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Trends in Science & Technology 17 (2006) 557e566

Review Flavour in

1995; Onno & Rouseel, 1994; Ottogalli, Galli, & Foschino, : a review 1996). The key groups of fermenting organisms, in addition to the , are strains of the bacteria (LAB). Salim-ur-Rehman*, Alistair In traditional sourdough systems initially part of the flour is mixed with all the and sufficient to make Paterson and John R. Piggott ‘‘sponge’’ that is allowed to ferment for some hours, typi- cally overnight, exposed to the atmosphere. The sponge is Centre for Food Quality, Department of Bioscience, then mixed with rest of the flour, water, and to a suit- University of Strathclyde, 204 George Street, Glasgow able consistency, and then given a short period for fermen- G1 1XW, Scotland, UK (Permanent address: Institute tation before final proving and . The outcome is of Food Science and Technology, University of considered to be of a richer flavour due to bacterial Agriculture, Faisalabad, Punjab 38000, Pakistan. of the sponge through airborne contamination or D D Fax: 92 41 9201105; 44 141 553 4124; incorporation of 2e5% of sponge from the previous batch e-mail: [email protected]) as a microbial inoculum (Bruemmer & Lorenz, 1991). In good practice, a sponge should contain meta- 8e 9 1 Flavour compounds are key elements for consumer accep- bolically active at 10 10 cfu g and 6e 7 1 tance and product identification in bread. One category of yeasts at 10 10 cfu g , primarily responsible for acidifi- speciality breads, the have a process cation and leavening action of , respectively. How- affected by a complex microflora of yeasts and lactic acid ever, the LAB may either originate from natural flour bacteria which confer specific flavour characteristics. Al- contaminant, a fermented dairy product or from a commer- though yeasts have the primary leavening role, lactic acid cial starter culture containing characterised strains of LAB bacteria (LAB), with trophic and non-trophic relationships, produced in batch fermentation (Vuyst & Neysens, 2005). produce important flavour components. Sourdoughs are be- coming important as consumers move away from pan breads towards speciality products. However, successful new product of sourdough sponges development requires an understanding of variations in carbo- The microbial ecology of the sourdough fermentation is hydrates’ metabolism, roles of endogenous enzymes and inter- complex and varied with identification of more than 50 spe- actions of microorganisms for generation of non-volatile and cies of LAB, mostly of the genus , and more volatile flavour compounds. The potential of sourdough baking than 20 species of yeasts, are dominated by the genera remains to be developed through specifications and optimisa- Saccharomyces and Candida. Sourdough microflora have tions of process conditions and introduction of exogenous primarily stable associations with lactobacilli and yeasts, enzymes and other ingredients. With effective new product having important metabolic interactions contributing to- development sourdough characteristics could be matched to wards production of flavour compounds (Vuyst & Neysens, relate with consumer tastes. 2005). Sourdough flavour components identified as pro- duced by yeast (Table 1) and LAB (Table 2) fall into many classes of compounds. Several yeasts are found in sourdoughs but Saccharomy- Introduction ces cerevisiae is considered the dominant organism for Sourdough is an important modern fermentation method leavening of bread (Corsetti et al., 2001)(Table 1). It has of cereal flours and water (Vogel et al., 1999) based upon been suggested that numbers of S. cerevisiae are overesti- the earlier spontaneous process. A slack flour dough is mated due to the lack of robust systems for quantifying inoculated with microbial starter, ‘‘mother culture’’, which individual yeast in this habitat (Vogel, 1997). Important is constantly renewed in a cyclical way, using specified yeasts in sourdough starters include Saccharomyces exiguus recipes and ripening conditions (Hammes & Ganzle, (physiologically similar to Candida milleri), Candida kru- sei, Pichia norvegensis and Hansenula anomala. Other * Corresponding author. yeasts isolated from sourdough sponges include 0924-2244/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tifs.2006.03.006 558 S.-ur-Rehman et al. / Trends in Food Science & Technology 17 (2006) 557e566

Table 1. Volatile and non-volatile compounds present in wheat flour sourdough fermented with various yeast strains (Damiani et al., 1996)

Compounds A B C D F Lactic acid Ethanol þþþ 1-Propanol þþþþ 2-Methyl-1-propanol þþþ Ethyl acetate þþþþþ 3-Methyl-1-butanol þþþ 2-Methyl-1-butanol þþþþ 1-Pentanol 2-Methyl-1-pentanol 1-Hexanol þþþþ 3-Hexen-1-ol 1-Heptanol 1-Octanol Acetaldehyde þþþþþ 3-Methyl-1-butanal þþþþþ 2-Methyl-1-butanal þþþþ Hexanal þþþþþ 3-Methyl-hexanal Heptanal Trans-2-heptenal Octanal þþþþþ Nonanal þþþþþ Benzaldehyde þ Hexane þþþþþ Heptane þþ Octane þþ þ, Present and , not present. A e ;Be Candida krusei;Ce Candida norvegensis;De Saccharomyces exiguus; and E e Hansenula anomala.

Saccharomyces delbrueckii, Torulopsis holmii and Torulop- sourdoughs (Gu¨l et al., 2005). In wheat sourdough, mixed sis unisporus (Gu¨l, O¨ zc¸elik, Sadıc¸, & Certel, 2005). cultures were estimated as: Lb. Sanfranciscensis e 20%; It is generally considered that in sourdoughs, the ratio of Lb. Alimentarius e 14%; Lb. Brevis e 12%; LAB to yeast should be 100:1 for optimal activities citreum e 7%; Lb. Plantarum e 6%; Lactococcus lactis (Gobbetti, Corsetti, Rossi, La Rosa, & De Vincenzi, 1994). subsp. lactis e 4%; Lb. fermentum, Lb. acidophilus and Although, Saccharomyces exiguus is a dominant species in Weissella confusa e each 2%; Lb. delbrueckii subsp. del- starters, strains do not ferment and grow by fermenting brueckii (Corsetti et al., 2001). Although, Lb. sanfrancisco released by heterofermentative lactic acid bacteria. In is considered a key sourdough lactic acid bacterium return, the yeast supplies an electron source, , to the (Gobbetti & Corsetti, 1997) in rice sourdough, Lb. fermen- LAB that assist both growth and acetic acid production: a tum, Lb. gallinarum, Lb. kimchii, Lb. plantarum, Lb. pontis, co-operation with important technological and sensory conse- Lb. paracasei, and Lb. paralimentarius have been reported quences. Production of acetic acid improves quality in leavened to dominate (Merotha, Hammesa, & Hertela, 2004). dough and other bakery products (Vernocchi et al., 2004). Mixed cultures of LAB and yeasts vary in composition Typical sourdough prokaryotes are members of the LAB in sourdough sponges. The use of mixed cultures has a num- genus Lactobacillus with obligately homofermentative, and ber of important advantages, such as improved flavour and facultatively or obligately heterofermentative strains with texture and retained freshness for longer compared to strains of Lb. sanfranciscensis (Catzeddu, Mura, Parente, baker’s yeast bread (Meignen et al.,2001). In such mixed Sanna, & Farris, 2006) (synonym Lb. brevis subsp. lindneri), cultures, yeasts act mainly as leavening agents, while Lb. plantarum and Lb. brevis most frequently isolated LAB contribute mainly to the flavouring compounds of (Tru¨per & de Clari, 1997). Certain strains, initially classified bread. Kefir is a natural mixed culture which can produce as Lb. brevis, are recently allotted to the new species bread of good quality. Many microorganisms have been iso- Lb. Pontis (Vogel et al., 1994). Other LAB strains including lated from kefir microflora, sharing symbiotic relationships Carnobacterium divergens (Lb. Divergens, Lb. amylophilus, including yeasts (Kluyveromyces, Candida, Torulopsis and Lb. sake, Lb. acetotolerans, L. plantarum, Pediococcus Saccharomyces sp.), lactobacilli (Lb. brevis, Lb. acidophi- pentosaceus and P. acidilactici, and Tetragenococcus halo- lus, Lb. casei, L. helveticus, and Lb. delbrueckii), strepto- philus (Pediococcus halophilus) have been isolated from cocci (Streptococcus salivarius) lactococci (Lc. Lactis ssp. S.-ur-Rehman et al. / Trends in Food Science & Technology 17 (2006) 557e566 559

Table 2. Volatile and non-volatile compounds present in wheat flour sourdough fermented with hetero- (AeE) and homofermentative (FeJ) LAB strains (Damiani et al., 1996)

Compounds A B C D E F G H I J Lactic acid þþþþþþþþþþ Acetic acid þþþþþþþ Ehanol þþþþþ 1-Propanol þ 2-Methyl-1-propanol Ethyl acetate þþþþþþþþþþ 3-Methyl-1-butanol 2-Methyl-1-butanol 1-Pentanol 2-Methyl-1-pentanol þþ 1-Hexanol þþþþþþþþþþ 3-Hexen-1-ol þ 1-Heptanol þþþ 1-Octanol þþþ Acetaldehyde þþþþþþþþþþ 3-Methyl-1-butanal þþþ 2-Methyl-1-butanal Hexanal þþþþþþþþþþ 3-Methyl-hexanal þ Heptanal þþþþþ Trans-2-heptenal þþþþ Octanal þþþþþþþþþþ Nonanal þþþþþþþþþþ Benzaldehyde þþþ Diacetyl þþþþþ Hexane þþþþþþþþþ Heptane þþþþþþþþþ Octane þþþþþþþþþ þ, Present and , not present. A e Lactobacillus brevis lindneri;Be Lactobacillus brevis;Ce Lactobacillus fructivorans;De ;Ee Lactobacillus cellobiosus;Fe ;Ge Lactobacillus farciminis;He Lactobacillus alimentarius;Ie Lactobacillus acidophilus; and J e Lactobacillus delbrueckii.

thermophilus, Leuconostoc mesenteroides and L. cremoris) lambica, three each as C. krusei and Candida valida and two and occasionally acetic acid bacteria (Plessas, Pherson, as Candida glabrata were identified (Succi et al., 2003). Bekatou, Nigam, & Koutinas, 2005; Simova et al., 2002). The phenotypic characterisation of the isolates identifies the presence of 80% as facultative heterofermentative Factors affecting sourdough volatiles LAB (Lb. plantarum, Lb. paracasei, Lb. casei) and 12% and non-volatile compounds as heterofermentative LAB (Lb. brevis and L. mesenter- Sourdough sponges vary in proportions of LAB in dif- oides) in sourdough of Altamura bread (Ricciardi, Parente, ferent speciality breads. The primary metabolic products Piraino, Paraggio, & Romano, 2005). of LAB are L-lactic and acetic acids with lesser amounts The conditions and substrates of the sourdough sponge are of citric and malic acids. Ratio of lactic acid to acetic fundamental with respect to microbiological stability of acid is important for final product flavour (Linko, Javanainen, dough. Mixed commercial starters containing Lb. brevis & Linko, 1997). LAB multiply and produce lactic and acetic and S. cerevisiae (baker’s yeast), fermented at 30 C for acids more slowly in the mixed culture with yeasts than in 20 h, showed reduced yeast growth and ethanol production monocultures (Merseburger, Ehret, Geiges, Baumann, & in dough but more glycerol (80%) and acetic acid (55%) Schmidt-Lorenz, 1995). whereas lactic acid production was unchanged (Meignen Heterofermentative LAB species can be discriminated et al., 2001). In contrast to the production of durum wheat from homofermentative on the basis of volatile compounds flour sourdough breads, more than 95% strains were (Table 2). Certain aldehydes are derived both from enzy- of the yeasts Candida humilis and C. krusei and S. cerevisiae matic oxidation or autoxidation of the lipid fraction of were dominant in rice sourdough (Merotha et al., 2004). the wheat (Frankel, 1982; Hann & Morrison, 1975) and While in Triticum aestivum wheat flour sourdough, 58 strains bacterial metabolism. Homofermentative LAB, with the as S. cerevisiae, five as Candida colliculosa, four as Candida exception of Lb. delbrueckii subsp. delbrueckii strains are 560 S.-ur-Rehman et al. / Trends in Food Science & Technology 17 (2006) 557e566 primary contributor of ethyl acetate and diacetyl (Hansen & due to the greater flour enzyme activity, which increases the Hansen, 1993; Hansen, Lund, & Lewis, 1989a). availability of soluble carbohydrates (Ro¨cken & Voysey, The composition of sourdough flavour compounds is, 1993). In wheat sourdough , maltose remains however, not only influenced by microbial composition, between 2 and 5 g/kg (Martinez-Anaya, Pitarch, & Bene- but also interactions between bread making processes and dito de Barber, 1993) as yeast preferentially depletes avail- ingredients (Collar, 1996). These are important for process able glucose and fructose (Barber, Baguena, Benedito de stability and production of variety of regional specialities Barber, & Martinez-Anaya, 1991). that meet consumer and commercial demands (Gobbetti, Co-fermentations are other metabolic strategies to 1998). In sourdoughs produced from Triticum aestivum enable sourdough LAB to use non-fermentable substrates, (Succi et al., 2003) and Triticum durum wheat flours, a num- increasing adaptability. Co-fermentation of fructose and ber of lactic acid bacteria and yeasts range from ca. log 7.5 maltose or glucose has been observed specifically in a to log 9.3 colony forming units (cfu)/g and from log 5.5 to fructose-negative strain of Lb. Sanfranciscensis (Gobbetti, log 8.4 cfu/g, respectively (Corsetti et al., 2001). In slack Corsetti, & Rossi, 1995; Stolz, Bo¨cker, Hammes, & Vogel, , there are changes in yeast and lactic acid bacterial 1995), as has co-metabolism of citrate and maltose or glu- growth with more abundant flavour compounds. Stimulat- cose (Gobbetti & Corsetti, 1996). Moreover, changes in the ing effects on yeasts of NaCl can indirectly be attributed availability of citrate and fructose can influence the ratios to a reduction in competition with lactic acid bacteria for of acetic acid and lactic acid and micro-structural rheolog- available (Neysens, Messens, & Vuyst, 2003), and ical features of the doughs (Gianotti et al., 1997), through with content of other soluble carbohydrates, in the cereal change in acetic acid production (Calderon, Loiseau, & flours at typical fermentation temperatures. Guyot, 2003). Certain strains of S. cerevisiae are sensitive Ingredients in sourdough bread influence generation of to the acetic acid produced by LAB, especially at the volatile compounds, volatile compounds of germinated, normal sourdough pH of 4.0e4.5 that favours the undisso- sourdough fermented, and native substantially different, ciated lipophilic and membrane-diffusible form of the variable even after the second treatment (Heinio¨ et al., organic acid. Thus, it is necessary to add baker’s yeast 2003). Type of cereal flours, endogenous and exogenous to compensate for poor survival of wild-type yeasts in cereal components and processing steps such as the heat consecutive sourdough fermentations (Suihko & Makinen, treatment in baking has significant effects on the generation 1984). of flavouring compounds in breads (Hansen, 1995). One key Lb. sanfranciscensis hydrolyses maltose and accumu- driver of flavour is the carbon source available to the micro- lates glucose in the medium in a molar ratio of about 1 - organisms, notably low-molecular weight sugars that is, at ose to 1 glucose (Gobbetti, Corsetti et al., 1994; Gobbetti, least in part, flavour precursors (Martinez-Anaya, 1996). Simonetti et al., 1994; Stolz, Bo¨cker, Vogel, & Hammes, addition to wheat dough stimulates both yeast 1993). A study of maltose uptake and glucose excretion and LAB growth and increases bacterial production of in Lb. sanfranciscensis (Neubauer, Glaasker, Hammes, lactic and acetic acids (Corsetti, Gobbetti, & Rossi, 1994). Poolman, & Konings, 1994) showed that once the maltose However, titratable acidity (TTA) of the dough increases is depleted, consumption of the excreted glucose is initi- in proportion to the addition of sucrose over the range of ated. Glucose excreted during fermentation may be used 0e6%, ascribed to acetic acid accumulation which affects by maltose-negative yeasts such as S. exiguus or induced the generation of flavouring compounds (Simonson, glucose repression of maltose catabolism in competitors Salovaar, & Korhola, 2003). sourdough from using maltose, thereby giving an ecological advantage breads are produced with specific cultures of Lactobacillus to Lb. sanfranciscensis (Nout & Creemers-Molenaar, 1987). (Linko et al., 1997; Seitz, Chung, & Rengarajan, 1998). However, among the Lactobacillaceae, only certain species e A lack of competition between Lb. sanfranciscensis and Lb. sanfranciscensis, Lb. pontis, Lb. reuteri, and Lb. S. exiguus for maltose consumption was noted which is fun- fermentum (Vogel et al., 1994) e phosphorylate maltose damental for the stability of both the microorganisms for and the enzyme maltose phosphorylase may be considered the production of good quality San Francisco French bread the key for LAB growth during sourdough fermentation. (Sugihara, Kline, & McCready, 1970). However, LAB Conversely, with sucrose and Lb. plantarum and S. cer- growth and lactic and acetic acid productions may be re- evisiae or S. exiguus in co-culture (Gobbetti, Corsetti, & duced by more rapid consumption of maltose and, notably, Rossi, 1994a) cell yield and lactic acid production increase of glucose by S. cerevisiae in an association with Lb. san- (Robert et al., 2006). Invertase hydrolysis of sucrose by franciscensis in synthetic media (Collar, 1996). Even so, yeasts into glucose and fructose, may enhance LAB metab- imbalances between yeast consumption and hydroly- olism (Aksu & Kutsal, 1986) because yeasts can hydrolyse sis by endogenous flour enzymes lead to rapid depletion of sucrose to about 200 times faster than the released mono- soluble carbohydrates during wheat sourdough fermenta- saccharides are fermented (Martinez-Anaya, 1996), with tion which, in turn, decreases LAB acidification due to rapid sucrose depletion during sourdough fermentation microbial competition (Rouzaud & Martinez-Anaya, 1993). (Seppi, 1984). S. exiguus preferentially utilises glucose or This situation is less pronounced in rye dough fermentation sucrose and shows a high tolerance for the acetic acid S.-ur-Rehman et al. / Trends in Food Science & Technology 17 (2006) 557e566 561 from heterolactic LAB metabolism (Suihko & Makinen, (Spicher, Rabe, Sommer, & Stephan, 1981; Spicher, Rabe, 1984). Sommer, & Stephan, 1982). Flavour component release may also be influenced by Temperature optima for co-culture of C. humilis and the environmental conditions such as temperature. Yeast Lb. sanfranciscensis are 28 and 32 C, respectively, but and LAB growth increase in a direct relationship with reduced production of acetate by Lb. sanfranciscensis has temperature between 15 C and 27 C. Optimal growth been observed at 35 C, although, both lactate and ethanol temperature of the two Candida milleri strains was between formations are not affected at this temperature (Brandt, 26 C and 28 C in monoculture. Nevertheless, decrease in Hammes, & Ganzle, 2004). Lactic acid production varied the growth of yeast may be accompanied by a similar from 3.11 to 5.14 g/kg and traces of acetic acid may be pro- reduction in the growth of LAB (Neysens et al., 2003; duced by some of the Lb. plantarum and Lb. farciminis Simonson et al., 2003). strains during fermentation (Table 1). Processing conditions such as proofing time, tempera- ture and slackness of sourdough may also affect the aroma Volatile compounds volatiles. Low temperature (25 C) and sourdough firmness Microbial metabolisms verify production of different are considered appropriate for LAB souring activities but volatile compounds for hetero- and homo-lactic LAB limited yeast metabolism. Raising the temperature to (Table 1) fermentations. Abundant products of yeast fer- 30 C and semi-fluid sourdough fermentation can generate mentation are 2-methyl-1-propanol, 2,3 methyl-1-butanol more complete volatile profiles. While at 3 h fermentation, and other iso-alcohols. Heterofermentative LAB products the sourdough may mainly be characterised by iso-alcohols are dominated by ethyl acetate and certain alcohols and but an increase of leavening time of up to 9 h can produce aldehydes (Spicher et al., 1982) whereas major homofer- volatiles about three times higher than that at 5 h as a result mentative LAB products are diacetyls and carbonyls of LAB contribution (Lund, Hansen, & Lewis, 1989). (Spicher et al., 1981)(Table 2). Lactobacillus brevis subsp. Lindneri and Lactobacillus Types of flavouring compounds in sourdough breads plantarum have been considered to have the most appropri- There are two categories of flavour compounds, produced ate profiles of flavour components. (Gobbetti, Corsetti, & during sourdough fermentation. Non-volatile compounds De Vincenzi, 1995a). However, with the exception of including organic acids produced by homo- (Gobbetti, Lb. plantarum DC400eSaccharomyces exiguus M14 asso- Corsetti, & De Vincenzi, 1995a) and heterofermentative bac- ciation, both hetero- and homofermentative LAB enhance teria (Gobbetti, Corsetti, & De Vincenzi, 1995b) whichacidify, yeast formation volatile compounds (Damiani et al., 1996; decrease pH and contribute aroma to the bread dough (Barber, Gobbetti, Corsetti, & De Vincenzi, 1995b). Benedito de Barber, Martinez-Anaya, Martinez, & Alberola, The LAB strains of sourdough vary in metabolism and 1985; Galal, Johnson, & Varriano-Marston, 1978). aroma compounds. Monoculture fermentation of dough The second category e volatile compounds of sour- for 15 h at 30 C, followed by mixing and a further 10 h dough bread e includes alcohols, aldehydes, ketones, esters fermentation has shown increase in the production of typi- and sulphur. All these compounds are produced by bio- cal sourdough volatile compounds (Merotha et al., 2004). logical and biochemical actions during fermentation and The production of volatile aroma compounds during contribute to flavour (Spicher, 1983). fermentation with combinations of single strains of S. cer- evisiae and C. guilliermondii, Lb. plantarum has been Non-volatile compounds investigated using both wheat doughs and simple substrates Interactions between lactic acid bacteria and exogenous including maltose and glucose (Stolz et al., 1993). Using enzymes influence microbial kinetics of acidification, only yeasts in wheat bread, seven volatiles were found acetic acid production and bread textural characters. It has abundant: acetaldehyde, acetone, ethyl acetate, ethanol, been shown to be desirable to augment endogenous hexanal, isobutyl alcohol, and propanol. In this fermenta- enzyme activities in flour and has been obtained from tion, S. cerevisiae produced more volatiles than C. guillier- LAB in association with microbial glucose oxidase, lipase, mondii, but quantity of volatile flavour compounds can be endo-xylanase, - and proteases. The growth of improved by the addition of glucose and sucrose less by Leuconostoc citreum 23B, Lb. lactis subsp. lactis 11M and maltose. It is still not clear whether yeasts produce more Lb. hilgardii 51B has been influenced by enzyme addition, flavouring components than LAB in simple substrate which which increases lactic acid production (Cagno et al., appeared to produce a greater range of volatiles from wheat 2003). However, in a continuous sourdough fermentation, flour (Hansen, 1995; Torner, Martı´nez-Anaya, Antun˜a, & an association between Lb. sanfranciscensis and S. cerevi- Benedito de Barber, 1992; Vollmar & Meuser, 1992). siae has showed optimal production of acetic acid (Vollmar Addition of enzymes to sourdough sponges can also & Meuser, 1992), whereas Torulopsis holmii improved enhance bread volatile compounds (Martinez-Anaya, dough acidification in association with Lb. sanfranciscensis 1996), lipid oxidations through addition of enzymes in the and S. cerevisiae and further enhanced acid production if form of active soya flour increase concentrations of hexanal, associated with Lb. sanfranciscensis and Lb. plantarum 1-hexanol, 1-penten-3-ol, 1-pentanol and 2-heptanone, and 562 S.-ur-Rehman et al. / Trends in Food Science & Technology 17 (2006) 557e566

2-heptenal and 1-octen-3-ol have only been reported in of bread (Bredie, Mottram, & Guy, 2002; Damiani et al., breads containing soya flour (Luning, Roozen, Moe¨st, & 1996). products from high temperature Posthumus, 1991). Addition of lipase, endo-xylanase and processing include pyrazines, pyrroles, furans and sulphur- a-amylases enhanced acetic acid production by Lb. hilgardii containing compounds and lipid degradation products such 51B. Textural analyses suggest that such sourdoughs with as alkanals, 2-alkenals and 2,4-alkadienals (Parker, Hassell, single enzyme have greater stability and crumb softening Mottram, & Guy, 2000). Moreover, at higher temperatures than breads produced with only exogenous enzymes (Cagno pyrroles, thiophenes, thiophenones, thiapyrans and thiazolines et al., 2003). Moreover, addition of fructose and citrate to increase and furans and aldehydes decrease (Bredie et al., sourdough enhanced the production of volatiles by LAB 2002). The pyrazines are nitrogen-containing ring compounds including ethanol, which was eliminated in baking and that have extremely powerful odour and flavour properties 2-methyl-1-propanal with no effect on lactic and acetic (Ji and Berhard, 1992). Hence, the production of these com- acids’ production (Gobbetti, Corsetti, & De Vincenzi, 1995a; pounds has a striking effect on the flavour of bread. Gobbetti & Corsetti, 1996; Gobbetti, Smacchi, Fox, Stepaniak, & Corsetti, 1996). In a French yeast sourdough, more than 40 flavour com- Sensory characteristics of bread ponents were identified: 20 alcohols, 7 esters, 6 lactones, 6 The sourdough fermentation is central to acceptability in aldehydes, 3 alkanes and a single sulphur compound flavour, as chemically acidified bread and breads prepared (Frasse, Lambert, Richard-Molard, & Chiron, 1993). Com- with pure commercial starter cultures are not well scored ponents, except for aldehydes and alkanes, increased in in sensory preference assessments (Lund et al., 1989; Rothe abundance due to greater activity of S. cerevisiae notably & Ruttloff, 1983). The synergistic metabolic activities of those related to formation of fusel alcohols. However, the microorganisms produce an acidification or souring in this fermentation 2,3 butanedione, 3-methyl-1-butanol, influencing the final character in the bread, notably the tex- 2-methyl 1-butanol, methional, 2-phenylethanol and 2 ture, increased shelf-life by reducing the mould growth unidentified compounds with a pungent and mushroom during storage (Corsetti et al., 2000; Oura, Soumalainen, odour were also apparent. & Wiskari, 1982; Ro¨cken & Voysey, 1995) and generate A number of other factors influence the production of typical flavour components yielding typical sourdough volatile compound during production of sourdough bread sensory attributes (Gobbetti, 1998; Katina et al., 2005). including free amino acids formed during fermentation The flavour of sourdough wheat bread is richer and more through proteolysis which influence volatile compound pro- aromatic than in wheat bread, a factor that can be attributed file (Collar & Martinez, 1993; Collar, Mascaros, Prieto, & to the long fermentation time of sourdough (Bruemmer & Benedito de Barber, 1991; Gobbetti, Corsetti, & Rossi, Lorenz, 1991). The most desirable sensory characteristics 1994b; Gobbetti et al., 1996; Spicher & Nierle, 1984). are obtained at pH 4.0e5.5 and at 140 C(Przybylski & Thiazolines, thiophenes, thiophenones, thiaziles, polythia- Kamin´ski, 1983). The loaves made with the addition of cycloakanes, pyrroles and pyrazines have been shown to 5e10% sourdoughs fermented with Lb. plantarum and relate to acidity in wheat extrusion (Hansen & 5e15% sourdoughs fermented with Lb. sanfrancisco are Hansen, 1994a) and sourdough fermentation (Hansen preferred in odour and taste (Hansen & Hansen, 1996). et al., 1989a). Sensory evaluation of sourdough of wheat bread crumb showed that bread made from sourdough fermented with Effect of Maillard and reactions the heterofermentative Lb. sanfranciscensis had a pleasant, on the flavour of bread mild, sour odour and taste, whereas sourdough bread The production of volatiles is also influenced by proof- fermented with the homofermentative Lb. plantarum had ing and baking processes (Hansen, Lund, & Lewis, an unpleasant metallic sour taste. But, when sourdough 1989b; Kamin´ski, Przybylski, & Gruchala, 1981). The was supplemented with sourdough yeast S. cerevisiae, the Maillard and Caramelization reactions are overwhelmingly wheat bread received a more aromatic flavour (Katina, responsible for flavour formation in cereal products. Upon Heinio¨, Autio, & Poutanen, in press). Chemical analyses heating, the reaction involves between simple the precur- of flavour compounds can be combined with sensory anal- sors such as amino acids and simple aldose or ketose of ysis of bread. Compounds that have been positively sugars (Rothe & Ruttloff, 1983). As shown previously, correlated with flavour of wheat crumb are acetaldehyde, the highest amount of free amino acids was produced 2-methylpropanoic acid, 2/3-methyl-1-butanol, 3-methyl- during sour dough fermentation with the association butanoic acid, isopentanal, 2-nonenal, benzylethanol, 2- between LAB and S. exiguus M14 (Gobbetti, Corsetti phenylethanol, 2,3-butandione and 3-hydoxy-2-butanone, et al., 1994; Gobbetti, Simonetti et al., 1994; Kratochivil dimethyl sulphide and 2-furfural (Hansen & Hansen, & Holas, 1983; Rothe, 1974). These amino acids are pre- 1996; Rothe, 1974). Sensory evaluation of sourdough rye cursors of iso-alcohols (Gobbetti, Corsetti, & De Vincenzi, bread crumb suggested that a most intense and bread-like 1995b; Hansen & Schieberle, 2005) that contribute directly flavour was related to propanone, 3-methylbutanal, benzyl to bread flavour during sourdough fermentation and baking alcohol and 2-phenylethanol (Hansen et al., 1989a). S.-ur-Rehman et al. / Trends in Food Science & Technology 17 (2006) 557e566 563

Effect of type and flour composition heterofermentative cultures, with the highest amounts de- on flavour compounds tected in sourdough made from wholemeal and low-grade The interactions between starter cultures and flour com- flours (Hansen & Hansen, 1994a). In contrast to white ponents play an important role in the production of flavour wheat bread, the starch granules are very much swollen compounds in the sourdough breads. A particular favoured in the bran sourdough bread with enzymatic mixture which cereal for sourdough is rye flour that contains flavour pre- improves the texture, volume, flavour and shelf-life of cursor amino acids, fatty acids and phenolic compounds bread (Katina, Salmenkallio-Marttila, Partanen, Forsell, & that are converted into flavour-active components in differ- Autio, 2006). ent production stages of baking (Schieberle & Grosch, With the high extraction rate (80e100%), the content of 1983; Schieberle & Grosch, 1985; Schieberle & Grosch, nutrients such as B vitamins and minerals increases com- 1987; Hansen, 1995). In rye, germination imparts cereal pared to low extraction rate flour (65e75%), as does the and fresh flavour in extruded products. Key flavour compo- buffering capacity of flour chiefly due to the nents are dimethyl sulphide and 2-methylbutanal which (Batifoulier, Verny, Chanliaud, Re´me´sy, & Demigne´, could be related to the sensory attribute intensities. Ex- 2005). These factors stimulate the growth and biochemical truded rye sourdough has been reported to have an intense, activity of the LAB followed by a higher production of sour flavour and porous texture with high contents of furfu- acids and flavour compounds (Martinez-Anaya, Benedito ral, ethyl acetate, 3-methylbutanol and 2-methylbutanol and de Barber, & Esteve, 1994; Salovaara & Valjakka, 1987). such extrudates of rye flour are rich in 2-ethylfuran, 2- Soya flour, containing a range of enzyme activities, also methylfuran, hexanal and pentanal (Heinio¨ et al., 2003). could influence the bread volatiles, isolated by dynamic Other contributors to the overall crumb flavour of headspace techniques and quantified by gas chromatogra- include 3-methylbutanal, 2-nonenal, 2,4-dcadienal, hexa- phy and gas chromatography/mass spectrometry (Luning nal, phenylacetaldehyde, methional, vanillin, 2,3-butan- et al., 1991). dione, 3-hydroxy-4,5-dimethyl-2(5H)-furanone and 2- and Wheat and rye flours are mostly used for sourdough 3-methylbutanoic acid (Kirchhoff & Schieberle, 2001). making but maize and rice flours can also be used (Buttery Sourdough breads may vary in flavouring compounds as & Ling, 1999; Buttery, Orts, Takeoka, & Nam, 1999; a result of flour composition and blending of the cereal Clarke & Arendt, 2005; Rocha & Malcata, 1999; Sanni, flours (Schieberle & Grosch, 1985; Schieberle & Grosch, Onilude, & Fatungase, 1998). Major volatiles of rice flour 1992; Schieberle & Grosch, 1994). Wheat flour composition include 1-hydroxy-2-propanone, furfuryl alcohol, 2,5-dime- influences the concentration of ethyl acetate and ethanol thylpyrazine, 2-methylpyrazine, pyrazine, hexanal, furfural, production in heterofermentative LAB sourdoughs which pentanol, 3-hydroxy-2-butanone and ethyl-3,6-dimethyl- are more abundant with high-grade flours (Hansen & pyrazine (Buttery et al., 1999) whereas major volatiles Hansen, 1994a). Ash contents from 0.55 to1.00% had identified in corn flour include 2-methylpyrazine, 1- positive influences on total volatiles. Wheat bread from hydroxy-2-propanone, 4-vinylguaicol, 2-acetyltetrahydro- sourdough sponges including Lb. plantarum or Lb. sanfran- pyridine (E,E )- and (E,Z )-2,4-decadienal, acetic acid, ciscensis had a higher content of 2,3-methyl-1-butanol; and 3-methylbutanal, g-butyrolactone, furfuryl alcohol and through association of LAB and yeasts, bread was consid- 2,5-dimethylpyrazine (Buttery & Ling, 1999). ered of enhanced flavour quality with higher contents of 2,3-methyl-1-butanol, 2-methyl-propanoic acid, 3-methyl- butanoic acid and 2-phenyl-ethanol (Hansen et al., 1989b; Conclusion Hansen & Hansen, 1996). It is considered that this is the In sourdoughs, flavour-active compounds are produced result of the combination of greater bacterial acidification by LAB and yeasts individually and through their interac- and proteolysis (Gobbetti, Corsetti et al., 1994; Gobbetti, tions. Heterofermentative LAB mainly produce ethyl Simonetti et al., 1994; Levesque, 1991) which may be acetate and certain alcohols and aldehydes, whereas homo- attributed to Lb. sanfranciscensis in the sourdough. Hansen fermentative LAB synthesise diacetyl and other carbonyls. & Hansen (1994b) have suggested an association of In contrast, iso-alcohols are products of yeast fermentation Lb. sanfranciscensis, Lb. plantarum and S. cerevisiae as but may contribute little towards final bread flavour. Addi- optimal for aroma balance in wheat sourdough breads. tion of fructose/glucose/maltose or citrate to the dough The amount of fermentable carbohydrate in the flour increases LAB contributions to volatile formation in baking varies with the type of cereal, but in particular with the but Maillard and Caramelization reactions are also respon- activities of amylases, xylanases and peptidases enzymes sible for flavour formation in baked cereal products. Inter- of flour. In wheat flours, totals of maltose, sucrose, glucose actions between microbial strains and ingredients, and fructose vary from 1.55 to 1.85% depending on extent fermentation temperature, pH and leavening time have all of starch hydrolysis, and activities of microbial enzymes been evaluated. Further research should be conducted on and microbial contaminants (Martinez-Anaya, 1996). optimization of fermentation process in wheat bread The flour type has a momentous effect on the production production by varying biotechnical parameters, levels of of ethyl acetate and ethanol in sourdough emulsifiers/improvers including lactose, solids and 564 S.-ur-Rehman et al. / Trends in Food Science & Technology 17 (2006) 557e566 oxidants. Effects of dried mixed cultures of yeasts-LAB wheat doughs started with pure culture of lactic acid bacteria. also need further attention. Cereal Chemistry, 68,66e72. Corsetti, A., Gobbetti, M., & Rossi, J. (1994). 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