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AIJobalert.comSarkari Naukri • Behtar Bhavishya

Reaction Kinetics and Microbial Inactivation

This lecture covers the fundamentals of reaction kinetics in thermal food processing, including zero, first, and second-order reactions. It demonstrates how to determine reaction order and rate constants through linear regression of concentration data. Additionally, it details microbial inactivation kinetics, explaining the concepts and practical applications of D-value and z-value for process optimization.

Science
English
· 6 topics
Checked against the video on 18 Oct 2026

Made by AI from the YouTube video Lec 7: Kinetics of reactions (opens YouTube in a new tab) by NPTEL IIT Guwahati, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Reaction Kinetics and Microbial Inactivation

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This lecture covers the fundamentals of reaction kinetics in thermal food processing, including zero, first, and second-order reactions. It demonstrates how to determine reaction order and rate constants through linear regression of concentration data. Additionally, it details microbial inactivation kinetics, explaining the concepts and practical applications of D-value and z-value for process optimization.

Introduction and Kinetic Expressions

Reaction kinetics describes how the concentration of a reacting species changes over time during thermal processing.
The parameter c can represent microbial count, enzyme activity, nutrient retention, color, texture, or flavor.
Must knowThe general kinetic expression is represented as -dc/dt = k_n × (cⁿ), where k_n is the rate constant and n is the reaction order.
The negative sign indicates that the reactant or viable microbial count depletes over processing time.
Thermal processing utilizes high temperatures primarily for sterilization, which involves killing harmful microorganisms.

Order of Reactions and Mathematical Models

Reactions are categorized into zeroth, first, second, or fractional orders based on the value of n.
For a zeroth-order reaction (n = 0), integration yields c = c_0 - k_0 × t, as seen in sugar caramelization and vitamin C destruction in meats.
Must knowMost microbial destruction and enzyme inactivation processes follow first-order reaction kinetics described by c = c_0 × e^{−k_1 × t}.
For a second-order reaction (n = 2), the integrated expression is 1/c = 1/c_0 + k_2 × t, applicable to thiamin destruction in milk.
Fractional reaction orders such as 0.3 or 1.2 also exist in complex food systems.

Graphical Determination of Rate Constant and Order

Taking the logarithm of both sides of the kinetic rate equation yields log(-dc/dt) = log(k_n) + n × log(c), matching the linear form y = mx + c.
At constant temperature, experimental concentrations c are measured at discrete time intervals t.
The rate term -dc/dt is calculated between successive time steps using differences like (c_0 - c_1) / (t_1 - t_0).
Must knowPlotting log(-dc/dt) on the y-axis versus log(c) on the x-axis produces a straight line where the slope equals reaction order n.
The y-intercept of the linear plot corresponds to log(k_n), allowing determination of k_n as e raised to the intercept value.

Microbial Growth and Inactivation Factors

Microbial growth and thermal inactivation are influenced by nutrient concentration, temperature, pH, oxygen availability, and water activity.
Temperature affects cells because vital metabolic reactions are catalyzed by enzymes that denature at elevated temperatures.
Microorganisms require a specific range of osmotic pressure to survive, which correlates directly with food water activity.
Lethal chemical compounds are categorized by Minimum Inhibitory Concentration (MIC), bacteriostatic effects, and Minimum Bactericidal Concentration (MBC).
A typical microbial growth curve consists of four phases: lag phase, exponential growth phase, stationary phase, and death phase.

Inactivation Kinetics and the D-value

When evaluating cell population N instead of concentration c, the first-order inactivation equation is expressed as N = N_0 × 10^{−t / D_T}.
Must knowThe D-value (decimal reduction time) is the heating time required at a given temperature to achieve a 1-log (90%) reduction in microbial count.
At a survival fraction of 10% (one-tenth of initial population), log10(N/N_0) equals -1, which makes heating time t equal to D.
A 99% kill corresponds to a 2-log reduction requiring 2D time, while a 99.99% kill represents a 4-log reduction requiring 4D time.
Must knowOn a semi-logarithmic survivor curve, organisms with a high D-value are more heat resistant and exhibit a flatter slope (-1/D_T).

Temperature Dependence and the z-value

Must knowThe z-value is defined as the temperature change required to achieve a 1-log (1 order of magnitude) change in the D-value.
Plotting log(D) on the y-axis versus temperature on the x-axis gives a straight line with a slope equal to -1/z.
Organisms exhibiting a high z-value possess greater thermal resistance across temperature variations and present a flatter slope.
The mathematical relation between D-values at different temperatures is D_T = D_ref × 10^((T_ref - T) / z).
In food sterilization design, optimizing temperature and time allows high microbial destruction (e.g., 6-log reduction) while minimizing nutrient degradation.

Parameter	Definition	Typical Graphical Slope
D-value	Time for 90% microbial destruction at constant temperature	-1 / D_T (log N vs time)
z-value	Temperature rise needed for 90% reduction in D-value	-1 / z (log D vs temp)

Key terms

D-value: Decimal reduction time; the duration of heating in minutes or seconds needed to destroy 90% of a microbial population at a specific temperature
z-value: The temperature change in degrees Celsius or Fahrenheit necessary to produce a ten-fold change in the D-value
Bacteriostatic: A condition or compound that stops bacterial growth without necessarily killing the cells
Minimum Inhibitory Concentration (MIC): The lowest concentration of an antimicrobial substance that prevents visible microbial growth
Minimal Bactericidal Concentration (MBC): The lowest concentration of an antimicrobial substance required to kill the microorganism

Formulas

c = c_0 - k_0 × t: Integrated rate law for a zeroth-order reaction
c = c_0 × e^{−k_1 × t}: Integrated rate law for a first-order reaction
1/c = 1/c_0 + k_2 × t: Integrated rate law for a second-order reaction
N = N_0 × 10^{−t / D_T}: Microbial population survival formula using decimal reduction time
D_T = D_ref × 10^((T_ref - T) / z): Relationship between D-value, reference temperature, and z-value

Quick revision

Zeroth-order kinetics describes sugar caramelization and meat vitamin C destruction.
Most microbial destruction and enzyme inactivation reactions follow first-order kinetics.
In a plot of log(-dc/dt) versus log(c), the slope directly gives the reaction order n.
One D-value represents killing 90% of the microbial population, leaving 10% survivors.
A microbial reduction of 99.99% corresponds to a 4-log reduction requiring 4 times the D-value.
Organisms with larger D-values have greater heat resistance and shallower survivor curves.
The z-value is obtained from the negative reciprocal of the slope of log(D) versus temperature.
Processing at higher temperatures often provides equivalent microbial kill with significantly lower nutrient loss.

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