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Food Microbiology Spring 201 8

Food Microbiology Spring 201 8. LECTURE Microbial Growth - Kinetics First Order. Growth Curve. Log CFU/ml. Optical Density. Lag. First Order Kinetics. Food microbiology is concerned with all phases

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Food Microbiology Spring 201 8

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  1. Food MicrobiologySpring 2018 LECTURE Microbial Growth -Kinetics First Order

  2. Growth Curve Log CFU/ml Optical Density Lag

  3. First Order Kinetics Food microbiology is concerned with all phases Of microbial growth (lag,log, stationary, death phase).Growth curves are normally plotted as the number of cells on a log scale or log10 cell number versus time.

  4. Growth of bacterial population

  5. One generation

  6. Growth Terminology and the concept of exponential growth • The interval for the formation of two cells from one is called a generation • The time requiredfor this to occur is called the generation time. • Generation timeis the time required for the cell population to double (the cell mass doubles during this period as well). • Because of this, the generation time is also called the doubling time.

  7. In nature, microbial doubling times may be much longer than those obtained in laboratory culture. • This is because in nature, ideal growth conditions for a given organism may exist only intermittently. • Depending on resource availability, physiochemical conditions (temperature, pH, and the like), moisture availability, and seasonal changes, bacterial populations in nature double only once every few weeks, or even longer.

  8. A mathematical relationship exists between the number of cells present in a culture initiallyand the number present after a period of exponential growth: N = N02n where N is the final cell number, No is the initial cell number, n is the number of generations that have occurred during the period of exponential growth.

  9. The Mathematics of Exponential Growth • As one cell divides to become two cells, 2° --'> 21. • As two cells become four, 21 --'> 22, and so on

  10. The generation time(g)of the exponentially growing population is (t / n), where tis the duration of exponential growth expressed in days, hours, or minutes, depending on the organism and the growth conditions. From a knowledge of the initial and final cell numbers in an exponentially growing cell population, it is possible to calculate n, and from nand knowledge of t, the generation time g.

  11. Relation equation of N and No to n The equation N = No2ncan be expressed in terms of n as follows: N = No2n log N = log No + n log 2 log N – log No = n log 2 n = log N – log No = log N – log No log 2 0.301 = 3.3 (log N – log No)

  12. example N = 108, No= 5 X 107, and t = 2 n = 3.3 (log N – log No) n = 3.3 [log(108) - log(5 X 107)] = 3.3(8 - 7.69) = 3.3(0.301) = 1 generation time, g = t/n = 2 / 1 = 2 h

  13. Related growth parameter • The generation time g of an exponentially growing culture can also be calculated from the slope of the line obtained in the semilogarithmic plot of exponential growth. • The slope is equal to 0.301 n/t (log 2n/t) and in the above example would be 0.301(1)/2, or 0.15. • Since g is equal 0.301/slope, we arrive at the same value of 2 for g. • The term 0.301nlt is called the specific growth rate, abbreviated k.

  14. The Growth cycle or phases of microbial growth • observed when microorganisms are cultivated in batch culture • culture incubated in a closed vessel with a single batch of medium • usually plotted as logarithm of cell number versus time • usually has four distinct phases

  15. Table First order kinetics to describe exponential growth and inactivation N=Final cell number(CFU/ml) N0=Initial cell number t=time (h) µ=Specific growth rate (h-1) g=Doubling time(generation time)(h) k=rate constant (h-1) D=Decimal reduction time(h) Ea=Activation energy(kcal/mol) T1 and T2,reference and test temperature(K) D0=Rate constant (h-1) Dd=Dose(Gy)

  16. The rate of growth is directly proportional to cell concentration or biomass- i.e. dx/dt αX dx/dt = μX ----------1 Where, X is the concentration of microbial biomass, t is time, in hours μ is the specific growth rate, in hours -1

  17. On integration of equation (1) from t=0 to t=t ,we have: xt= xo eμt --------- 2 Where, • Xo is the original biomass concentration, • Xt is the biomass concentration after the time interval, t hours, • e is the base of the natural logarithm.

  18. On taking natural logarithms of equation (2) we have : In Xt = In Xo + μt (3)

  19. Therefore, a plot of the natural logarithm of biomass concentration against time should yield a straight line, the slope of which would equal to μ. • During the exponential phase nutrients are in excess and the organism is growing at its maximum specific growth rate, ‘μmax ‘ for the prevailing conditions.

  20. Typical values of μmax for a range of microorganisms are given below in the Table.

  21. Three causes for lag: physiological lag low initial numbers Lag phase appropriate gene(s) absent growth approx. = 0 (dX/dt = 0)

  22. 20 21 22 23 24 20 21 22 23 24 20 21 22 23 24 20 21 22 23 24 20 21 22 23 24 20 21 22 23 24 2n 2n 2n 2n 2n 2n Exponential phase Nutrients and conditions are not limiting growth = 2n or X = 2nX0 Where X0 = initial number of cells X = final number of cells n = number of generations

  23. X = 2nX0 Cells grown on salicylate, 0.1% Example: An experiment was performed in a lab flask growing cells on 0.1% salicylate and starting with 2.2 x 104 cells. As the experiment below shows, at the end there were 3.8 x 109 cells. This is an increase is 5 orders of magnitude!! How many doublings or generations occurred? 3.8 x 109 = 2n(2.2 x 104) 1.73 x 105 = 2n log(1.73 x 105)= nlog2 17.4 = n

  24. dX/dt = uX where u = specific growth rate (h-1) y = mx + b (equation for a straight line) Calculating growth rate during exponential growth dX/dt = uX where u = specific growth rate (h-1) Rearrange: dX/X = udt Integrate: lnX = ut + C, where C = lnX0 lnX = ut + ln X0 or X = X0eut Note that u, the growth rate, is the slope of this straight line

  25. dX/dt = uX where u = specific growth rate (h-1) y = mx + b (equation for a straight line) Calculating growth rate during exponential growth Rearrange: dX/X = udt Integrate: lnX = ut + C, where C = lnX0 lnX = ut + ln X0 or X = X0eut Note that u, the growth rate, is the slope of this straight line

  26. lnX = ut + ln X0 or u = lnX – lnX0 t – t0 u = ln 5.5 x 108 – ln 1.7 x 105 8.2 - 4.2 = 2 hr-1 Find the slope of this growth curve

  27. Growth Kinetics g can be calculated by g== Example: Initial population is 103 CFU/ml and increasedto 106 cells in 300 min. What is generation time? g==30 min or you can first calculate µ and then calculate g. 2.3log(N/N0)= µ t µ =0.023min-1 and g=0.693/µ g=30.13 min µ can be obtained by slope of straight line when the log numbers of the cell isplotted against time.

  28. Ex:Ground meat manufactured with N0=1.2*104 CFU/g. How long it be held at 7°C before reaching a level of 108CFU/g (for µ=0.025 h-1) N=N0eµt 108 =1.2*104e0.025t t=361.12 h

  29. Death Kinetics- Killing can be by heat,radiation,acid,bacteriocin and other lethalagents is also governed by first order kinetics. Dvalue=amount of time required to reduce N0 by 90% is the most frequently used constant. The relationship between k and temperature is explained by arrhenius equation k=A eEa/RT

  30. Z value Zvalue= a number of degrees required to change in the D values by a factor 10, or It is the temperature required for one log10 reduction in the D-value.

  31. z-valueis used to determine the time values with different D-values at different temperatures with its equation shown below: where T is temperature in °F or °C. This D-value is affected by pH of the product where low pH has faster D values on various foods. The D-value at an unknown temperature can be calculated knowing the D-value at a given temperature provided the Z-value is known.

  32. For example: If Dvaue at 121 °C is 1.5 min and z value is 10 °C. The D value at 131 °C will be 0.15 min. Example: if it takes an increase of 10°F to move the curve one log, then our z-value is 10. Given a D-value of 4.5 minutes at 150°F, the D-value can be calculated for 160°F by reducing the time by 1 log. The new D-value for 160°F given the z-value is 0.45 minutes. This means that each 10°F increase in temperature will reduce our D-value by 1 log. Conversely, a 10°F decrease in temperature will increase our D-value by 1 log. So, the D-value for a temperature of 140°F would be 45 minutes.

  33. Microbial Growth Characteristics in Foods 1.Competition 2.Metabiotic Growth 3.Symbiotic Growth 4.Synergistic Growth 5.Commensalism 6.Antagonistic Growth 7.Predation

  34. 1.Competition Energy and nutrient sources are often present in limiting concentrations; microorganisms compete each other for nutrients and results in exclusion of slower growing species. Foods contain a mixed population of microorganisms.Competition among the different kinds of microorganisms in food determines which one will outgrow the others and cause its characteristics types of changes.

  35. 2. Metabiotic (Sequential)Growth Different types of microorganism present normally in foods, but the predominant types can change with time during storage. Ex: If the food is packaged in a bag with a little bit of air(e.g. ground meat),the aerobes will grow first and utilize O2. The environment will become anaerobic, in which anaerobes grow favorably. Ex: In most food fermentations metabiotic growth is observed. In Sauerkraut fermentation,4 different bacterial species grow in succession, one creating the favorable conditions for the next one. First ,coliform grow produce acid and activate the growth of lactic acid bacteria. second,Leuconoctocmesenteroides; third Lb. plantarum Last, acid tolerant Lb. brevis

  36. 3.Symbiotic Growth Two or more microorganisms help one another during growth in food. In yogurt;there are two types of lactic acid bacteria. 1. S. thermophilus 2.Lb. bulgaricus S. thermophilusproduces small quantities of formic acid and stimulates Lb. bulgaricus. Lb. bulgaricusproduce aminoacidinturn these products stimulate the growth Str. thermophilus

  37. 4.Synergistic Growth When two types of microorganism grow together and may able to bring changes which could not produce alone. Acetaldehydeis desirable flavor component in yogurt. S. thermophilus produce 8 ppm Acetaldehyde Lb. bulgaricus produce 10 ppm Acetaldehydein milk independently,when they grow together, they produce 30ppmAcetaldehyde.

  38. 5.Commensalism Microorganisms may not effect each other but one organisms uses the substrate whic isproduced by other. For ex:cellulose hydrolyzing microorganisms produce glucose and cellulose non hydrolyzing micoorganims use this glucose. One population benefits while latter remain unaffected.

  39. 6.Antagonistic Growth Microorganisms can adversely affect each other, one kill the other. Some Gr(+) bacteria produce antimicrobial components that can kill many other types. For ex: L. lactisssp. lactisproduce bacteriocin called nisin and inhibits Gr(-)bacteria. 6.Predation Growth The example for predatory growth is the attachment of bacteria of the genus Bdellovibrio,Daptobacter and Stenotrphomonasmaltophiliato Gr(-) bacteria,penetrating the cell wall, and subsequently multipyingwitin the periplasmic space.

  40. III. Chemical Changes Caused my microorganisms • Changes in nitrogenous organic compounds • Changes in organic carbon compounds a) Carbohydrates b)Organic acids c)Other compounds d)Lipids e)Pectic substances

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