. 1996).
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2- Lipolysis and metabolism of fatty acids
Milk fat is essential for the development of the correct flavour in cheese during ripening. Cheddar and other
cheeses normally made from whole milk do not develop correct flavour when made from skim milk or milks in which
milk fat has been replaced with other lipids (Wijesundera et al., 1998, McSweeney 2004). Indeed, satisfactory
flavour development is one or the principal problems encountered in the manufacture of reduced- fat variants of
established cheese varieties.
Lipolysis is particularly extensive in hard Italian verities, surface bacterially-ripened (smear) cheese and blue
mould chesses and is essential to correct flavour development in these cheeses. Extensive lipolysis is considered
undesirable in other types of cheese varieties such as Cheddar, Gouda and Swiss cheeses; high levels of fatty
acids in these cheeses lead to rancidity. However, low concentrations of FFA contribute to the flavour of these
cheeses, particularly when they are correctly balanced with the products of proteolysis or other reactions (Bosset
and Gauch 1993, Rychlik et al. 1997).
Rennet extracts used in the production of most cheese varieties should be free from lipase activity, but
rennet pastes used to coagulate the milk for certain Italian varieties (e.g., Provolone, Romano) contain pregastric
esterase (PGE) which is responsible for the extensive lipolysis in these cheeses (Nelson et al. 1977).
Fig. 3. Pathways for citrate-positive strains of Lactococcus and Leuconostoc sp. (Reprinted from Cheese:
Chemistry, Physics and Microbiology, Vol 1 (3rd edition) Fox P F, McSweeney P L H, Cogan T M &
Guinee T P (eds). McSweeney P L H & Fox P F. Metabolism of residual lactose and of lactate and
citrate, pp 361–371, Copyright 2004, with permission from Elsevier).
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The lipase esterase system of starter bacteria has received much less attention than their proteolytic system
lactococcus sp. are only weakly lipolytic, but may be for liberation of quite high levels of FFA when present in high
cell numbers or over extended ripening periods. Lipases/ esterases of lactococcus strain, which appear to be
intracellular, have been studied (Chik et al. 1997, Fox and Wallace 1997, holland and Coolbear 1996).
Obligately homofermentative lactobacilli used as starters (Lb. helveticus, Lb. delbrueckii subsp. bulgaricus and Lb.
delbrueckii subsp. lactis) also produce esterases, some of which have been studied (El- Soda et al. 1986, Khalid
and Marth 1990). Facultatively heterofermentative lactobacilli (e.g., Lb. casei, Lb. paracasei and Lb. plantarum),
which dominate the NSLAB flora of many cheese varieties, are weakly lipolytic. Micrococcus and Pediococcus are
also weakly lipolytic (Bhownik and Marth 1990). Psychrotrophic bacteria (e.g., Pseudomonas sp.) produce heat-
stable lipases which adsorb onto the fat globules in milk and survive pasteurization. They may contribute to lipolysis
in cheese made from milk containing high numbers of psychrotrophic bacteria prior to pasteurization (Cousins et
al. 1977). Penicillium sp. produce potent extracellular lipases which are primarily responsible for the extensive
lipolysis in mould-ripened cheeses (Gripon 1993 and Smit et al. 2005). The impact of FFA on the flavour of blue
mould-ripened cheeses is less than for hard Italian varieties, possibly due to neutralization as the pH increases
during ripening, and because of the dominant influence of methyl ketones on the flavour of blue mould cheeses.
Furthermore, FFA act as precursor molecules for a series of catabolic reactions which lead to the production
of other flavour compounds (Fig. 4). The flavour of blue mould cheeses is dominated by alkan-2-ones (2-methyl
The pathway by which alkan-2-ones are produced (β-oxidation) involves the release of fatty acids by
lipolysis, their oxidation to β- ketoacids and decarboxylation to alkan-2-ones with one less C-atom. Akan-2- ones
may be reduced to the corresponding secondary alcohols (alkan-2-ols), a step which is reversible under aerobic
conditions. The production of alkan-2-ones in blue mould cheese has been discussed by some researchers
(Gripon 1993, McSweeney 2004, Molimard and Spinnler 1996).
Lactones are cyclic compounds formed by the intramolecular esterification of hydroxy fatty acids. The
principal lactones in cheese are γ- and δ- lactones which have 5- and 6-sided rings, respectively, and are stable,
strongly flavoured and could be formed from the corresponding γ – or δ-hydroxy fatty acids. (Urbach 1993)
reported that in full fat cheeses δ -decalactone increased to a maximum concentration at about 14 weeks and then
decreased, whereas in low-fat cheeses, the level of δ -decalactone remained fairly constant throughout ripening.
However, the fact that the Cheddar cheese flavour actually improved when the δ -decalactone level decreased may
indicate that δ –decalactone plays very little part in Cheddar cheese flavour (Dimos et al. 1996). Hydroxylation of
fatty acids can result from the normal catabolism of fatty acids, and/or they can be generated from unsaturated fatty
acids by the action of lipoxygenases or hydratases (Dufossé et al. 1994). FFA can react with alcohols to yield
esters (which are highly flavoured) or with free sulphydryl groups to give thioesters.
Fig. 4. General pathways for the catabolism of free fatty acids in cheese (McSweeney and Sousa 2000).
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Fourteen different esters have been found in Emmental cheese (Bosset et al. 1995and1997, Imhof and
Bosset 1994, Rychlik et al.1997) and esters have also been claimed to be important contributors to the flavour of
Parmigiano-Reggiano cheese (Meinhart and Schreier 1986).
Proteolysis: during cheese ripening; is the most complex and important events which occurred and has been
discussed by several reviews (Fox and law 1991,Fox and McSweeney 1996, Fox et al., 1995b).
Proteolysis plays a vital role in the development of : (1) textural changes in the cheese curd, due to
breakdown of the protein network, decrease in water activity through water binding by liberated carboxy and amino
groups and increase in pH (in particular in surface mould –ripened varieties); (2) direct contribution to flavour and
perhaps to off-flavour (e.g., bitterness) of cheese through the formation of peptides and free amino acids, (F.A.A);
(3) liberation of substrates (amino acids) for secondary catabolic changes (e.g., deamination, decarboxylation,
transamination, desulphuration catabolism of aromatic compoubnds such as phenylalanine, tyrosine, tryptophane
and reactions of amino acids with other compounds); and (4) changes to the cheese matrix, which facilitate the
release of the flavoured aromatic compounds. The methodology for assessment of the extent and pattern of
proteolysis in cheese is of interest as an index of cheese maturity and quality, and has also been reviews (Fox and
Law1991, Fox et al. 1995 a, Fox and McSweeney 1996).
During ripening, proteolysis in cheese is catalyzed by enzymes from:
(1) the milk (plasmin, cathepsin D and perhaps other somatic cell proteinases);
(2) the coagulant (e.g., chymosin, pepsin, or plant or fungal acid proteinases);
(3) the starter;
(4) the nonstarter;
(5) the secondary starter (e.g., P. camemberti, P. roqueforti, Propionibacterium spp., Br. linens and other
(6) exogenous proteinases and/or peptidases used to accelerate ripening.
In most cheese varieties, the initial hydrolysis of caseins is caused by the coagulant and to a lesser extent
by plasmin and perhaps somatic cell proteinases (e.g., cathepsin D), which results in the formation of large (water-
insoluble) and intermediate- sized (water-soluble) peptides which are subsequently degraded by the coagulant and
enzymes from the starter and non-starter flora of the cheese. The production of small peptides and FAA is caused
by the action of microbial proteinases and peptidases.
The final products of proteolysis are FAA, the concentrations of which depend on the cheese variety, and
which have been used as indices of ripening (McSweeney and Fox 1997). The concentration of FAA in cheese at
any stage of ripening is the net result of the liberation of amino acids from casein and their transformation to
catabolic products. The principal amino acids in Cheddar cheese are Glu, Leu, Arg, Lys, Phe and Ser
(Wijesundera et al. 1998). Concentrations of amino acids generally increase during ripening, with the exception of
Arg., the concentration of which is reported to decrease later in ripening (Puchades et al. 1989). The level of
peptides and FAA soluble in cheese in 5% phosphotungstic acid (PTA) has been considered to be a reliable
indicator of the rate of flavour development (Ardo and petterson 1988) and the composition of the amino acid
fraction and the relative proportions of individual amino acids are thought to be important for the development of the
characteristic flavour (Broome et al. 1990).
Medium & small peptides and FAA contribute to the background flavour of most cheese varieties (Urbach
1995) and some individual peptides have brothy, bitter, nutty and sweet tastes. Fox and Wallace (1997) have
suggested that flavor and the concentration of FAA could not be correlated, since different cheeses (e.g., Cheddar,
Gouda and Edam) have very different flavours, although the concentration are relative proportion of FAA are
6. CATABOLISM OF AMINO ACIDS
Catabolism of free amino acids (FAA) can result in a number of compounds, including ammonia, amines,
aldehydes, phenols, indole and alcohols, all of which may contribute to cheese flavour. Catabolism of FAA probably
plays some role in flavour development in all varieties, but it is particularly significant in mould and smear-ripend
cheese (Fox et al. 1995 b). The first stage in amino-acid catabolism involves decaboxylation, deamination,
transamination, desulphuration or perhaps hydrolysis of the amino-acid side-chains. The second stage involves
conversion of the resultion compounds (amines and α- ketoacids), as well as amino acid themselves, to aldehydes,
primarily by the action of deaminases on amines, the final stage of amino- acid catabolism is the aldehydes to
alcohols, or their oxidation to acids. Sulphur-containing -amino acids can undergo-exrensive conversion, leading to
the formation of a number of compounds, including methanethiol and other sulphur derivatives (Fox and
Wallace1997). General pathways for the catabolism of FAA are summarized in (Fig. 5).
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Fig. 5. General pathways for the catabolism of FAA (modified from
Hemme et al. 1982 and McSweeney and Sousa 2000).
7. BITTERNESS AND OTHER OFF-FLAVOURS
Bitterness in cheese is due mainly to hydrophobic peptides and is genrally regarded as a defect, although
bitter notes may contribute to the desirable flavour of mature cheese. The literature concerning bitterness in dairy
products has been reviewed by (Lemieux and Simard 1991,1992), (McSweeney et al. 1997, Singh et al., 2003).
Certain sequences in the caseins are particularly hydrophobic and, when excised by proteinases, can lead to
bitterness. The action of coagulant has been implicated in the formation of bitter peptides in cheese, and thus
factors that affect the retention and activity of rennet in the curd may influence the development of bitterness. The
starter and rennet type are considered important in the development of bitterness. (Lawrence et al.1972) have
suggested that the major role of rennet in the development of bitterness may be the production of long peptides that
will be subsequently degraded to small bitter peptides by starter proteinases.
Off-flavours (rancidity) can be due to excessive or unbalanced lipolysis caused by lipases/ esterase from
starter or non-starter LAB, enzymes from psychrotrophs in the cheese milk, or indigenous milk lipoprotein lipase.
Late gas blowing and off-flavours in certain hard cheese result from the metabolism of lactate (or glucose)
by Clostridium sp. to butyric acid and H2 (Fox et al., 1995b). These defects may be avoided by good hygiene,
addition of NO3 or lysozyme, or by the physical removal of spares by bactofugation or microfiltration.
Carbon dioxide produced by citrate fermentation can cause undesirable openness and the defect in Cheddar
and Cottage cheeses.
In Cheddar, fruity flavours is regarded as a defect by professional cheese graders, although consumers may
be prepared to pay a premium for fruit Cheddar (McSweeney and Sousa 2000).
8. APPLICATION OF STRAINS WITH SELECTED ENZYME ACTIVITIES FOR IMPROVING
For the application of selected lactococci, it was found that strains possessing a specific flavour-forming
enzyme do not necessarily possess other enzymatic activities of the complete pathway. In addition, strains might
lack other characteristics for application as cheese starter (e.g., fast acidification). In order to be able to use such
strains and to overcome problems, it is required to combine selected strains with industrial strains in order to obtain
a starter with both good flavour generating potential as well as good acidifying and proteolytic activities (Ayad et al.
2000 and Ayad et al. 2001a).
It was found by (Ayad et al. 2001b) that different strains could influence each other in formation of flavour
components. Strains, which each had only a limited set of enzymes in a certain pathway, could complement each
other. For instance, the combination of L. lactis B1157 and SK110 strains in milk resulted in the formation of high
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levels of 3-methylbutanal. In SK110, a highly proteolytic strain from industrial origin, the complete pathway from
casein via leucine to 3-methylbutanal cannot proceed due to the lack of a decarboxylating enzyme. L. lactis strain
B1157. On the other hand is a non-proteolytic wild strain and thus unable to produce enough free amino acids that
can serve as substrate for the subsequent transamination and decarboxylation steps. However, when B1157 and
SK110 are cultivated together, the strains complement each other with regard to their enzyme activities resulting in
a high production of the chocolate flavour component 3-methylbutanal. This proto-cooperation between strains as it
is called offers new possibilities for the construction of tailor-made starter cultures, because it makes it clear that not
all the desired enzyme activities in a certain flavour pathway leading to flavour need to be present in one strain.
An example of the application of knowledge of proto-cooperation, but also of population dynamics of starter
cultures for the optimisation of a cheese flavour is given by (Ayad et al. 2002 and Smit et al. 2005). A selected L.
lactis strain (strain B851) with high (in vitro) activity to form 3-methylbutanal was used to improve the taste of
Proosdij cheese. Proosdij cheese is a Gouda-type cheese, prepared with a mesophilic starter culture in
combination with a thermophilic adjunct culture. This cheese has a flavour profile, which has characteristics
between Gouda and Parmesan cheese. One of the key flavour components in this type of cheese is 3-
methylbutanal (Engels 1997 and Neeter and De Jong1992). The selected L. lactis strain B851was used in
combination with the regular cultures used for this type of cheese. The cheeses made with and without the selected
adjunct strain were analysed for the production of 3- methylbutanal by headspace gas chromatography(Ayad et al.
2001b) and graded by an expert panel (Ayad et al. 2003). It was found that the use of the selected adjunct strain in
cheese resulted in both an increase in the key flavour production as well as in the intensity of the Proosdij cheese
This culture had previously been developed to prevent crack formation in Proosdij cheese. In this cheese
type, the addition of culture B851 led to an increase in the overall flavour intensity, indicating that it is possible to
tailor the flavour of cheese by using specifically selected cultures, even in combination with complex starter cultures
(Smit et al. 2005 and Mikelsone& Ciprovica 2011).
The general pathways for the formation of volatile and nonvolatile compounds are well characterized for
most cheese varieties, and detailed knowledge is available on the production of the primary products of lipolysis
(FFA), glycolysis (lactate and products of citrate metabolism) and proteolysis (FAA) in certain varieties (e.g., Gouda
and Cheddar). However, much work remains to be done in order to understand the mechanisms by which these
primary products are converted to volatile flavour compounds.
Medium and small peptides and FAA contribute to the background flavour of most cheese varieties.
Catabolism of free amino acids (FAA) can result on a number of compounds, including ammonia, amines,
aldehydes, phenols, indole and alcohols, all of which may contribute to cheese flavour.
Off- flavour may be due to: excessive or unbalanced lipolysis; lat gas blowing and off- flavour in certain hard
cheeses result from the metabolism of lactate (or glucose) and CO2 produced by citrate fermentation can cause
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