FE 411 FOOD BIOTECHNOLOGY

1. FE 411 FOOD BIOTECHNOLOGY 1-Biochemical Pathways

2. The Nature of Microorganisms • Microorganisms can be found almost anywhere in the taxonomic organization of life on the planet. Bacteria and archaea are almost always microscopic, while a number of eukaryotes are also microscopic, including most protists, some fungi, as well as some animals and plants. Viruses are generally regarded as not living and therefore are not microbes, although the field ofmicrobiology also encompasses the study of viruses.

3. The Nature of Microorganisms • All microorganisms are allocated to a specific group with respect to growth temperature. • Obligate psychrophiles are defined as those organisms capable of growth at ornear 0°C but not at 20°C. Such organisms usually have a maximum growth temperatureof 15–17°C. • Psychrotrophic organisms are capable of growth at or near 0°C but exhibitoptimum growth at approximately 25°C and are frequently unable to grow at 30°C. • Mesophiles exhibit growth from 20–45°C with an optimum growth temperature usuallyin the range of 30–35°C. Thermophiles exhibit growth in the range of 45–65°C. • Hyperthermophiles are organisms from oceanic thermal vents and hot springs that arerestricted to growth temperatures from 70–120°C. Hyperthermophiles have not yet beenisolated from foods.

4. Nutritional Requirements All biological systems, from microorganisms to man, share a set ofnutritional requirements, which are: 1. Sources of energy a. Phototrophsorganisms which are capable of employingradiant energy. b. chemotrophs organisms which obtain the energy for theiractivities and self-synthesis from chemical reactions thatcan occur in the dark. 2. Sources of carbon a. autotrophs organisms which can thrive on an entirelyinorganic diet,using CO2 or carbonates as a sole source ofcarbon. b. heterotrophs organisms which cannot use CO2 as a solesource of carbon but require, in addition to minerals, one ormore organic substances, such as glucose or amino acids, as sources of carbon. 3. Sources of nitrogen: atmospheric nitrogen, inorganicnitrogen compounds, or other derived nitrogen. 4. Sources of sulfur and phosphorus: elementary sulfur, inorganicsulfur, or organic sulfur. 5. Sources of metallic elements: sodium, potassium, calcium,magnesium, manganese, iron, zinc, copper, and cobalt. 6. Sources of vitamins.

5. Physical Conditions • After determining the proper nutrients forthe cultivation of bacteria, it is necessary to determine the physicalenvironment in which the organisms will grow best. Three majorphysical factors to be taken into consideration are temperature, thegaseous environment, and pH. • Since microbial activity and growth are manifestations ofenzymatic action, and since the rates of enzyme reactions increasewith increasing temperatures, the rate of microbial growth istemperature dependent. Depending on the temperature range overwhich they grow, bacteria are called psychrophiles,mesophiles, orthermophiles.

6. Physical Conditions • The principal gases in the cultivation of bacteria are oxygen andcarbon dioxide. There are four types of bacteria, according to theirresponse to oxygen: 1. Aerobic bacteria grow in the presence of free oxygen. 2. Anaerobic bacteria grow in the absence of free oxygen. 3. Facultatively anaerobic bacteria grow in either the absence or thepresence of free oxygen. 4. Microaerophilic bacteria grow in the presence of minutequantities of free oxygen. For most bacteria the optimum pH for growth lies between 6.5 and7.5. Although a few bacteria can grow at the extremes of the pHrange, for most species the minimum and maximum limits fallsomewhere between pH 4 and pH9.

7. Common Pathways of Energy Metabolism for Bacterial,Fungal, and Animal Cells Catabolicpathwaysprovideenergyforbiosynthesisand in mostinstancesalsoprovidereducingequivalentsforcarbonreductionreactions. Therearethreemajorgroups of compoundsthat link anabolicandcatabolicpathways: • adenosinephosphates (AMP, ADP, ATP), which link energyyieldingandenergyrequiringreactions; • nicotinamideadeninedinucleotide (NAD, NADH); and • nicotinamideadeninedinucleotidephosphate (NADP, NADPH). The Embden–Meyerhof–Parnas (EM) pathway, thepentose-phosphate (PP) pathway, and the Entner–Doudoroff(ED) pathway are three routes for the utilization of hexoses such as glucose.

8. Embden–Meyerhof–Parnas pathway. • This is the glycolysispathway in which 1 mol glucose is degraded to 2 mol pyruvate. • The net energy gain is of 2 mol ATP. The pyruvate is eitherreduced to lactate under anaerobic conditions or converted intoacetyl-CoA for entry into the TCA cycle.

9. Entner–Duordoff pathway • Glucose is degraded in theabsence ofphosphofructokinase to pyruvate and glyceraldehyde-3-phosphate. The latter can be degraded further to pyruvate bythe enzymes of glycolysis. • This pathway is less efficient that glycolysis,yielding only 1 mol ATP/mol glucose.

10. Pentose-phosphate pathway • This pathway is divided intothree parts: • (a) An oxidative sequenceof reactions in which glucoseis converted to the five-carbon,ribulose-5-phosphate. • (b) Epimerizationand isomerization reactionsin which the important nucleic acidprecursor, ribose-5-phosphate isformed. • (c) A nonoxidative sequenceof transaldolase and transketolasereactions.

11. Tricarboxylic acid cycle • The TCA pathway operates in the mitochondria of eukaryotes. • It allows the complete oxidation of pyruvate, which enters the cycle from glycolysis. • The reducedcoenzymes are reoxidized via the electron transport chain. The complete aerobic metabolism ofglucose via glycolysis and the TCA cycle yields between 30 and 38 mol ATP/mol glucose, dependingon the extent of the coupling of phosphorylation in the electron transport chain.

12. Glucose is not the only carbohydrate that can be converted to pyruvate by glycolysis

13. Anaerobic Breakdown of Carbohydrates • The terms glycolysis and fermentation have been applied to the anerobic decomposition of carbohydrate to the level of lactic acid. • The 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. • The two most common forms of fermentation are lactic and alcoholic.

14. Fermentation • Phosphoketolase pathway. This pathway allows heterolactic fermentation in certain bacteria and fungi. The key enzyme is phosphoketolase, which degrades the five-carbon xylulose into a two-carbon, acetyl and three-carbon, glyceraldehyde. Lactate and ethanol are carbon products of the pathway. The net energy gain is 1 mol ATP/mol glucose utilized.

15. Lactic Fermentation • A familiar fermentation product is lacticacid, which is derived from the reduction of pyruvatewith NADH to lactate by lactate dehydrogenase. A furthermeans of energy conservation observed in several homolacticfermenting organisms, but which is also found inother organisms, is the generation of a proton motive forcein association with the excretion of lactic acid out of thecells via a secondary transport mechanism.

16. Alcoholic Fermentation • The formation of ethanol as the sole organicfermentation product is a two-step process. The first stepis the decarboxylation of pyruvate to acetaldehyde plusCO2 by pyruvate decarboxylase. NAD is recycled in thenext step, the NADH-dependent reduction of acetaldehydeby alcohol dehydrogenase. Only organisms possessing pyruvatedecarboxylase are capable of producing ethanol asa major fermentation product.

17. Butanol/acetone fermentation. • This anaerobic metabolism is typicalof the genus Clostridium. Variousfermentation products are formed byreduction using NADH derived fromglycolysis. The proportion of each productformed is dependent on the fermentationconditions.