Thermal Processing of Food Page 1 Safefood 360° is the provider of food safety management software for industry leading businesses. 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 3.2 Method for pasteurizing The use of high tempera- tures to preserve and ensure 4. Sterilization the safety of food is based 4.1 Canned foods on the effect of icrm obial 4.2 Conditions affecting the growth of destruction. Thermal pro- micro-organisams 4.3 Micro-organisms in retorted foods cessing is one of the most 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) 4.10 Thermal death time (TDT) curve covers the main science be- 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 Safefood 360, Inc. 2014 Part of Our Professional Whitepapers Series 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 effcienciess For small products es gs causes quality loss due to increased enzyme 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 Safefood 360, Inc. 2014 Part of Our Professional Whitepapers Series 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 the destruction of all disease causing 3. Pasteurization 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 63°C For 30 min (low unit operation in food processing it can be temperature long time LTLT) used to destroy enzymes and relatively 72°C For 15 sec (primary high heat sensitive micro-organisms (e.g. non 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 Thermal Processing of Food Page 5 Thermoduric: organisms that can survive High-Temperature-Short-Time (HTST) exposure to relatively high temperatures but do not necessarily grow at these In this method the heating of every particle temperatures e.g. Streptococcus and of milk to at least 72°C and holding Lactobacillus. for at least 15 seconds. Carried out as a continuous process. Ultra Heat Treatment Thermophilic: organisms that not only (UHT) a sterilisation treatment, can also survive relatively high temperatures be performed using higher temperatures but require high temperatures for their and shorter times e.g. 1 s at 135°C growth. Typical Equipment employed for this method includes: 3.2 Method for Pasteurizing • Plate heat exchanger (PHE) There are number of basic methods of pasteurization widely used in the industry. • Holding tube – sized to ensure the correct treatment time is achieved Batch (holding) Method • Holding tanks – for storage of the In this method every particle (e.g.