DEFINITION OF FOOD FERMENATION:\r\nMicrobial(1) activities(2), usually anaerobic(3), on suitable substrate under controlled(4) or uncontrolled (5) conditions resulting(6) in the production of desirable (7) foods or beverages which are characteristically more stable(8), palatable(9) and nutritious(10) than the raw substrate.\r\nWhat do fermentation scientist do:\r\n1-Fermentation industry beer, wine, dairy fermentation,\u00a0 pickling, oriental fermented foods\r\n2- Industrial fermentations organic acids, antibiotics, medicine\r\n3-New product development\u00a0genetic engineering\r\n4-Teaching, Research, Extension University, Government,\u00a0 Industry\r\nHistory of Fermentation\r\n\r\n1-Pre-scientific ERA- before\u00a017th century\r\na-traditional microbiological process( beer, wine, bread, vinegar, cheese, pickling\r\nb-traditional food preservation\u00a0\u00a0 sun drying, salting, freezing\r\nc-food borne diseases and spoilage\u00a0\u00a0\u00a0 accepted as fate or act of God\r\n2-ScientificEra\r\na- Elucidation of biological processes through scientific developments in microbiology ,\r\n-improvement of traditional processes\r\n-development of new industries based on m.o. not previously exploited\r\n-antibiotics, enzyme biotechnology\r\nb-new preservation procedures\u00a0\u00a0 freeze drying, microwave, radiation\r\nc-control of food spoilage and food borne\u00a0 diseases by understanding mechanism better\r\n3-Future\r\na- continue to improve traditional methods\r\nb-biotechnology\r\nc-new technologies\r\nSome cornerstones of fermentation\r\nDiscovery of microorganism by Antonie von Leewenhoek\r\n1810 Canning by Appert\r\n1854 Pasteur showed that microbes are responsible for fermentation.\r\nFermentation is the conversion of sugar to alcohol to make beer and wine.\r\nMicrobial growth is also responsible for spoilage of food. Bacteria that\u00a0 use alcohol and produce acetic acid spoil wine by turning it to vinegar (acetic acid).\r\nGeneral classification of microorganisms\r\nMicrobial activity\r\nMicroorganisms that carry out their metabolism using oxygen are referred to as aerobic microorganisms.\r\nSome microorganisms can\u00a0 substitute nitrate, others sulfate or ferric ion, for oxygen and thus grow in the absence of oxygen. These microorganisms are referred to as anaerobic.\r\n\r\nObligate (or strict) aerobes: the presence of oxygen is required\r\nObligate (or strict) anaerobes: the absence of oxygen is required, oxygen is toxic to the cells\r\nFacultative anaerobes: can survive with or without oxygen\r\nMicroaerophiles: require low concentrations of oxygen and don't do well either at atmospheric oxygen concentrations or without oxygen\r\nBiotechnology: Biotechnology is the scientific activity concerning the integrated application of biochemistry, microbiology and process technology on biological systems on the behalf of industrial\u00a0 processes and environmental management..\r\nBiochemical engineering The application of engineering principles to biologically based processes ( in fermentation processes: manipulating living cells so as to promote their growth and production in a desired way );\r\nMaximize productivity Minimize costs\r\nFermentation , the act or process of fermenting; a slow decomposition process of organic substances\u00a0 induced by microorganisms, or by complex nitrogenous organic substances (enzymes) of vegetable or animal origin, usually accompanied by evolution of heat and gas, e.g. alcoholic fermentation of sugar and starch, and lactic fermentation.\r\nNutrient requirements\r\nAll microorganisms need for their microbial activity the presence of several nutrients.\r\nWater: Major component of all fermentation process is consistent, clean water. PH, dissolved salts and microbial contamination are important factors. Long before plants are set up near by good water sources.\u00a0 Now water treatment is applied. There might be different\u00a0 characteristic water requirement for different type of food production.\r\nHard water for English Burton beer, high carbonate water for darker beers.\r\nIn SCP production large amount of water is used so recycling should be applied.\r\nCarbon source:\r\n-Carbohydrates Carbohydrates are capable of being used by all microorganisms,.\r\nGlucose is the most readily metabolized sugar.\r\nMost fungi can use disaccharide\u2019s.\r\nMost common carbohydrate is starch( from maise, cereals and potatoes)\r\nSugar cane, sugar beet.\r\nCorn steep liquor which is by product of starch production from corn.\r\n-Hydrocarbons : petroleum products\r\n-Lipids\r\nEnergy source:\r\neither from light(photosynthesis)or oxidation of medium components\r\nMost industrial organisms are chemoorganotrophs( common energy source organic materials.\r\nPurines and pyrimidines :\r\nIt is generally only in bacteria that cases of purine and\u00a0 pyrimidine metabolism have been reported.\r\nVitamins and growth factors\r\nThere is considerable species variation in the requirements of vitamins and related factors by other microorganisms.\r\nMany of the natural carbon and nitrogen sources contain all or\u00a0 some of the required vitamins. Generally, vitamins A, C, D, and K are not necessary for growth. In glutamic acid production biotin is required.\r\n\r\nNitrogen sources\r\nInorganic nitrogen sources: ammonia gas and ammonium nitrates( cause alkaline drift),\u00a0 ammonium salts ( cause acidic drift)\r\nOrganic nitrogen amino acids, proteins, urea\r\nChemical elements and inorganic ions\r\nMineral nutrients required by microorganisms are species dependent but consists generally of Fe, K, Mg, Mn.\r\nSometimes S, N, Ca, Co, Cu, P, Zn are required.\r\nAEROBIC CATABOLISM\r\nMicrobial cells consist of a wide variety of chemical substances which have to be synthesized or taken up from outside the cell.\r\nThese processes require a lot of energy.\r\nEach cell has to provide the necessary energy and different possibilities\u00a0 for its supply are developed: a number of organisms can use light energy; most microorganisms, of course, obtain it from the oxidation of chemical compounds.\r\nChemicals used as sources for energy are metabolized and energy is conserved either by substrate-level phosphorylation or by building up an electrochemical gradient across the cytoplasmic membrane.\r\nThe major carbohydrate-metabolizing pathways are:\r\nEmbden\u2013Meyerhof\u2013Parnas (EMP) pathway, also called glycolysis\r\nEntner\u2013Doudoroff (ED) pathway\r\npentose phosphate (PP) pathway.\r\nThe three pathways differ in many ways, but two generalizations can be made:\r\n1. All three pathways convert glucose to glyceraldehyde 3-phosphate (GAP) by different routes.\r\n2. The GAP is converted to pyruvate via reactions that are the same in all three pathways.\r\nTransport into the Cell\r\n1. Passive transport: facilitated diffusion (e.g. Zymomonas mobilis and erythrocytes). As facilitated diffusion is only found in a few organisms or cell types, it will not be discussed further here.\r\n2. Active transport: symport with a proton, protons or sodium ions. In this process of active transport the substrate can accumulate to a high concentration in the cytoplasm in a chemically unaltered form. Active transport requires energy and is linked to energy available from ion gradients or ATP hydrolysis.\r\n3. Group translocation: phosphotransferase system (PTS). Group translocation is the process whereby a substance is transported while simultaneously being chemically modified, generally by phosphorylation.\r\nEmbden\u2013Meyerhof\u2013Parnas\u00a0Pathway\r\nThe process of sugar breakdown is called glycolysis. The enzymatic reactions of a glycolytic pathway will form pyruvate\u00a0 coupled to ATP synthesis by substrate-level phosphorylation.\r\nThe overall reaction is:\r\nGlucose+2 ADP3\u2212+2 Pi2\u2212+2 NAD+\u21922 Pyruvate\u2212+2 ATP4\u2212+2 NADH+2 H++2 H2O\r\nEntner\u2013Doudoroff Pathway\r\nThere is a second important pathway for the breakdown of carbohydrates which is only found in prokaryotes.\r\nIt was first discovered in 1952 by Entner and Doudoroff in Pseudomonas saccharophila.\r\nThe ED pathway viewed as an alternative to the EMP pathway\r\nProkaryotes, which carry out the ED pathway, lack the key enzyme phosphofructokinase of the EMP pathway.\r\nBecause of the net production of only 1 mole ATP per mole glucose fermented, this pathway is usually found in aerobic bacteria\r\nThe overall reaction is:\r\nGlucose+NADP++NAD++ADP3\u2212+Pi2\u2212\u21922 Pyruvate\u2212+NADPH+NADH+3H++ATP4\u2212\r\nPentose Phosphate Pathway\r\nIn bacteria, about 80% of the glucose is degraded aerobically via the EMP and ED pathway,about 20% enters the PP pathway for generation of ATP, regeneration of NADPH and the synthesis of precursors for nucleotide and aromatic amino acid biosynthesis. So many variations of the PP pathway are\u00a0possible, depending on the need of the growing cell.\r\nThe overall reaction is:\r\nGlucose+ATP4\u2212+2 NADP++H2O\u2192Pentose\u00a05-P2\u2212+CO2+ADP3\u2212+2 NADPH+3 H+\r\nDistribution of Embden\u2013Meyerhof\u2013Parnas (EMP),\r\nEntner\u2013Doudoroff (ED), and pentose phosphate (PP)\u00a0pathway in bacteria and some eukarya\r\nOxidation of Pyruvate to Acetyl Coenzyme A\r\nIn all the major carbohydrate catabolic pathways, pyruvate is a common product,\r\noxidized during aerobic growth by the pyruvate\u2013dehydrogenase complex:\r\nPyruvate\u2212+NAD++CoASH\u2192AcetylCoA +CO2+NADH\r\nThe Tricarboxylic Citric Acid Cycle\r\nTo finish\u00a0the respiratory metabolism of glucose, acetyl coenzyme A enters the\u00a0tricarboxylic acid (TCA) cycle to produce carbon dioxide, water, reduced\u00a0coenzymes and ATP\r\nThe overall reaction is:\r\nAcetyl CoA+ADP3\u2212+Pi2\u2212+Q+2 H2O+NADP++2 NAD+\u00a0 \u2192\u00a02 CO2+ATP4\u2212+QH2+NADPH+2 NADH++H++3 H++CoASH\r\nOther Substrates as Sources for Metabolic Activity\r\nLiving organisms can use a variety of substrates for growth:\r\nalmost every naturally occurring organic compound can serve as a source for cell carbon or energy.\r\nThese can be low-molecular-mass compounds or polymers,\u00a0 such as glycogen, starch, cellulose, polysaccharides, lipids, fatty acids and proteins.\r\nPolymers cannot enter the cell; they must be cleaved outside into monomers and dimers which are small enough to be transported into the cell and then enter the metabolic pathways.\r\n\r\nCarbohydrates\u00a0\u00a0Glucose is not the only carbohydrate that\u00a0can be converted to pyruvate by glycolysis\r\nAnaerobic Breakdown of Carbohydrates\r\nThe terms glycolysis and fermentation have been applied to the anerobic decomposition of carbohydrate to the level of lactic acid.\r\nThe final product in some organisms is lactic acid; in others, the lactic acid is further metabolized anaerobically to butyric acid, butyl alcohol, acetone and propionic acid.\r\nThe two most common forms of fermentation are lactic and alcoholic Fermentation\r\nIn glycolysis, the same reactions occur whether oxygen is present or not.\r\nThe products are primarily pyruvic acid, NADH and ATP.\r\nThe essential difference between aerobic and anaerobic processes occurs with pyruvic acid and NADH.\r\nIn the case of fermentation reactions, pyruvic acid is converted to a variety of organic compounds.\r\nThese reactions involve the transfer of electrons and hydrogen from NADH to organic compounds.\r\nFermentation is a major source of energy for those organisms that can only survive in the absence of air (obligate anaerobes).\r\nOther fermentative organisms that can grow in the presence or absence of air (facultative anerobes) use fermentation as a source of energy only when oxygen is absent.\r\nIn fermentation, energy gain is very low and occurs as a result of substrate-level phosphorylation.\r\nThe synthesis of ATP in fermentation is restricted to the amount formed during glycolysis.\r\nDuring glycolysis, glucose is oxidized to pyruvic acid, which is the physiologically important first intermediate product in the aerobic or anaerobic dissimilation of glucose.\r\nPyruvate may also be reached via the metabolism of sugars other than glucose or the metabolism of fatty acids and amino acids.\r\nPyruvate is a sort of Grand Central Station, in that it is the point of arrival and departure of a wide variety of metabolic substrates and products.\r\nPyruvate is reduced to lactic acid.\r\nIt may also be decarboxylated and reduced to ethyl alcohol.\r\nConversely, it may serve as the\u00a0 source of amino acids, fatty acids and aldehydes.\r\nThe anaerobic system of biological oxidations that does not use oxygen as the final acceptor of electrons is called anaerobic respiration.\r\nIn anaerobic respiration, compounds such as carbonates, nitrates and sulphates are ultimately reduced.\r\nMany facultative anaerobic bacteria can reduce nitrate to nitrite under anaerobic conditions.\r\nThis type of reaction permits continued growth when free oxygen is absent, but the accumulation of nitrite which is produced by the reduction of nitrate is eventually toxic to the organisms.\r\nCertain species of Bacillus and Pseudomonas are able to reduce nitrite to gaseous nitrogen. This process occurs when aerobic organisms are grown under anaerobic conditions.\r\nThe organisms which reduce sulphate and carbonate are strictly anaerobic\r\nDesulfovibrio desulfuricans reduces sulphate to hydrogen sulphide as it oxidizes carbohydrate to acetic acid.\r\nMethanobacterium bryanti is able to couple the reduction of CO2 to methane with the oxidation of carbohydrate to acetic acid.\r\nSome organisms (strict aerobes) are enzymically equipped to use only free oxygen as the final hydrogen (e\u2212) acceptor, but others (facultative aerobes) are equipped to use as the final hydrogen (e\u2212) acceptor either free oxygen or some reducible inorganic substrate, commonly a nitrate.\r\nIn fermentations, usually only NAD or NADP functions as the hydrogen (e\u2212) carrier. Flavine adenine dinucleotide (FAD) and cytochrome systems are not required since the final hydrogen (e\u2212) acceptor is not oxygen but an organic substance \u2013 commonly pyruvic acid.\r\nFermentation may be caused by facultative organisms under anaerobic conditions, e.g. Saccharomyces cerevisiae, by strictly anaerobic organisms, e.g. Clostridium or by organisms that do not utilize free oxygen, e.g. species of Lactobacillus.\r\nDepending on the conditions of growth, the substrate and the organisms involved, the end products of fermentation vary greatly.\r\nLactic Fermentation\r\nThe products of glucose fermentation by all species of Streptococcus,\u00a0 many species of Lactobacillus and several other species of bacteria are mainly lactic acid with minor amounts of acetic acid, formic acid\r\nand ethanol.\r\nSeveral species of Streptococcus produce more than 90% of lactic acid based on the sugar used, and hence this type of fermentation is referred to as homolactic fermentation.\r\nThe homolactic fermentation which forms only lactate is the characteristic of many of the lactic acid bacteria, e.g. Lactobacillus casei, Streptococcus cremoris and pathogenic streptococci, a heterolactic fermentation converts only half of each glucose molecule to lactate.\r\nBoth these fermentations are responsible for the souring of milk and pickles.\r\nThe heterofermentative metabolic sequence found in Leuconostoc and some species of Lactobacillus ferments glucose according to the equation: Glucose\u2192Lactate+Ethanol+CO2\r\nThe\u00a0heterofermentative metabolic sequence in Leuconostoc and some species of\r\nLactobacillus.\r\nAlcoholic Fermentation\r\nThe major substrates yielding ethanol are the sugars which in yeasts are degraded to pyruvate by the EMP or glycolytic pathway.\r\nThere is a net yield of one ATP for each pyruvate formed from glucose.\r\nThe identical metabolic route for ethanol formation is found in the bacterial species Sarcina ventriculi, Erwinia amylovora and Zymomonas mobilis, which also possess the enzymes pyruvate decarboxylase and alcohol dehydrogenase.\r\nButyric Fermentation\r\nThe butyric fermentation is initiated by a conversion of sugars to pyruvate through the EMP pathway.. The ATP yield per mole of glucose fermented is 2.5 moll\r\nn\u00a0Fermentation of glucose to butyrate by Clostridium\u00a0butyricum, C. kluyveri and C. pasteurianum. FdF, Ferredoxin F\r\nMixed Acid (Formic) Fermentation\r\nThis is a characteristic of most Enterobacteriaceae.\r\nThese organisms dispose of their substrate in part by lactic fermentation but mostly through pyruvate breaking down into formate \u00a0and acetyl-CoA, which in turn generates an ATP.\r\nThe formic fermentation yields three ATP per mole of glucose fermented (compared with two in lactic fermentation.\r\nPropionic Fermentation\r\nThis pathway extracts additional energy from the substrate.\r\nPyruvate is carboxylated to yield oxaloacetate, which is reduced to yield succinate and is then decarboxylated to yield propionate.\r\nThe lactate is first oxidized to pyruvate; part is then reduced to propionate and the rest is oxidized to acetate and CO2:\r\nThis process of extracting energy from lactate yields only one ATP per nine carbon atoms fermented.\r\nHence, propionic acid bacteria grow slowly\r\nGeneral\u00a0pathways for the formation of fermentation products from glucose by various\u00a0organism\r\nOverview of\u00a0fermentation products formed from pyruvic acid by different bacteria.\r\nButanol and\u00a0acetone fermentation in Clostridium acetobutylicum.(strict anaerobe)\r\nMetabolic\u00a0 regulators added\r\nto media\u00a0Precursors\r\n:\u00a0 Some intermediary chemicals, when added to\u00a0fermentation medium directly incorporated to product Example :\u00a0phenylethylamine, phenylacetic acid in penicillin fermentation\r\nInducers\r\n:\u00a0\u00a0 Compounds that increase production of final\u00a0products during fermentation.( Common in enzyme fermentations)Example: To\u00a0induce alphaamylase production via fermentation add starch( an inducer and\u00a0substrate for the enzyme )\r\nInhibitors:\u00a0\u00a0 When certan inhibitors are added to fermentations more of a\u00a0specific product may be produced or a metabolic intermediate which is normally\u00a0metabolised is acumulated. Example : Ethanol fermentation may be\u00a0modified to produce glycerol by adding sodiumbisulphite (acetaldehyde is\u00a0electron acceptor\u00a0 in forming NAD+\u00a0from\u00a0 NADH2 , if sodiumbisulphite is\u00a0added it\u00a0 forms complex with acetaldhyde\u00a0and it is no longer electron acceptor, instead dihydroxyacetonephosphate act as\u00a0electron acceptor then when it is reduced\u00a0glycerol is formed)\r\nTypical\u00a0Inoculum Preparation\r\nCLASSIFICATION OF\u00a0FERMENTATION \u00a0PROCESSES\r\nBASED ON DEPENDENCE OF PRODUCT\u00a0FORMATION ON ENERGY METABOLISM\r\nType I:\u00a0The\u00a0product is derived directly from primary energy metabolism. ( growth,\u00a0carbohydrate metabolism and product formation runs parallel.\r\nExamples: Single\u00a0cell production, ethanol, gluconic\u00a0acid production\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 time\r\n________________\u00a0\u00a0\u00a0\u00a0 specific growth rate\r\n-------------------------\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 specific carbohydrate consumption rate\r\n.................................\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 specific product formation rate\r\nType II:\u00a0The product is also derived from the\u00a0 substrate used for the primary energy\u00a0metabolism, but production takes place in a secondery pathway which imary\u00a0meatabolism\r\nExamples : citric, itaconic and some amino\u00a0acids productions\r\nType III:\r\nPrimarymetabolism functions\u00a0first,accomponied by substrate consumption and growth. Afterwards the product\u00a0is formed by the rections of intermediary metabolites.\r\nExample:\u00a0antibiotics and some vitamin productions\r\nSUBMERGED FERMENTATIONS\u00a0\r\nIndustrial fermentations may be carried out either batchwise, as fed-batch operations, or as continuous cultures.\r\nBatch and fed-batch operations are quite common, continuous fermentations being relatively rare.\r\nFor example, continuous brewing is used commercially, but most beer breweries use batch processes.\r\nFermentation methodologies.\r\n(A) Batch\u00a0fermentation.\r\n(B)\u00a0Fed-batch culture.\r\n(C)\u00a0Continuous-flow well-mixed fermentation.\r\n(Feed: F,\u00a0Xf, Sf\u00a0 ,Harvest : F, X, S, P,\u00a0 Volume: V\u00a0time: t)\r\n(D) Continuous\u00a0plug flow fermentation, with and without recycling of inoculum.\r\nTypical\u00a0growth profile of microorganisms in a batch submerged culture\r\nA batch fermentation can be considered to be a closed system.\r\nAt time t=0 the sterilized nutrient solution in the fermentor is \u00a0inoculated with microorganisms and incubation is allowed to proceed.\r\nIn the course of the entire fermentation, nothing is added, \u00a0except oxygen (in case of aerobic microorganisms), an antifoam agent, \u00a0and acid or base to control the pH.\r\nThe composition of the culture medium, the biomass concentration, and the metabolite concentration generally change constantly as a result of the metabolism of the cells.\r\nLag phase\r\nPhysicochemical equilibration between\u00a0microorganism and the environment following inoculation with very little\u00a0growth.\r\nLog phase\r\nBy the end of the lag phase cells have adapted to the new conditions\u00a0of growth. Growth of the cell mass can now be described quantitatively as a doubling\u00a0of cell number per unit time for bacteria and yeast\u2019s, or a doub\u00adling\u00a0of biomass per unit time for filamentous organisms as fungi.\r\nBy plotting the number of cells or biomass against time on a semilogarithmic\u00a0graph, a\u00a0straight line results, hence the term log phase.\r\nAlthough the cells alter the medium through uptake of substrates and\u00a0excretion of metabolic products, the growth rate remains constant during the\u00a0log phase. \u00a0Growth rate is independent of substrate\r\nconcentration as long as excess\u00a0substrate is present.\r\nStationary phase\r\nAs soon\u00a0as the substrate is metabolized or toxic substances have been formed, growth\u00a0slows down or is completely stopped.\r\nThe biomass\u00a0increases only gradually or remains constant during this stationary\u00a0phase, although the composition of the\u00a0cells may change.\r\nDue to\u00a0lysis, new substrates are released which then may serve\u00a0as energy sources for the slow growth of survivors.\r\nThe\u00a0various metabolites formed in the stationary phase are often of great biotechnological\u00a0interest.\r\nDeath phase\r\nIn this phase the energy reserves of the cells are exhausted. A straight\u00a0line may be obtained when a semilogarithmic plot is made of\u00a0survivors versus time, indicating that the cells are dying at an exponential\u00a0rate.\r\nThe length of time between the stationary phase and the death phase is dependent\u00a0on the microorganism and the process used.\r\nThe fermentation is usually interrupted at the\u00a0end of the log phase or before the death phase begins\r\nCell growth modeling in a batch reactor\r\nThe simplest way to model cell growth will be to consider an unstructured,\u00a0unsegregated model for cell growth. For this kind of model,\r\nrx = dX\/dt = \u00b5X\r\n(1)\r\nwhere,\r\nrx = rate of cell generation (g\/l-hr)\r\nX = cell concentration (g\/l)\r\n\u00b5 = specific growth rate (hr-1)\r\nMonod's equation\r\n\u00b5 = \u00b5maxS\/(Ks +\r\nS)\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0(2)\r\nwhere,\r\n\u00b5= specific growth rate (hr-1)\r\n\u00b5max = maximum specific growth\r\n(hr-1)\r\nS = substrate concentration (g\/l)\r\nKS = saturation constant for\r\nsubstrate (g\/l) ( substrate concentration at \u00bd of \u00b5max)\r\nFigure depicts the dependence\u00a0of \u00b5 on S according to Monod\u2019s equation.\r\nOne should note that Monod's equation is empirical and does not\u00a0have any mechanistic basis. The equation is only valid for an exponentially\u00a0growing culture\u00a0 under condition of\u00a0balanced growth.\r\nThe equation\u00a0\u00a0 does\u00a0\u00a0 not\u00a0work\u00a0\u00a0 well\u00a0\u00a0 in\u00a0\u00a0 transient\u00a0conditions. Despite\u00a0\u00a0 its\u00a0\u00a0 simplicity\u00a0 and no\u00a0fundamental basis, it works\r\nDiauxic\u00a0growth :\r\nWhen two\u00a0fermentable substrates\u00a0are available\u00a0readily\u00a0fermentable\u00a0 one will be used first\u00a0and\u00a0 when it is exhausted second one will\u00a0be used.\r\n(when\u00a0 glucose and galactose are exist in lactic fermentation\u00a0glucose will be\u00a0used first and then galactose)\r\nFed-batch fermentation\r\nAn\u00a0enhancement of the closed batch process is the fed batch fermentation.\u00a0In the fed-batch method the critical elements of the nutrient solution\u00a0are added in small concentrations at the\u00a0beginning of the\r\nfermen\u00adtation \u00a0and these substances continue\u00a0to be added in small doses during the production\u00a0phase.\r\nd(VX)\/dt= V\u00b5X\r\nX* dV\/dt+ V*dX\/dt= \u00a0V\u00b5X\r\nSince F= dV\/dt\r\n\u00b5= F\/V+ 1\/X*(dX\/dt)\r\nD=F\/V ( dilution rate)\r\n\u00b5=\r\nD+ 1\/X*(dX\/dt)\r\nContinuous fermentation\r\nIn continuous fermentation, an open system is set up.\r\nSterile nutrient solution is added to the bioreactor continuously and an equivalent amount of converted nutrient solution with microorganisms is simultaneously taken out of the system.\r\nIn the chemostat in the steady state, cell growth i controlled by adjusting the concentration of one substrate.\r\nIn the turbidostat, cell growth is kept constant by using \u00a0turbidity to monitor the biomass concentration and the rate of feed of nutrient solution is appropriately adjusted.\r\nContinuous bioreactor dynamics\r\nFor a continuously fed bioreactor, the cells are continuously supplied substrate at growth limiting level,\u00a0 and hence they remain in the exponential phase.\r\nnSince the cells remain in the exponential phase, Monod's equation can be applied. A cell balance on the reactor can be written as\r\nFX - FXf +\r\nV(dX\/dt) = rx\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0(3)\r\nwhere,\r\nF = volumetric flow rate (l\/hr)\r\nX = cell concentration inside the reactor and in the outlet stream\r\n(g\/l)\r\nXf = cell concentration in the feed (g\/l)\r\nV = reactor volume (l)\r\nrx = rate of cell generation (g\/l-hr)\r\nFor a sterile feed (Xf = 0), and noting that the reaction rate can be written in terms of the specific growth rate (rx = \u00b5X), \u00a0equation (3) can be reduced to\r\ndX\/dt = (\u00b5-\r\nD)X\r\n(4)\r\nwhere,\r\nD = dilution rate = F\/V (hr-1)\r\nA balance on the substrate yields the following equation\r\nFS - FSf + V(dS\/dt) =\r\nrsV\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 (5)\r\nwhere,\r\nF = volumetric flow rate (l\/hr)\r\nS = cell concentration inside the bioreactor and in the outlet stream (g\/l)\r\nSf = substrate concentration in the feed (g\/l)\r\nV = reactor volume (l)\r\nrs = rate of substrate consumption (g\/l-hr)\r\nA yield parameter (Yx\/s) is defined that relates the amount of cell mass produced per amount of substrate consumed, and is mathematically represented as\r\nYx\/s = mass\r\nof cells produced\/mass of substrate consumed = rx\/-rs\r\nCombining\r\nequations (1), (5), and (6) yields\r\ndS\/dt = D(Sf - S) -\r\n\u00b5X\/Yx\/s\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0(7)\r\nThe CSTBR (continuous stirred tank bioreactor) is now completely described by equations\r\n(4) and (7)\r\nAt SS (with fixed Sf and D), \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00b5\r\n= D\u00a0\u00a0\u00a0 (8)\r\nS = DKS\/(\u00b5max \u2013\r\nD)\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0(9)\r\nX = Yx\/s (Sf \u2013 S)\r\n(10)\r\nD\u00a0 must be less than \u00b5max for a realistic value of S to be achieved.\r\nRelationship between dilution rate and specific growth rate for a steady\r\nstate CSTBR\r\nX = 0\u00a0\u00a0 (11) and S = Sf\u00a0\u00a0(12)\r\nEquation (11) and (12) define a situation called washout.\r\nThis situation is encountered whenever the value of dilution rate equals or exceeds \u00b5max.\r\nnA\u00a0rigorous discussion of washout would point to the fact that whenever \u00b5 (Sf),\u00a0i.e., \u00b5 evaluated at Sf , is less than \u00b5max, then the critical dilution rate\u00a0for washout will occur at D = \u00b5 (Sf), and not at D = \u00b5max.\r\nnStart\u00a0up is an important consideration as well. The general procedure in the start up\u00a0avoiding washout would be to initiate cell growth in a batch mode until the\u00a0exponential phase is reached.\r\nAt this point, the sterile feed would be started with a dilution rate such that D < \u00b5 (Sf).\r\nA non washout steady state would be reached after a transient phase.