Thermal Processing of Food                                                                                                                    Page 1
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                                                                            WHITEPAPER
                                      Thermal Processing of Food
                         CONTENTS                                                                                                 SUMMARY
                         1.         Introduction
                         2.         Blanching
                         2.1        Blanching and enzyme inactivation
                         2.2        Methods of blanching
                         2.3        Testing of the effectiveness of blanching
                         3.         Pasteurization
                         3.1        Purpose of pasteurization
                                                                                                                                                tempera-
                         3.2        Method for pasteurizing                                                            The use of high 
                         4.         Sterilization                                                                      tures to preserve and ensure 
                                                                                                                       the safety of food is based 
                         4.1        Canned foods                                                                       on the effect of microbial 
                         4.2        Conditions affecting the growth of                                                 destruction. Thermal pro-
                                    micro-organisams                                                                   cessing is one of the most 
                         4.3        Micro-organisms in retorted foods
                         4.4        Microbial spoilage of canned foods                                                 widely used unit operations 
                         4.5        Sterilisation process and equipment                                                employed in the food indus-
                         4.6        Containers for thermally treated products                                          try and is frequently deter-
                         4.7        Cleaning of containers prior to filling                                            mined as a Critical Control 
                         4.8        Seaming of cans                                                                    Point (CCP). This whitepaper 
                         4.9        Death rate curve (D value)                                                         covers the main science be-
                         4.10       Thermal death time (TDT) curve
                         4.11       Some factors affecting heat resistance                                             hind the unit operation and 
                         4.12       Design of heat sterilization processes                                             should be used to underpin 
                         4.13       The “F0 value”                                                                     the development and design 
                         4.14       The lethality factor “l”                                                           of thermal processing steps.
                  Safefood 360, Inc. 2014                                                           Part of Our Professional Whitepapers Series
             Thermal Processing of Food                                                                       Page 2
             1.      Introduction                                   2.1     Blanching and enzyme 
                                                                     inactivation
             There are two main temperature 
             categories employed in thermal processing:             Freezing	and	dehydration	are	insuffcient 
             Pasteurization and Sterilisation. The basic            to inactivate enzymes and therefore 
             purpose for the thermal processing of foods            blanching can be employed. Canning 
             is to reduce or destroy microbial activity,            conditions	may	allow	suffcient time for 
             reduce or destroy enzyme activity and                  enzyme activity. Enzymes are proteins 
             to produce physical or chemical changes                which are denatured at high temperatures 
             to make the food meet a certain quality                and lose their activity. Enzymes which 
             standard. e.g. gelatenization of starch &              cause loss of quality include Lipoxygenase, 
             denaturation of proteins to produce edible             Polyphenoloxidase, Polygaacturonase and 
             food. There are a number of types of heat              Chlorophyllase. Heat resistant enzymes 
             processing employed by the food industry.              include Catalase and Peroxidase
              Mild processes            Blanching                   2.2   Methods of Blanching
                                        Pasteurisation
              More severe               Canning                     Blanching is carried out at up to 100°C 
              processes                 Baking                      using hot water or steam at or near 
                                        Roasting                    atmospheric pressure.
                                        Frying 
                                                                    Some	use	of fluidised	bed	blanchers,	
             2.      Blanching                                      utilising	a mixture of air and steam, has 
                                                                    been reported. Advantages include faster, 
             The primary purpose of blanching is                    more uniform heating, good mixing of 
             to destroy enzyme activity in fruit and 
                                                                    the	product,	reduction	in	effluent,	shorter	
             vegetables. It is not intended as a sole               processing time and hence reduced loss of 
             method of preservation, but as a pre-                  soluble and heat sensitive components. 
             treatment prior to freezing, drying and 
             canning. Other functions of blanching                  There is also some use of microwaves for 
             include:                                               blanching. Advantages include rapid 
                                                                    heating and less loss of water soluble 
             •       Reducing surface microbial                     components. Disadvantages include high 
                     contamination
                                                                    capital	costs	and	potential	diffculties in 
                                                                    uniformity of heating. 
             •       Softening vegetable tissues to
                     facilitate	filling	into	containers
             •       Removing air from intercellular
                     spaces prior to canning
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            Thermal Processing of Food                                                             Page 3
            Steam Blanchers                                   drum, partially submerged in the hot water. 
                                                              It	is	carried	along	by	internal	flights,	
            This is the preferred method for foods            residence time being controlled by the 
            with large cut surface areas as lower             speed of rotation.
            leaching losses. Normally food material 
            carried on a mesh belt or rotatory cylinder       Pipe blanchers consist of insulated tubes 
            through a steam atmosphere, residence             through which hot water is circulated. Food 
            time controlled by speed of the conveyor or       is metered into the stream, residence time 
            rotation. Often poor uniformity of heating        being controlled by the length of the pipe 
            in the multiple layers of food, so attaining      and velocity of the water. 
            the required time-temperature at the 
            centre results in overheating of outside          The blancher-cooker has three sections, a 
            layers.                                           preheating stage, a blanching stage, and 
                                                              a cooling stage. As the food remains on a 
            Individual Quick Blanching (IQB) involves         single belt throughout the process, it is less 
            a	first	stage	in	which	a	single	layer	of	the	     likely to be physically damaged. With the 
            food	is	heated	to	suffcient temperature to        heat recovery incorporated in the system, 
            inactivate enzymes and a second stage in          16 to 20 kg of product can be blanched for 
            which a deep bed of the product is held for       every kg of steam, compared with 0.25 to 
            suffcient time to allow the temperature at        0.5kg per kg stream in the conventional 
            the centre of each piece to increase to that      hot water blanchers. 
            needed for inactivation.
                                                              2.3   Testing of the Effectiveness of 
            The reduced heating time (e.g. for 10 mm           Blanching
            diced carrot, 25 s heating and 50 s holding 
            compared with 3 minutes conventional              Over blanching causes quality loss due 
            blanching) results in higher energy               to overheating while under blanching 
                                                              causes quality loss due to increased enzyme 
            effcienciess  For small products  es gs  
            peas, sliced or diced carrots), mass of           activity because enzymes activated and 
            produce blanched per kg steam increases           substrates released by heat. The 
            from 0.5kg for conventional steam                 Peroxidase test in vegetables is used to 
            blanchers to 6-7kg for IQB.                       detect enzyme inactivation. This enzyme is 
                                                              not in itself implicated in degradation, but 
            Hot Water Blanchers                               is relatively heat resistant and easily 
                                                              detected. It consists of adding guaiacol 
            Includes various designs which hold               solution and hydrogen peroxide solution 
            the food in hot water (70 to 100°C)               and observing the development of a brown 
            for	a	specified	time,	then	moves	it	to	a	         colour indicating peroxidase activity. 
            dewatering/cooling section. In blanchers 
            of this type the food enters a slowly 
            rotating 
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            Thermal Processing of Food                                                            Page 4
            Complete inactivation is not always              of the process, alkaline phosphatase is a 
            essential – green beans, peas and carrots        naturally occurring enzyme in raw milk 
            with some residual peroxidase activity           with a similar D value to heat-resistant 
            have shown adequate storage quality at           pathogens and so is routinely used as an 
            -20°C through with other vegetable (e.g.         indicator of adequate pasteurisation. If 
            Brussels sprouts) zero peroxidase activity is    phosphatase activity is found, it is assumed 
            essential.                                       that pasteurisation is inadequate.
                                                             Pasteurization is normally used for 
            3.     Pasteurization                            the destruction of all disease causing 
                                                             organisms (e.g. pasteurization of milk) or 
            3.1    Purpose of Pasteurization                 the destruction or reduction in the number 
                                                             of spoilage organisms in certain foods e.g. 
            Pasteurization is a relatively mild heat         vinegar. 
            treatment in which food is heated to             Table:  Milk Pasteurizing Temperatures
            <100°C. It is widely used throughout the 
            food industry and is frequently employed          Temperature Time
            as a CCP in various HACCP plans. As a 
            unit operation in food processing it can be       63°C             For 30 min (low 
            used to destroy enzymes and relatively                             temperature long time LTLT)
            heat sensitive micro-organisms (e.g. non          72°C             For 15 sec (primary high 
                                                                               temperature short time, 
            spore forming bacteria, yeast and moulds).                         HTST method)
            In this regard is it used to extend shelf life    89°C             For 1.0 sec
            by several days e.g. milk or months e.g.          90°C             For 0.5 sec
            bottled fruit.                                    94°C             For 0.1 sec
                                                              100°C            For 0.01 sec
            The severity of treatment and resulting 
            extension of shelf life is determined            These temperatures are equivalent and are	
            mostly by pH of the food. In low acid foods      suffcient to destroy the most heat sensitive 
            (pH<4.5), the main purpose is destruction        of the non-spore-forming pathogenic 
            of pathogenic bacteria, while below pH 4.5       organisms. Milk pasteurization 
            the destruction of spoilage microorganisms       temperatures	are	also	suffcient to destroy 
            or enzyme deactivation is usually more           all yeasts, moulds, gram negative bacteria 
            important. The extent of heat treatment          and many gram positive. The two groups 
            required is determined by the D value            of micro-organisms that survive 
            (Decimal reduction time or time to reduce        pasteurisation temperatures used in milk 
            numbers by a factor of 10 or 90% of the          are:
            initial load) of most heat resistant enzyme 
            or micro-organism which may be present. 
            In	terms	of	checking	the	effectiveness	
           Safefood 360, Inc. 2014                            Part of Our Professional Whitepapers Series