Bread is one of the most consumed food products known to humans, and for some people, it is the principal source of nutrition. Bread is an inexpensive source of energy: it contains carbohydrates, lipids, and proteins, and it is important as a source of essential vitamins of the B complex and of vitamin E, minerals and trace elements. There are numerous variations of the breadmaking process; tradition, cost, the kind of energy available, the kind of flour, the kind of bread required, and the time between baking and consumption of bread.
Principal ingredients Flour The breadmaking quality of flour: f (the variety of the grain, all agricultural and climatic conditions, including the harvest, and the milling process.)
The most important characteristics of flour:
1. the protein content,
2. the quantity and quality of gluten,
3. the water absorption capacity,
4. the diastatic activity.
The kneading of the flour and water gives the dough a cohesive, viscoelastic mass that retains the gas formed during fermentation.
YEAST
Commercial yeast production worldwide exceeds 1.8 million t per year. The yeasts are used mostly by the baking industry, but also by the brewing and distilling industries. Yeast is also a commercial source of natural flavourings, flavour potentiators and the dietary supplements. The first yeasts used for baking were obtained from the mashes produced in the manufacture of beer.
The first compressed yeasts used for baking and brewing were made in England in about 1792, and by 1800 they were available throughout northern Europe. The large-scale commercial production of bread in the US was facilitated by the introduction of an improved strain of compressed yeast in 1868, by Charles Fleischmann. The vigorous research and development effort that ensued yielded yeast strains suited to each type of fermentation and leavening. For example, some of these strains were able to tolerate high sugar or salt concentrations The vigorous research and development effort that ensued yielded yeast strains suited to each type of fermentation and leavening. For example, some of these strains were able to tolerate high sugar or salt concentrations, or the high temperatures used in fermentation and the proving of dough. Anton van Leeuwenhoek (1632–1723) of Holland was possibly the first human to set eyes on a yeast, when he observed a droplet of fermenting beer with the aid of one of the first microscopes, capable of 250–270-fold magnification. Louis Pasteur (1822–1895) carried out extensive systematic studies which revealed the nature of yeasts and their extraordinary biochemical capabilities. Currently more than 500 species of yeasts, belonging to around 50 genera, are known.
Bakers’ yeast and the yeasts used in brewing, wine-making and distilling are strains of Saccharomyces cerevisiae, belonging to the family Saccharomycetaceae in Ascomycotina. The genus Saccharomyces (translation ‘sugar fungus’) derives its name from its common occurrence in sugary substrates such as nectar and fruits. Strains of S. cerevisiae have been isolated from diverse sources, including breweries, wine, berries, cheese, pear juice and must, honey, eucalyptus leaves, kefir. Reproduction Asexual reproduction usually occurs by budding. The vegetative cells are diploid or polyploid, and this phase predominates in the life cycle of the yeast. Sexual reproduction involves the production of asci, within which ascospores develop directly following meiosis of the diploid nucleus. The sporulation of S. cerevisiae is encouraged by media containing acetate, such as acetate agar. Sporulation also occurs on potato–dextrose agar.
Hybrid Strains
Cells of opposite mating types can be fused to produce hybrid yeast strains.( or new techniques like protoplast fusion and the construction of recombinant DNA),
This technique can be used to combine industrially desirable traits such as high growth rates, high yields, resistance to drying and CO2 production. rapidly fermenting strains: produce high volumes of CO2 for automated bakeries strains with intermediate activity, for traditional bakeries; and strains which ferment more slowly, for in-store bakeries. Bakers’ yeast is Saccharomyces cerevisiae.
• Based on moisture content: compressed (most common), granular, dried in the form of a pellet, instantaneous, encapsulated, frozen, or in the form of a ‘cream.’
• Based on maltose adaptation: standard and rapid
• Rapid yeast is more active than standard yeast, and is adapted for use in accelerated bread making systems.
• Tolerance to high osmotic pressure : important in frozen dough.
Commercial Production of Bakers’ Yeast Sources
• Industries requiring yeast cultures can either obtain them from culture collection centres or isolate and develop their own cultures.
• Saccharomyces can be isolated from natural sources, and maintained in pure culture by conventional microbiological techniques.
• The source material (fermenting sugary materials, fruit juices or soil) is usually serially diluted and plated onto potato–dextrose agar (PDA) or yeast extract–peptone–dextrose agar (YEPDA).
• The growth of yeasts in preference to bacteria is achieved by the pH of the medium being below neutral (usually 4–6) and the incorporation of antibacterial antibiotics.
• Enrichment culture is a technique by which strains with characteristics required by industry (e.g. tolerance to high temperatures) can be isolated from natural habitats.
• The required strains are selected either by gradually increasing exposure to the factors to which tolerance is required, or by cultivation with very high levels
• Maintenance of Cultures
• If yeasts are used regularly( for batch process): the simplest method is to use agar slopes or broth.
• Normally, slope and broth cultures are subcultured once every 2 months.
• After allowing for adequate growth at ambient temperature, they are kept at 4–8°C until use.
• The drawback of this simple technique is the risk of contamination and the development of genetic variants .
• These undesirable effects can be prevented by the incorporation of a ‘selection pressure’: the incorporation of a high sugar or salt concentration in the maintenance medium, to retain yeasts which will be tolerant to high concentrations in the fermentation.
Preservation of Cultures
• Yeasts that are sensitive to dehydration in slope or stab cultures may be maintained by overlaying with mineral oil.
• All microorganisms except non-sporulating bacteria sporulate in soil, and in this form remain viable and functional for up to 2 years.
• The inclusion of glycerol (5–20%) in the suspension medium and storage at −20°C are recommended.
• Well-equipped culture collection centres have facilities for the extended storage of cultures in liquid N2 at −196°C or by lyophilization.
Growth Requirements
• S. cerevisiae is a heterotroph, i.e. requires organic compounds for growth. a mesophile, growing best in the temperature range 25–40°C. bakers’ yeast has basic nutritional requirements for carbon and nitrogen sources, minerals and vitamins.
• Carbon
• A limited range of sugars is utilized as a carbon source by S. cerevisiae.
• Glucose and fructose are readily utilized, and of the disaccharides, sucrose and maltose are preferred.
• Other malto oligosaccharides can also be utilized, but less readily.
• S. cerevisiae cannot utilize pentoses, other hexoses, the disaccharides lactose or cellobiose or the polysaccharides.
• In industry, the preferred carbon sources are cane or sugar-beet molasses, which have a fermentable sugar concentration of 50–55% and around 80% total soluble solids.
Nitrogen
• S. cerevisiae can utilize
• inorganic nitrogenous compounds ( ammonium sulphate, ammonium chloride , ammonia. Urea can be used to provide N2 in the commercial production of yeasts. )
Minerals
• The major minerals required: phosphorus (an important component of nucleic acids) potassium calcium sodium magnesium sulphur. Iron, zinc, copper, manganese and cobalt are required as trace elements. These requirements are largely met by the molasses, with the exceptions of phosphorus and magnesium.
• Phosphorus is supplied in the commercial production of yeasts as phosphoric acid or as phosphate of sodium, potassium or ammonium.
• Magnesium is supplied as magnesium sulphate in the growth medium.
Vitamins
• S. cerevisiae requires : biotin, pantothenic acid, inositol and thiamin These (except for thiamin)are available from molasses.. Sugar-beet molasses, however, is deficient in biotin A mixture of -aspartate and oleic acid was found to completely eliminate the requirement for biotin.
• Molasses must be supplemented with thiamin to enable maximum growth of the yeast.
Oxygen
• S. cerevisiae possesses a remarkable ability to adapt to thrive in varying levels of available O2.
• In very low levels of O2: shuts off the respiratory enzymes. The yeast then leads a fermentative life, in which sugar is partially and non-oxidatively utilized for energy and the ‘waste’ product is ethanol.
• adequate O2 is available; sugar is converted by the respiratory enzymes to CO2 and H2O, as well as to intermediates needed for the cell biomass.
• in microaerophilic conditions (often erroneously termed ‘anaerobic’ conditions), the yeast grows significantly more slowly than in aerobic conditions.
• The maximum theoretical yields of yeast solids:
• microaerophilic (anaerobic) :7.5 kg/100 kg of sugar utilized. aerobic conditions : 54.0 kg/100 kg of sugar utilized In the case of S. cerevisiae: the conc. of the sugar influences the availability of O2. If the medium contains > 5% glucose: glucose utilization via the TCA (tricarboxylic acid) cycle is almost completely blocked.
Even if the culture is aerated, glucose can only be fermented, this phenomenon being known as the ‘glucose effect’ or the ‘Crabtree effect’.
• For aerobic growth, therefore, the sugar has to be supplied incrementally so that the rate of growth of the yeast (μ) does not exceed 0.2 and the respiratory quotient (RQ) is maintained at the value of 1.
• In aerobic conditions, as much as a third of the available sugar is metabolized via the hexose monophosphate pathway, generating NADPH, which is mainly utilized in synthetic reactions.
• aerobic growth conditions: the yeast’s metabolism is elegantly balanced, generating chemical energy for cellular metabolism and precursor molecules for cell growth and proliferation.
• The TCA cycle is important: involved in the production of both the biomass precursor molecules and the chemical energy.
• S. cerevisiae also possesses the enzymes involved in the ‘glyoxylate shunt’. This replenishes the TCA intermediates taken up for biomass formation.
pH
• S. cerevisiae grows optimally at pH 4.5–5.0, although it can tolerate a pH range of 3.6–6.0.
• At higher pH values, the yeast’s metabolism shifts, producing glycerol instead of ethanol in microaerophilic conditions.
• In the production of bakers’ yeast an initial pH at the lower end of the range inhibits the growth of bacterial contaminants.
• As the process progresses towards harvesting, the pH is raised slightly so that any colouring matter taken up by the yeast from the molasses is desorbed.
Temperature
• S. cerevisiae has one of the shortest generation times amongst yeasts, 2.0–2.2 h at 30°C.
• Bakers’ yeast production is optimal: at 28–30°C.
Preparation of Medium
• The molasses (containing around 50% fermentable sugars and 80% soluble solids) is usually diluted with an equal weight of water, and the pH is adjusted to 4.5–5.0 with sulphuric acid.
• The diluted molasses is clarified using a desludger centrifuge.
• Clarification by filtration is recommended for beet molasses, but is not necessary for cane molasses.
• The clarified molasses is then sterilized by the high temperature short time (HTST) process.
• Other sterilization methods, which involve prolonged heating at low temperatures, cause caramelization and hence a decrease in the fermentable sugar content.
• Medium supplements are added: a nitrogen source (e.g. urea, 2.5 g l−1); potassium orthophosphate (KH2PO4), 0.96 g l−1; hydrated magnesium sulphate (epsomite,MgSO4.7H2O), defoamer (silicone, fatty acid derivatives or edible oil), The molasses is then transferred to the fermenter, filling it to about two-thirds of its total volume, and pitching yeast is added.
• More clarified molasses is added incrementally in the fed-batch cultivation.
• A substrate of starch (from corn, sorghum or tubers), hydrolysed by acids or enzymes, or of sugar cane juice, does not require elaborate clarification.
• Supplementation with a nitrogen source, minerals and vitamins is, however, necessary.
Cultivation
• In industry, bakers’ yeast is produced in fermentation tanks with a capacity of 200 m3 or more.
• Tanks, and all connecting tubes, should preferably be made of stainless steel.
• The design and operation of fermenters for the production of bakers’ yeast must be carefully considered given the interrelationship between aeration, the specific growth rate of yeast and the substrate concentration.
• The facility for bubbling compressed air (as a source of O2) into the medium, to ensure effective aeration, is therefore particularly important.
• In practice, O2 transfer in a fermentation system can be manipulated by adjusting the bubble size and by the dispersion of air in the cultivation medium, by using mechanical agitation close to the point of entry of air into the medium.
• The facility for bubbling compressed air (as a source of O2) into the medium, to ensure effective aeration, is therefore particularly important.
• In practice, O2 transfer in a fermentation system can be manipulated by adjusting the bubble size and by the dispersion of air in the cultivation medium, by using mechanical agitation close to the point of entry of air into the medium.
• The level of dissolved O2 during fermentation can be determined by using O2 electrodes
• a typical cultivation tank used for the production of bakers’ yeast, an airlift fermenter.
• It consists of a cylindrical vessel, provided with a sparger (aerater) at the bottom for producing air bubbles in the medium.
• Aerobic growth results in the generation of almost 14 650 J of heat per gram of yeast solids, and because cultivation has to be carried out at 28–30°C, cooling is necessary.
• This is achieved by cooling coils.
• Directional flow of the cultivation medium is achieved by pumping air in at the bottom of a ‘draft tube’. If the depth of the broth exceeds 3 m, the use of compressed air, to overcome the hydrostatic pressure of the liquid, is recommended.
• The manufacture of bakers’ yeast begins with a number of stages which build up production, and involve the inoculation of the medium with pitching yeast. This development process is usually divided into eight stages, in which the yeast solids are gradually built up from the slope or flask culture of yeast.
• Over the eight stages, 0.2 kg of yeast solids give a final yield of about 100 000 kg of yeast.
• In the first two stages, sterilized medium and pure yeast cultures are employed in pressurized tanks, but the subsequent stages are operated in open tanks.
• The entire process is known to involve 24 generations of the yeast
Maturation
At the end of the final stage of yeast cultivation, the feed rate is greatly reduced. This allows the yeast cells to mature and results in a low proportion of budding cells, which confers higher stability on compressed yeast in storage.
Finishing Stages
• Yeast Cream
• The culture broth can be centrifuged in a continuous centrifuge (with a vertical nozzle) at 4000–5000 g, leading to almost complete recovery of the yeast cells.
• In the first run, around two-thirds of the fluid can be removed and in subsequent runs further concentration of the cells is achieved, producing a slurry called ‘yeast cream’, which contains about 20% yeast solids.
• Yeast cream can be stored at 4°C for a number of days, with good retention of viability.
• Compressed Yeast
• This is prepared from yeast cream by filtration or by pressing in a filter press.
• Rotary continuous vacuum filters can also be used.
• The pressed cake thus obtained is mixed with 0.1–0.2% of emulsifiers such as monoglycerides, diglycerides, sorbitan esters and lecithin, and then extruded through nozzles.
• The extruded material, in the form of thick strands, is cut into suitable lengths and packaged (usually in packs of about 500 g) in wax paper or polythene sheet.
• The compressed yeast must be rapidly cooled, and stored at 5–8°C.
• Active Dry Yeast (moisture content of 4.0–8.5%. )
• Active dry yeast is useful in situations (e.g. homes) where storage at low temperatures is not possible.
• It is prepared by spreading out the pressed yeast cake to produce thin strands or small particles, which are then dried.
• Generally, a tunnel drier is used, taking 2–4 h with the air inlet temperature maintained at 28–42°C.
• continuous drying or fluidized-bed drying (airlift drying), is also available.
• Emulsifiers such as sucrose esters or sorbitan esters (0.5–2.0%) are mixed with the dried yeast to facilitate rehydration.
• Antioxidants, such as butyl hydroxyanisole(BHA) at 0.1%, are also added, to prevent undesirable oxidative changes.
• 1 g compressed yeast = 0.4-0.45 g dry yeast
• Rehydration:
• X g yeast + 4X g water at 37.8-44.5 C
• T < 32 C …..or T>46 C
– The glutothione leaches out of yeast and causes to dough become soft and sticky
– Cell contents ( a acids, vitamins, nucleic acids, coenzymes,glutothione and inorganic salts) leaches out and fermentative power of yeast decreases. To produce dry yeast products heat resistant strains are used and heat resistant strains generally carries low gassing power.
Lysine-enriched Yeast
• Strains of bakers’ yeast which can convert precursor molecules such as 5-formyl-2-oxovalerate or 2-oxo-adipate to lysine, with high efficiency, have been reported.
• The use of such strains for the fortification of lysine-deficient cereals has potential.
Vitamin-enriched Yeast
• The addition of thiazole and pyrimidine to the cultivation medium has been shown to cause bakers’ yeast to synthesize high levels of thiamin (around 600 μg g−1).
• The irradiation of bakers’ yeast with ultraviolet light has been shown to convert ergosterol to calciferol (vitamin D2), with a vitamin potency reaching as much as 180 000 USP units per gram of yeast.
• Such strains will be useful for the fortification of food, feed and pharmaceuticals.
• High µ(sp g rate)..high potein..high activity..low carbohydrate..low stability Low µ.. low p..low a…high c…high stability
• Should be free of Salmonella, Listeria , E. coli and low coliform
• Normal level of bacteria and wild yeast available –no problem because flour and other ingredients contain these ( bakers yeast outnumbers them)
• Low PH gives higher purity but darker color
• Storage time, temp makes yeast darker, gummier and lower performance
• Tendency of yeast to grow via respiration when high level of oxygen are present is known as
Pasteur effect
• commercial bakers yeast fermentations use high aeration and incrimental feeding to maintain high oxygen and low sugar levels throughout the process.
• Maltose Adaptation
• for good production of gas: yeast requires a plentiful and continuous supply of glucose as an energy source.
• The action of α- and β-amylases of flour in damaged starch gives maltose ; transported into the cell; broken down into two molecules of glucose.
• metabolic system active ;when yeast detects maltose ;takes some time (lag phase).
• maltose adaptation is different for rapid and standard yeast strains.
• The choice of one over another depends on the breadmaking process to be used.
Temperature
• at 4 °C, no activity, no fermentation
• at 10–15 °C, yeast activity, fermentation; slow
• at 20–40 °C, fermentation ; very active
• at 45 °C yeast activity slows down
• at 55 °C, the yeast dies.
• the best storage temperature: 4 °C (0–10 °C).
• an increase of 1 °C gives an 8–12% increase in fermentation speed.
• at 38 °C, more gas production, also lactic and butyric fermentation (which is undesirable).
• pH
• very tolerant to fluctuations in pH (2–8),
• but the optimum pH :4 and 6.
• Usually, the pH of dough is 5
Presence of Dough Inhibitors
• The copper and chlorine in water, fermentation inhibitors.
• Organic acids, acetic and propionic acid, fermentation inhibitors.
Sources of Sugar used by Yeast
• Yeast uses
• (1) trehalose (Trehalose is a non-reducing sugar formed from two glucose units joined by a 1-1 alpha bond very resistant to acid hydrolysis) endogenous
• (2) free sugar from flour
• (3) sugars obtained by the action of enzymes in oligosaccharides and polysaccharides in the flour, or added as additives
• Kneading ; air is incorporated into the dough, and air bubbles are formed.
• These bubbles contain mainly nitrogen;low solubility in water, and oxygen is quickly used up
• Yeast cannot create new bubbles, as the pressure (P) in a bubble is related to the radius (r) of the bubble and the interfacial tension (γ) by the law of Laplace: P = 2γr.
• The more bubbles are formed, the finer the grain is.
• carbon dioxide is produced in the aqueous phase, the pH decreases, and this phase becomes saturated with carbon dioxide.
• the dough is leavened, because the aqueous phase is saturated by CO2 and the newly formed CO2 is retained in preexisting bubbles.
• the quality of gluten is also very important to retain the gas
• carbon dioxide is retained in the dough in two phases: contained within the gas cells and dissolved in the aqueous phase.
• At the pH of dough, most of the carbon dioxide is present as CO2 and a small amount of this as CO32− HCO3−, or H2CO3.
• Only 45% of the total gas produced is present at the end of fermentation. (lost in initial fermentation, punching, rounding, molding, and final fermentation.
Salt
• to provide flavor,
• influence the rheological properties of dough.
• Higher concentrations of salt inhibit enzymatic reactions and also inhibit the fermentation activity of yeast.
• In general, the proportion of salt used is 1–2% (based on flour weight).
Water
• Mineral constituents of the dough water (mainly carbonates and sulfates)
• give a firmer, more resistant gluten;
• the doughs do not collapse during fermentation,
• the gas retention is improved, and with a normal volume,
• the grain is finer and more elastic
Optional Ingredients
• Fat
• fat increases the shelf-life,
• produces a finer grain,
• yields a greater volume of baked foods (10%).
• The crust is more elastic and softer.
• The shortening effect ( film between the starch and protein)
• mono and diglycerides or lecithin, promote the formation of this film
• The shortening effect is greater for fat with a lower melting point than for harder fats. ( hydrogenated vegetable fats)
• the use of fat requires less water in the formulation.
Sugar
• promotes fermentation,
• browning of the crust,
• sweeter taste.
• dough more stable, more elastic, and shorter, and the baked goods more tender.
Milk and Dairy ProductsMilk, skim-milk powder, whey containing lactosepromotes browning, a softer crust, and a longer shelf lifeOxidantsthe oxidation of -SH groups of protein to -SS-groups,improved gas retention. disulfide bonds: between protein chains lead to a firmer gluten The time of dough maturation is shorter, the oven spring is greater, the volume is large, the quality of the grain is better.The oxidant commonly used is ascorbic acid
