Paper No. : 04 Unit Operations in Food 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 1 Unit Operations in Food Processing Food Technology 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 foods, 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 2 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, canning 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 Microorganisms 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 microorganism 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, 3 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) 4 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. 5 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). 6 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.