Biotechnology of bread baking

35 Sakamoto, H., Kumazawa, Y. and Motoki, M. (1994) ‘Strength of Protein Gels Prepared with Microbial Transglutaminase as Related to ReactIon Condition,’ I” j. Food%!. 59, 866-871 36 Matsumura, Y., Chanyongvorakul, Y., Mori, T . and Motoki, M. (1995) ‘Gelation oi Protein Solutions and Emulsions by Transglutaminase’ in Food Macromolecules and (‘olloldr (Dickmson, E and Lorient, D., eds), pp. 410417, Royal Society of Chemistry. Cambridge, UK 37 Chanyongvorakul, Y. et al. (1994) ‘Gelation of Bean 11 S Globulins by Cal+Independent Transglutaminase’ in Biosci Riotechnol. &ochem. 58, 864-869 38 Kang, 1.1. (1994) ‘Celation and Gel Properties of Soybean Clycinin in a Transglutamlnase Catalyzed System’ in J. Agric. Food Chem. 42, 159-165 39 Fzrgemand, M , Otte, J. and Qvist, K.B (1997) ‘Enzymic Crosslinking of Whey Proteins by a Ca’+-independent Microbial Transglutaminase From Streptomyces lydicus’ in Food Hydrocollords 11, 19-25 40 Dickinson, E. and Yamamoto, Y. (1996) ‘Rheology of Milk Protein Gels and Protwn-stablllred Em&Ion Gels Crosshnked with Transglutammase’ m J. Agric. Food Chem. 44, 1371-1377 41 Dickinson, E. (1992) An Introduction fo Food Colloids, pp. 66-70, Oxford Unlverslty Press 42 Ross-Murphy, S.B. (1995) ‘Rheology of Biopolymer Solutions and Gels’ in New Phyxo-chemical Techniques for the Characrerization of Complex Food Systems (Dickinson, E., ed.), pp 139-156, Blackie 43 Tanimoto, S.Y. and Kinsella, I.E. (1988) ‘Enzymatic Modification of Proteins Effects of Transglutaminase Crosslinking on Some PhysIcal Properties of P-Lactoglobulin’ in J. Agric. Food Chem. 36, 281-285 44 Nio, N. and Motoki, M. (1986) ‘Celation of Protein Emulsion by Transglutaminase’ in Agric. Biol. Chem. 50, 1409-l 412 45 Ma&mum, Y., Kang, I-J., Sakamoto, H , Motoki, M. and Mod, T (1993) ‘Filler Effects of Oil Droplets on the Vlscoelastlc Properties of Emulsion Gels’ in Food Hydrocolloids 7, 227-240 46 Dickinson, E., Murray, B.S. and Stainsby, G. (1988) ‘Protein Adsorption at Air-Water and Oil-Water Interfaces’ in Advanre~ /R Food Emulcmm and foamy

eta/.

(Dickinson, E. and Stainsby, G., eds), pp. 123-l 62, Elsevier 47 Murray, B.S. and Dickinson, E. (1996) ‘Interfacial Rheology and the Dynamic Properties of Adsorbed Films of Food Proteins and Surfactants’ in Food SC;. Technol. Int (Japan) 2, 131-l 45 48 Courthaudon, J-L., Dickinson, E., Matsumura, Y. and Clark, DC. (1991) ‘Competitive Adsorption of P-Lactoglobulin + Tween 20 at the Oil-Water Interface’ in Co/bids Suri. 56, 293-300 49 Chen, j. and Dickinson, E. (1995) ‘Surface Shear Viscosity and Protein-Surfactant Interactions in Mixed Protein Films Adsorbed at the, Oil-Water Interface’ in Food Hydrocolloids 9. 35-42 50 Dickinson, E., Rolfe, S.E. and Dalgleish, D.G. (1988) ‘Competitive Ad,orption of a,,-Casein and p-Casein in Oil-in-water Emulsions’ in Food Hydrocolloids 2,397-l05 51 Han, X-Q. and Damodaran, S. (19961 ‘Thermodynamic Compatibility of Substrate Proteins Affects Their Crosslinking by Transglutaminase’ in j. Agric. Food Them. 44, 121 l-l 217 52 Dickinson, E. (1993) ‘Towards Natural Emulsiiiers’ in Trends FoodSci. Technol. 4, 330-334 53 Kato, A., Wada, T., Kobayashi, K., Seguro, K and Motoki, M. (1991) ‘Ovomucin-Food Protein Conjugates Prepared Through the Transglutaminase Reaction’ in Agric. Bio/. Chem. 55, 1027-1031 54 Ikura, K., Sasaki, R. and Motoki. M. (1992) ‘Use of Transglutaminase in Quality Improvement and Processing of Food Proteins’ in Comments Agrk Food Chem. 2,389-409 55 Sakamoto, H , Kumazawa, Y., Kawajiri, H. and Motoki, M. (1995) ‘e-(7.GIutamyl)lyslne Crosslink Distrlbutton in Foods as Determined by Improved Method’ in J. Food SC;. 60, 416419 56 Hurrell, R.F., Carpenter, K.J., Sinclair, W.J., Otterbum, M S. and Asqulth, R.S. (1976) ‘Mechanisms of Heat Damage I” Protems’ in Br. 1. 35, 3t#3-395 57 Seguro, K., Kumazawa, Y., Kuraishi, C., Sakamoto, H. and Motoki, M. (1996) ‘The c-(y-Glutamyl)lyrine Moiety in Crosslinked Casein is an Available Source of Lyrmefor Rats’ I” J. Nutr. 126, 2557-25hL

Nutr.

Review

Biotechnology Biotechnology

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The baking of yeast-leavened and sourdough breads, together with the brewing of beer, sake and wine, and the production of yoghurt and cheese, etc., is among the oldest biotechnical processes. All involve either yeast or lactic acid bacteria, or both. For thousands of years, these microorganisms have been essential to the manuYu-Yen

Linko,

Piivi

Javanainen

and

Susan

Linko

are

Trends

in Food

Science

& Technology

October

at the

Laboratory

Technology, FIN-02015,

1997

Yu-Yen Linko, PSivi Javanainen Susan Linko

and

of the most

technologies.

Bioprocess Engineering, Department of Chemical Unwrrlty of Technology, PO Box 6100 HUT, (fax: +358-g-462373; e-mail: [email protected]).

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bioengineering,

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[Vol.

Helsinki Finland

81

of

facture of several types of foods. The role of biotechnology in food production ranges from the genetic modification of raw materials such as wheat and other cereals, yeast and lactic acid bacteria, to applications in raw material preservation and the production of food additives such as enzymes, and to the bioprocess itself. Increasing the productivity of cereal grains, together with improving their nutritive, functional and process quality., is the key issue in raw material production. Genetic engineering also offers the possibility to improve the insect, disease and cold resistance of cereal crops. This stateof-the-art overview illustrates how the bread baking process can take advantage of both ancient and modern developments in biotechnology.

Sourdough baking Spontaneous souring with natural microflora and the use of special starter cultures are both used in sourdough bread baking. Conventionally, 2-5% of old dough is used as an inoculum for the new batch. Sourdough breads vary from a large number of different rye breads and flat breads to San Francisco sourdough bread made from wheat flour. Interest in whole-grain sourdough breads has recently rapidly increased in many countries. This has been partly due to realization of the increased importance of nutritive fiber in the diet. In Finland, -40% of bread on the market is sourdough bread, mostly made from rye. Recent years have witnessed the development from continuous propagation of mixed-strain cultures in bakeries to commercial, well-defined starter cultures, and from simple, manual batch operations to continuous processes with advanced control’“. Meuser has extensively discussed various methods that are used in sourdough production. A typical commercial sourdough starter culture contains -lOi microbial cells per gram. When properly stored, such starter cultures may stay active for extended periods, and keep their mixed culture composition intact throughout years of propagation5. Sourdough inoculums were used for the leavening of bread even before baker’s yeast was available. The decrease in pH, owing to lactic acid fermentation, to levels that are inhibitory to a-amylase activity made it possible to bake tasty bread from rye with a low falling number (high a-amylase activity). Sourdough starter culture strain development has initially been based mainly on their capacity for acid formation, but sourdough baking also gives bread a longer shelf life and improved flavor. Together with yeast, heterofermentative lactic acid bacteria also aid leavening. According to Spicher et aL6, Lactobacillus sanfrancisco is a superior microorganism for sourdough bread baking. However, its fastidious requirements with regard to growth, freezing and freeze drying have limited its use in defined starter cultures’. Other important sourdough bacterial strains include Lactobacillus plantarum, Lactobacillus brevis var. lindneri and Lactobacillus fermenturn. In a mixed culture of L. sanfrancisco and L. brevis, the former species dies off quickly during extended incubation, unless supplied with fresh baker’s yeast and wheat bran’. The dominating yeast in sourdoughs is often Candida milleri; however, in San Francisco sourdough bread Saccharomyces exiguus is also important, whereas in frozen dough products and in very sweet dough products Saccharomyces rosei and Saccharomyces rouxii, respectively, are dominants. A major problem associated with the use of frozen dough is the reduced viability of yeast after thawing. According to Japanese studies, a mixed culture of Saccharomyces cerevisiae and S. rouxii markedly improves the fermentation stability of frozen dough. Special sweet dough yeasts behave poorly in doughs that are low in sugar (‘lean’ doughs), which require yeasts that have a high maltose-fermenting capability. In the main, sourdoughs contain both lactic acid bacteria and yeasts. Lactic acid bacteria produce mainly 340

L-lactic acid and acetic acid, with lesser quantities of other acids such as citric and malic acid, and flavor compounds. The traditional method is to mix equal quantities of rye flour and water, and let the mixture preferment spontaneously for 2-3 d. Alternatively, some old bread, ready sourdough or commercial starter can also be mixed into the dough. This step is followed by the addition of some more flour and water for another day’s fermentation. Owing to the relatively long fermentation periods, there has been great interest in ways ,of reducing the time required for sourdough preparation; for example, in Germany an accelerated method with a fermentation period of only 3 h has been developed. However, the very short fermentation time results in certain changes in the end product, and also requires the addition of fresh baker’s yeast to ensure satisfactory leavening. The use of well-defined commercial or proprietary starter cultures allows the improved control and reproducibility of bread quality. A good starter culture should lower the pH rapidly down to -4. The ratio of lactic acid to acetic acid is also important for the flavor of the final bread. Typical methods that are used in Germany are the ‘Detmolder Einstufensauerfiihrung’ and ‘Berliner Kurzsauerfiihrung’. In Sweden and Finland, a fermentation period of -20 h is typically preferred. Propionate production in situ for improved shelf life Modern consumers prefer minimally processed highquality foods with few or no additives. Biotechnical production of propionates in situ through well-balanced and controlled mixed-culture fermentation provides a convenient means to retard mold growth and thus to extend shelf life9,10. Propionic acid formation in sourdough fermentation was studied as early as the 195Os”, but few details have been published, and the bacterial strains were not identified. Wutzel’* has referred to the possible use of propionic acid bacteria in conjunction with lactic acid bacteria and yeasts in continuous sourdough fermentation. The problem with this approach is the different growth requirements of the different microorganisms and the relatively slow growth and propionic acid production rate of propionibacteria. hp[!4itions

of genetic engineering

The first transgenic, fertile wheat was reported only five years ago by Indra and Vimla Vasil and their colleagues at the University of Florida (Gainesville, FL, USA) and Michael From of Monsanto (St Louis, MO, USA), which was obtained by direct microprojectile bombardment of regenerable embryogenic calli to introduce a bacterial herbicide resistance gene13.They chose as a marker the bar gene, which encodes the enzyme phosphinothricin acyltransferase, instead of the commonly used gene that encodes an enzyme that inactivates kanamycin, to which wheat is likely to be res&tant. Although cl% of the bombarded calli could be recovered as transformed lines, their result was clearly a milestone. The researchers then developed a method Trends in Food Science

& Technology

October

1997 [Vol. 81

to recover flowering, transgenic wheat plants within 9 months or less by direct delivery of DNA into the scutellar tissue of immature embryos or l-2-month-old embryogenic calli r4. One of the main problems was that the regeneration of plants after the transformation of somatic cells with the Ti plasmid of an Agrobacterium sp. appeared not to work with wheat; the attainment of transgenic wheat was therefore dependent on in vitro cell cultures that could both take up and integrate exogenous DNA and remain capable of differentiation into whole plants15. Similar techniques have also been used by Weeks et al. I6 and Becker et al.” Weeks et al. I6 introduced DNA into S-d-old calli that were derived from immature embryos by bombarding them with microprojectiles that were coated with DNA containing a bar gene marker. The bombardment was under the control of the maize ubiquitin promoter. The transformants were recovered from a selective medium, and once the transformed plantlets had developed a dense network of rootlets, they were planted in soil. Nine independent fertile wheat lines were obtained as a result. Genetic engineering also offers several possibilities for the improvement of grain quality18-20. For example, Plant Breeding International (Cambridge, UK) has been working on wheats of improved baking quality21. A viscoelastic gluten network forms from wheat proteins during dough mixing to produce dough with properties that facilitate the entrapment of the carbon dioxide that is formed during fermentation. It has been shown that the high molecular weight subunits of wheat glutenins are likely to play a major role in glutenin functionality during the baking processz2. Modification of the composition of gluten protein fractions to affect baking quality is an interesting possibility. Quite recently, Shewry et al.” demonstrated that this is, indeed, technically feasible. Their transformation protocol was based on a culture system in which somatic embryogenesis and shoot formation are induced as quickly as possible after culture initiation in order to minimize the frequency of variant regenerants. The cultures were bombarded 1 d after initiation and antibiotic (geneticin) selection was applied at -3 weeks, when defined somatic embryos were present. Rooted plants could be transferred to a greenhouse within 4-5 months of culture initiation. Starch structure and content could be modified by altering the expression of genes that are involved in starch synthesis. Nutritional quality may also be improved by altering the amino acid composition of the grain by adding genes coding for storage proteins from plants that contain high levels of lysine and threonine24, although it should be remembered that changes that are beneficial from the nutritional point of view may have adverse effects on baking quality. Nevertheless, the bread baking industry will probably still have to wait a while before new, improved wheat strains that have been developed using recombinant DNA technology become commercially availableZ5. Recent advances in DNA analysis have made it relatively simple to identify the species and variety composition of cereal grain and grain-derived productsz6. The Trends in Food Science

& Technology

October

1997 [Vol. 81

polymerase chain reaction (PCR) has made it possible to amplify and detect specific DNA sequences through the use of synthetic oligonucleotide primers2’. PCR techniques are particularly attractive, because of their simplicity and the need for only one grain or less for identification. Baker’s yeast Genetic engineering can also be used to modify the properties of baker’s yeast and sourdough starter cultures for improved performance and bread quality:Ls. The quality requirements of baker’s yeast are many. In addition to excellent and uniform dough leavening ability, a typical, good baker’s yeast should be able to tolerate a fairly wide range of temperatures, and sometimes also varying pH and the presence of sugar, fat and preservatives, as well as being a good aroma producer, etc. A commercial yeast strain should also be able to grow on a wide range of substrates at a high yield, and have a high optimal temperature and excellent processing characteristics. Some of the recent developments will be briefly mentioned here. In 1988, Korhola and co-workers of Alko (Finland) developed a stable a-galactosidaseproducing baker’s yeast strain, which is capable of utilizing raffinose in molasses 2q. The Mel1 gene, which encodes extracellular o-gaiactosidase, from S. cerevisiae var. uvarum was either integrated into the chromosomal DNA or introduced via an autonomously replicating 2p. plasmid into the cells of a commercial meZ” baker’s yeast strain. Consequently, more carbohydrate was available for yeast growth during baker’s yeast production, with no adverse effects on yeast quality. Two years later, in 1990, the first genetically modified baker’s yeast, developed at Gist-Brocades (Delft, The Netherlands)““, was given approval by the UK’s Advisory Committee on Novel Foods and Processes for mass production and use in bakeries2’. Although this was an example of the development of a self-cloned organism, that is, a gene and promoter were transferred from a closely related yeast isolate into another yeast strain (intra-specific transfer) that was well adapted to industrial culture and production, this was the first time that a genetically modified organism (GMO) that was intended for eating had been approved for use. The news also immediately prompted lively discussion on the safety, regulatory and labeling issues relating to GMOS*‘*~‘, although self-cloned organisms are not now regarded as true GMOs. The yeast was shown to behave in the environment like the parent baker’s yeast, and the safety precautions taken included the complete removal of foreign DNA (e.g. antibiotic resistance genes necessary for the construction work), keeping the synthetic DNA linkers to an absolute minimum, taking measures to prevent any possibility of fusion protein expression, and incorporating the construct into the yeast chromosomes to obtain genetic stability3’. Nakagawa and Ouchii32 of Kyowa Hakko Kogyo Co. (Japan) have recently announced the development of a baker’s yeast that has improved freeze tolerance and high fermentative activity in both lean and sweet doughs. 341

Enzymes in baking Although the use of enzymes in baking has been studied since the 195Os, the costs and unavailability of enzymes in large quantities delayed widespread applications until recently. While only 5 years ago food uses represented nearly 60% of the total use of industrial enzymes, in 1996 only -33% of the total worldwide enzyme market of about US$1.3 billion was used by the food industry, with an estimated US$52 million (-4%) used in baking applications. An estimated 2.4% annual growth in food enzyme use is expected in the near future33. With the rapid developments in automation and mechanization, enzymes have today a very central role in baking technology, and the applications of enzymes in baking are expected to grow faster than in any other food area. Several commercial enzyme preparations and enzyme-containing bread improvers are available, and amylases, proteases, lipoxygenase, hemicellulases and even lactase are used in the baking industry34,35. Several enzymes are now specifically engineered and produced on a commercial scale by GMOS~‘,~~.~~, which has markedly increased production yields, reduced costs, and made possible the production of specialty enzymes by host organisms that are well suited for large-scale use. Enzyme-catalyzed reactions already play an important role in growing wheat and in stored grains. As soon as water is added to flour or grain is moistened, enzymes are activated as in the germination process. However, most of these enzyme-catalyzed reactions are deleterious from the point of view of baking quality. In yeasted doughs, several yeast enzymes are of importance, and in sourdough fermentations enzymes from lactic acid bacteria are important. Amylolytic enzymes The supplementation of bread doughs with o-amylase, sometimes together with protease and other enzymes, affects the functional properties of dough3X. Wheat and rye flours contain only -OS-LO% of fermentable sugars; thus, the enzymic breakdown of starch to maltose is invaluable if the yeast is to grow, unless sufficient sugar is added to the baking formula. If the or-amylase activity of flour is insufficient, the maltose content remains too low and the dough rises poorly. In such a case, -0.3% of malt flour or fungal o-amylase, usually from Aspergillus or Rhizopus species, may be addedeither at the mill or at the bakery. Fungal amylasesare heat labile and do not survive the baking process. Together with glucoamylase, it is possible to control the reducing-sugar content of the final bread so that, for example, the golden brown color of sliced white bread is obtained on toasting. o-Amylase also influences dough viscosity and the crumb structure of the final bread. Treatment with amylaselowers the viscosity of dough, improves handling, and results in a softer and larger loaf of bread. During the baking process,starch gelatinizes, and protein denatures and forms a rigid structure, releasing water to the gelatinizing starch. If ol-amylase activity is too high (i.e. the falling number

is too low), excessive starch breakdown during the early stages of oven baking results in a moist, sticky and rubbery bread crumb, and a small volume. Fungal cxamylaseis inactivated at -75°C in 10m:inand can, therefore, be used with less risk to improve bread quality. The controlled addition of an c;u-amylase of intermediate thermostability can be used to reduce starch retrogradation and, thus, to approximately double the bread’s shelf life39.The useof a-amylase to retard staling can be enhancedby the addition of pullulanase, a debranching enzyme of Bacillus acidopullulyticus. Native, undamagedstarch granules are quite resistant to a-amylase. Therefore, during dough mixing and fermentation, amylolytic enzymes are abbeto break down only the starch of granulesthat have been damagedduring milling, or starch that has becomegelatinized during oven baking. The starch of hard wheat experiencesmore mechanical damage than that of soft wheat, and the extent of damage during milling can be controlled to some degree by proper adjustment of the brake rolls. During the baking of doughs containing little or no addedsugar, yeast fermentation is very dependenton pamylase providing the excess reducing sugar necessary for browning and flavor development through the Maillard reaction. Increasingly, modern, young and busy consumerslike to buy ready-mixed frozen dough instead of preparing dough themselves.This trend has openedup yet another application for enzymes in baking. In such a case,flour with a high falling number is usually preferred and amylolytic enzymes are added for high bread quality. If the a-amylase activity of bread wheat is too high, it is not possibleto avoid problems by mixing such wheat with higher-quality wheats. It has been shown that falling numbers, which are widely used for quick characterization of wheat quality, are not additive, and that even aslittle as 5% of wheat with high cr-amylaseactivity can spoil the whole batch. Food-grade wheat or rye with a low falling number can, however., be used for the production of flat bread by extrusion cocking40. The traditional method of reducing a.-amylaseactivity, especially with rye breads, has been sourdough baking with lactic acid bacteria. As soon as the pH of the dough drops below -4.5, a-amylase is inactivated. Theoretically, a-amylase could be irreversibly inactivated by lowering the pH to 3.0; the flour or dough could then be neutralized. This, however, is impractical at the industrial level. Proteases The proteolytic activity of sound, ungerminated grain is normally low. Most cereal proteases are of the papainasetype, that is, they become activated (as does a-amylase) in the presence of compounds that reduce disulfide bonds. The native proteasesin flour have little importance on bread quality. However, :it is known that proteolytic enzymes that are capableof hydrolyzing gluten proteins can have a drastic effect on baking quality. For example, the saliva of an insect (Ezqgaster sp.) contains a potent proteasethat hydrolyzes key grain proteins,

making the grain unfit for baking. Nevertheless, the controlled addition of proteolytic enzymes can also be used to advantage”. Dough viscosity and optimal mixing time decrease, resulting in improved viscoelastic properties. With the use of fungal proteases, it has been possible to decrease mixing time by one third without adversely affecting the viscoelastic properties of the dough. Both the machinability of the dough and the bread crumb were improved. The influence of native proteolytic enzymes in rye bread baking is even less marked, because in this case proteins play a lesser role owing to the high pentosan content of rye flour. Added proteolytic enzymes are increasingly used in the baking of chemically leavened biscuits and soda crackers to control the viscoelastic properties of the dough. Proteases have largely replaced bisulfite, which was previously used to control consistency by reducing gluten protein disulfide bonds, whereas proteolytic activity breaks down peptide bonds. In both cases, however, gluten structure is weakened in a similar fashion. Hemicellulases Rye flour contains pentosans, which prevent gluten formation during dough mixing. Partial enzymic hydrolysis of pentosans results in a marked improvement in dough handling and increases loaf volume. It has been realized recently that non-starch polysaccharides such as arabinoxylans and P-glucans may play an important role in the baking process and that marked quality improvements can also be obtained in wheat bread baking by the use of hemicellulases and 13-glucanases41%42. White flour contains -2.5--3%, wholemeal wheat flour -S%, and rye as much as 8% of hemicellulose, which has an important role in the water binding of dough. Pentosans can bind up to -6.5 times their weight of water, and may contribute approximately one third of the water-binding capacity of dough. Following the observation that xylanase present as an impurity in commercial amylase preparations improved both the loaf volume and crumb quality, the interest in enzymes that act on gluten and starch gradually shifted to those that hydrolyze hemicelluloses or pentosans. This, alone, can be considered a revolution in baking enzyme technology. Adding pentosanases to dough has been claimed to have a marked bread anti-staling effect, and in rye bread baking to decrease markedly dough development time and energy consumption during mixing43. In biscuit baking, the viscoelastic properties of dough can be manipulated by the controlled application of hemicellulases and proteases. With the ever-increasing consumer demands for high-quality, low-fat products, enzyme technology makes it possible to control the dough consistency and machineability of tough, low-fat doughs44. Lipases Lipases are present in all cereal grains. Lipases hydrolyze water-insoluble esters at the lipid-water interface. However, under suitable conditions many lipases also catalyze ester synthesis and transesterification reactions. The lipase activity of white wheat flour is usually low enough Trends

in Food Science

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1997 [Vol. 81

not to cause problems, but the lipase activity of wholegrain flour can be sufficient to result in hydrolytic rancidity both of the native lipids and of baking fat. Lipoxygenase and sulfhydryl oxidase Oxidizing enzymes can play an important role in the modification of gluten proteins. Lipoxygenase from soy was discovered in the 1920s as an agent that is capable of bleaching wheat flour. It was originally called carotene oxidase until it was realized that it oxidized polyunsaturated fatty acids, not carotenoids. When it was observed relatively recently that lipoxyg.enase also affects dough properties, interest in lipoxyg,enase as a potential bread improver was resumed34x35. Lipoxygenases oxidize polyunsaturated fatty acids during dough mixing. The hydroperoxides formed can oxidize the sulfhydryl groups of gluten proteins and thus be advantageous in the formation of the gluten network of dough. The presence of added lipoxygenase also decreases dough mixing time. Finally, lipoxygenase has been reported to give bread a nut-like flavor in some systems. Sulfhydryl oxidase from Aspergihs niger added to dough either alone or together with glucose oxidase has been claimed to strengthen weak doughs4’. Polyphenol oxidase Polyphenol oxidases are responsible for the so-called enzymic browning. In baking, this occurs only with pumpernickel-type rye breads with very long baking times at relatively low temperatures. Future outlook The benefits to the baking industry of the rapid developments in biotechnology during the past few years are expected to be seen in the near future. The first genetically engineered baker’s yeast was approved for production and test use in 1990, and the first transgenic, fertile wheat plants were reported in 1992. These developments have opened up limitless possibilities not only for basic research in genetics, but also in the introduction of the modern achievements of molecular biology to the benefit of consumers, subject to consumer acceptance and economic feasibility. Interest in sourdough baking is increasing with the increased interest in ‘natura.1’ and ‘green’ technologies, and because of the recent health claims, particularly those relating to rye. The ability to produce propionates in situ as preservatives during wellcontrolled mixed-culture pre-fermentation in orlder to extend the mold-free shelf life of bread by several days is certainly in the interest of consumers, and could have marked economic importance. Perhaps the biggest impact of biotechnology will, however, result from the rapidly increasing use of enzymes by the baking industry to improve bread quality. This is largely due to the increasing availability of new and cheaper enzymes as a result of the rapid developments in recombinant DNA technology and bioprocess engineering, and it has been estimated that the baking enzyme market will increase at a faster rate than the food enzyme market in general. 34.3

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