Paper No. : 04 Unit Operations in Processing

Module : 09 Fundamentals of Thermal Processing of Food

Development Team

Principal Investigator Prof. (Mrs.) Vijaya Khader, Ph.D Former Dean, Acharya N G Ranga Agricultural University Dr. Vijaya Khader Former Dean, Acharya N G Ranga Agricultural University Paper Coordinator Er. Dibyakanta Seth Asst. Professor, Tezpur University, Assam

Dr. Kshirod Kumar Dash Content Writer Asst. Professor, Dept. of Food Engg. & Tech., Tezpur University, Assam Prof. (Mrs.) Vijaya Khader, Ph.D Content Reviewer Former Dean, Acharya N G Ranga Agricultural University Dr. MC Varadaraj , Chief Scientist CSIR-CFTRI, Mysore Dr. Vijaya Khader Dr. MC Varadaraj

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Unit Operations in Food Processing Fundamentals of Thermal Processing of Food

Description of Module

Subject Name Food Technology

Paper Name 04 Unit Operations in Food Processing

Module Name/Title Fundamentals of Thermal Processing of Food

Module Id FT/UOFP/9

Pre-requisites Heat transfer

Objectives To study the relationship between the microbial destruction and thermal treatment and to get acquainted with the related terms like D value, z value and F value. Keywords Thermal death time, z- value, F-value

Biological materials, particularly , are perishable in nature and therefore, it becomes critical to apply certain preservation techniques in order to increase their shelf life. Physical and chemical means

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

of preservation like drying, smoking, salting, chilling, freezing and heating are most commonly used in food industries. Thermal processing is a technique that causes surface destruction of the pathogenic as well as /or spoilage micro-organisms leading to an increase in the shelf life of the product. This includes treatments like pasteurization and sterilization, etc. Sterilization is a term referring to any process that eliminates or kills all forms of life, including transmissible agents (such as fungi, bacteria, viruses, spore forms, etc.) present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media. Sterilization can be achieved by applying heat, chemicals, irradiation, high pressure, and filtration or combinations thereof. Generally the food industries use Clostridium botulinum as the index micro-organism i.e. complete sterility with respect to this spore is critical. Sterilization process can be divided into two main categories. First one is ‘in-container’, which is used for foods, which are placed in containers such as cans, bottles and plastic pouches. Second one is ‘Continuous flow system for ultra-high treatment (UTH) processes, which generally involves heating at 140 °C to 150 °C for 1 to 3 seconds. Sterilization as a definition terminates all life; whereas sanitization and disinfection terminate selectively and partially. Both sanitization and disinfection reduce the number of targeted pathogenic organisms to what are considered "acceptable" levels - levels that a reasonably healthy, intact, body can deal with. It is a widely known fact that temperature influences the growth rate of micro-organisms and each organism has a certain optimal temperature range at which it grows best. Exposure beyond this temperature for relatively long durations of time causes microbial death. Thermal Death Rate Kinetics of When microorganisms are subjected to temperatures beyond their optimum, they lose their viability. Assuming that, a single enzyme inactivation will inactivate the cell, and then in a suspension of of a single species at a constant temperature, the death rate i.e. the rate of destruction is proportional to the number of organisms:

dN dt = -kN (1) The above is a first order equation where,

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

N = the number of viable organism at a given time, t = time (min), and k = reaction velocity constant (min-1) which is a constant for a particular temperature and the type of organism. Rearranging and integrating eq. (1) we obtain: N dN t ∫ = − ∫ kdt (2) N0 N t=0 Nο ln = kt (3) N Where,

N0 = the original number at t=0, also called the concentration level (initial concentration of microbes) and N = the number at time t, also called the sterility level. Also, Eqn. (3) can be written as: −kt N=N0e (4) Decimal reduction time (D Value=Time) D-value refers to decimal reduction time and is the time required at a certain temperature to kill 90% of the organisms being studied. Hence Decimal reduction time is the time (in min) during which the original number of viable microbes is reduced by 1/10 of its initial concentration i.e. it is the time necessary for 90% reduction in microbial population. Substituting t by D into Eqn. (4) 푁 1 = = 푒−푘퐷 (5) 푁0 10

Or, taking log10 on both sides and solving for D we get: 2.303 D = (6) k Combining Eqns. (3) and (4) we obtain the following: Nο t = Dlog (7) 10 N

A plot of log10 (N/N0) versus t gives a straight line (from Eqn.3)

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

It is observed that sometimes a plot of Bacterial spore deviates somewhat from the logarithmic rate of death, particularly during a short period immediately following thermal processing. However, for our convenience we use a logarithmic type curve for spores such as Cl. botulinum. To determine the microbial death rate, the cell suspension is sealed in a test tube which is then subjected to a high temperature in a hot water bath. After a specific duration of time, it is removed and chilled immediately. The number of viable organisms is then determined by the standard plate count method. The effect of temperature on the reaction rate k may be expressed by an Arrhenius type equation κ = αe−E/RT (8) Where α = an empirical constant, R is the gas constant in kJ/g mol.K, T is absolute temperature in K, and E is the activation energy in kJ/g mol whose value ranges from 210 to about 418 kJ/g mol for vegetative cells and spores and much less for enzymes and vitamins. Substituting the value of ‘k’ in Eq. (3) and integrating, Nο t ln = α ∫ e−E/RTdt (9) N t=0 From above, it can be concluded that at constant temperature T Eqn. (9) becomes (3). Since k is a function of temperature, the decimal time D, which is related to k by Eq. (6), is also a function

of temperature. Hence, D is often written as DT to show its temperature dependence.

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

Fig 1: Decimal Reduction Time Figure 1 shows the effect on an organism held at specified temperature for a period of time, and time is the only variable. We start with 10,000 (104) organisms, and after 10 minutes, 90% of them have been destroyed, leaving 1,000 organisms (103). This is referred to as a one decimal log reduction. In this instance D Value = 10 minutes. For every additional 10 minutes at same temperature this process kills an additional 90% of the organisms present. It is important to remember that the D Value for an organism is significantly different at different temperatures. D Value changes dramatically when the temperature is varied. It is easy to understand that, as the temperature increases, it takes less time to destroy a population of organisms. Therefore, the D Value (the time needed to kill 90% of the population) becomes smaller as the temperature rises. 12-D Concept For canned foods, Cl. Botulinum is the primary organism of concern and it has been established that the minimum heating process should reduce the number of the Spores by a factor of 10-12. This means that -1 -12 since D is the time to required to reduce the original number by 10 , substituting N/N0 =10 into eq.(4) we get, 2.303 t = 12 = 12D ………………………….. (10) k This is often called the 12D concept and the time is called the thermal death time.

This time does not represent complete sterilization. When a plot of decimal reduction time DT at a given T with the temperature T in °F is made on a semi log plot, we obtain a straight line. A typical thermal destruction curve is shown in Fig (2).

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

Fig. 2 : Thermal destruction curve The survivor curves for microbial populations are influenced by external agents. As the magnitude of the preservation agents such as temperature, pressure, PEF increase, the rate of the microbial population reduction increases. Thermal Death Time (F) Thermal death time is the amount of time that is necessary to kill a specific number of microbes at a specific temperature. This value is obtained by keeping temperature constant and measuring the time necessary to kill the amount of cells specified. This time is expressed as a multiple of D values as long as the survivor curve follows a first-order model. Z-value (=Temperature) Z Value is the number of degrees of temperature necessary to reduce the D Value one log cycle, that is, to kill 90% of the population of a microorganism. For Z Values, temperature is the variable, with time being constant. It is the reciprocal of the slope resulting from the plot of the logarithm of the D-value versus the temperature at which the D-value was obtained. While the D-value gives the time needed at a certain temperature to kill an organism, the z-value relates the resistance of an organism to differing temperatures. The z-value calculates a thermal process of equivalency, if given one D-value and the z- value.

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

Fig 3: Z value and D value Traditional approach to thermal processing has used the term thermal resistance constant ‘z’ to describe the influence of temperature on Decimal Reduction time for microbial population. Thermal resistance constant is defined as the increase in temperature, necessary to cause a 90% reduction in the Decimal Reduction time (D) i.e. T – T 푧 = 2 1 (11) log D푇1 – log D푇2

Letting T1=250°F (121.1°), which is the standard temperature against which thermal process are

compared, and calling T2=T, eq. (11) becomes

DT = D250.10(250 − T)⁄z (12) For Cl. botulinum , z=18°F. This means that increase in temperature of 18°F (10°C) will increase the death rate by a factor of 10.. Using eq. (7), Nο t = DTlog (13) 10 N

Substituting T=250°F (121.1°C) as the standard temperature into this equation and substituting F0 for t 8

Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

Nο Fο = D250log Where the F0 value of process is the time t in min at 250°F that will produced the same 10 N

degree of sterilization as the given process at its temperature T. The F0 of the given process at temperature T is Fο = t. 10(T°C−121.1)/(z°C) (14)

For instance, the values for F0 and z for adequate sterilization with Cl.botulinum vary somewhat with

the type of food. For several different temperature stages, each having different times t1, t2,….the F0

values for each stages are added to give the total F0. (T1−250)/z (T2−250)/z Fο = t1.10 + t2.10 +……… (15) Pasteurization The term pasteurization applies to mild heat treatment of foods that is used to kill organisms that are relatively low in thermal resistance than others for which the more drastic sterilization processes are required. Pasteurization includes the destruction of vegetative microorganisms and not heat-resistant spores. Pasteurization of milk is, for instance, done to kill Listeria monocytogenes, which is a non spore forming bacterium. This does not sterilize the milk, instead reduces the other bacterial count sufficiently so that the milk can be stored for a longer duration, if refrigerated. The times involved in pasteurization are much shorter and the temperatures used are much lower. ° ° ° The F0 value is given at 150 F (65.6 C) or a similar temperature rather than at 250 F as in sterilization. Also the concept of the z value arises, where a rise in temperature of z°F will increase the death rate by a factor of 10. 10 ° ° An F0 value written as F 150 means the F value at 150 F with a z value of 10 F. For pasteurization of milk,

z Nο F = DT1 log10 (16) T1 N Rewriting the above equation, we get: z (T−T1)/z FT1= t. 10 (17)

Where T1 is the standard temperature being used, z is the temperature in 0F for a tenfold increase in death rate, and

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

T is the temperature of the actual process. Effects of Thermal Processing of food constitutes Thermal processing produces certain undesirable effects in food which primarily involves the reduction of certain nutritional values. For example, certain heat-labile constituents like Ascorbic acids, thiamin

and riboflavin (Vitamin B1 and B2) are partially destroyed by thermal processing. Thermal processing also contributes flavor change in a food product and this same kinetics can be applied to predict the time for detecting a flavor change in a product.

Example 1: A Thermal process is accomplished by instantaneous heating to 130°C followed by a 6 second hold and immediate cooling. Estimate the lethality at 121°C when the thermal resistance for the microorganism is 7.5°C. Solution 1: Using equation 14 we get 퐹 130 = 10(131−121)/7.5 = 15.848 퐹121

Or, F130 = (6) (15.848) = 95.1 seconds. Example 2: The results of a thermal resistance experiment gave a D value of 7.5 minutes at 110°C. If 4 there were 4.9 * 10 survivors at 10 minutes, determine the ratio (N/N0) for 5, 15, 20 minutes. Solution 2: D= 7.5 minutes Therefore, k= 2.303/7.5 = 0.307 -kt N/N0 = e = exp (-0.307*10) 4 6 N0 = 4.9*10 /0.04642 = 1.055*10

For 5 minutes, N/ N0= exp (-0.307*5) = 0.215

N/ N0= exp(-0.307*15)= 0.01

Example 3 Pooled raw milk at the processing plant has bacterial population of 4x10exp5/mL. It is to be processed at 79°C for 21 seconds. The average D value at 65°C for the mixed population is 7 min. The Z value is

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food

7°C. How many organisms will be left after pasteurization? What time would be required at 65°C to accomplish the same degree of lethality? Answer: At 79°C, the D value has been reduced by two log cycles from that at 65°C since the Z value is 7°C. Hence it is now 0.07 min. The milk is processed for 21/60=0.35 min, so that would accomplish 5 log cycle reductions to 4 organisms/mL. At 65°C, you would need 35 minutes to accomplish a 5D reduction.

-3 N/ N0= exp(-0.307*20)= 2.15*10

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Unit Operations in Food Processing Food Technology Fundamentals of Thermal Processing of Food