Processing and Preservation UNIT 13 PROCESSING BY HEAT APPLICATION Structure 13.0 Objectives 13.1 Introduction 13.2 Effect of Heat on Texture and Composition 13.3 Effect of Heat on Microorganisms and Enzymes 13.4 Role of Heat Application – Peeling, Juice Processing, Syrup / Brine Preparation & Filling Use of Heat in Peeling of Fruits and Vegetables Use of Heat in Juice and Pulp Processing Syrup / Brine Preparation and Filling 13.5 Blanching and Exhausting Blanching Exhausting 13.6 Pasteurization and Sterilization Pasteurization Sterilization 13.7 Combination of Time, Temperature, pH/Acidity Time and Temperature pH or Acidity 13.8 Role of Heat Application during Product Preparation Use of Heat in Jam and Jelly Production Use of Heat in Ketchup and Sauce Production 13.9 Let Us Sum Up 13.10 Key Words 13.11 Answers to Check Your Progress Exercises 13.12 Some Useful Books 13.0 OBJECTIVES After reading this unit, you should be able to: • define types of heat applications; • describe the role of heat application during processing and product preparation; • state the effect of heat application on quality of food, microorganisms and enzymes; and • discuss the effect of heat in combination with time, temperature, pH or acidity. 13.1 INTRODUCTION Fruits and vegetables are living entities and highly perishable horticulture produce. Enzymes and microorganisms can easily spoil these commodities. In order to keep fruits and vegetables for a longer period, they are protected by several means. Heat application is one of the most important methods of preservation of fruits and vegetables. The main purpose of processing of fruits and vegetables by application of heat is to inactivate enzymes and killing of microorganisms. Heat can be applied in different forms such as blanching, pasteurization and sterilization. Heat application influences the texture and composition of fruits and vegetables, and microorganisms and 28 Processing by Heat Application enzymes. Heat application plays a great role during various unit operations of processing of fruits and vegetables viz., peeling, preparation of juice and pulp; syrup and brine, during blanching, exhausting, pasteurization or sterilization. There are certain factors like time, temperature; pH and acidity in combination with heat application will affect the quality of the processed products. There are several types of processed products, generally preserved by heat application such as jam, jelly, juices, sauces, candies, canned and bottled products etc. Heat processing has advantages such as to improve eating quality and availability of some nutrients and a simple control of processing conditions. 13.2 EFFECT OF HEAT ON TEXTURE AND COMPOSITION Carbohydrates, proteins, fats, vitamins, minerals and fibre are the major dietary constituents of fresh vegetables in addition to a large quantity of water. Fruits are held in high public regard as sources of wholesome food and are valued for their taste, aroma and texture. Fresh fruits appeal virtually to all the senses, smell, taste, touch, sight and even sound as when one bites into crunchy apple. Vitamins and minerals are the major contributions of fruits to the human diet. Although some fruits are also considered good energy sources and some may contribute notable amount of fat (e.g. Avocados and nuts), sugar (e.g. Dates and figs) and proteins (e.g. tucuma). Fruits may play an important role in the diet by supplying fibre. Heat processing is one of the most important methods for extending the storage life of fruit and vegetable products. Because of this extended life, the processed products are made available throughout the year. This has increased the availability of nutrients to the consumer. However, heat processing also has a detrimental effect on nutrients since there is thermal degradation of nutrients. Therefore, heat processing makes it possible to extend and increase availability of a food product to the consumer, but the food product may have a lower nutrient content compared to the fresh food product. This is a great challenge to the food processing industry is to minimize the loss of nutrients during thermal processing. Heat processes are used in terms of blanching, pasteurization and sterilization. The primary objective of heat processing is to increase the palatability of the food and to increase the storage life of food product besides minimizing food- borne diseases. For example, the cooking, which includes roasting, boiling, frying etc. The heat processes viz. blanching, pasteurization and sterilization has already dealt in Para 2.5 and 2.6 in a greater details, here we are discussing their effect on texture and composition of fruits and vegetables. Heat processing generally includes the terms blanching, pasteurization and sterilization and their principal purpose is given here. Blanching is a heat process frequently applied to fruit and vegetables prior to freezing, drying or canning. The objective of the blanching process depends on the subsequent treatment of the foodstuffs. The adequacy of the blanching operation is enzyme inactivation. Generally, if enzymes are inactivated, the heat treatment was sufficient to accomplish the objectives of blanching prior to canning. Pasteurization is a heat process meant to inactivate part, but not all, of the vegetative microorganisms present in the food product. Since the food is not sterile, the pasteurization must also be used in conjunction with other 29 Processing and Preservation preservation techniques like fermentation, refrigeration, and maintenance of anaerobic conditions. Sterile is a term, which used to a condition in which no viable microorganisms are present. A viable organism being one that is able to reproduce under conditions optimum for its growth. Sterilization is a term used to apply to a heat process that produces a sterile condition in food product. Some microorganisms and their spores are extremely heat resistant and cannot be destroyed completely. Severe heat treatment can only make the food sterile, but it will influence the organoleptic and nutritive value of food. Therefore, the sterilization process is also used in conjunction with other preservation techniques, such as packaging and control of storage temperature. Thus, the remaining dormant microorganisms or their spores will not grow in the environment of the food under the storage. Foods that have been thermally processed and meet this requirement are said to be “commercially sterile”. Heat processing may change not only the chemical composition of the food but also its structure by softening tissues, releasing or tying up moisture, destroying or forming colloidal suspensions, gels or emulsions. Protein may become denatured and therefore more available to some organisms than it was in the native state. Starch or protein may become gelated, releasing moisture and becoming more easily decomposed. For the same reasons, cooked food usually is more easily decomposed than the original fresh food. Preservation is a convenient method of storing fruit for use in periods when the fresh products are not available. The characteristics of fruit are usually altered to such an extent during processing that the processed products do not necessarily resemble the fresh products. If processed and stored properly, the nutritive value of these processed fruit is comparable to that of the fresh products. The thermal processes viz. blanching, pasteurization and commercial sterilization have to be optimized for nutrient retention, given in the box below: Process Method of optimization Blanching Thermal losses, leaching losses, oxidative degradation, damage to product Pasteurization HTST if heat resistant enzymes are not present Commercial Convection – heating foods and aseptic processing. Sterilization HTST until heat-resistant enzymes become important Conduction heating foods; not necessarily HTST; difficult but not impossible calculation. Effect of blanching on nutrients The effect of blanching on food nutrients is generally considered based on thermal, leaching and oxidative losses. Blanching is done either by dipping in hot water or exposing to steam. In water blanching the loss of water –soluble vitamins increases with contact time, and fat – soluble vitamins are relatively unaffected. Steam Blanching results in greater retention of water – soluble nutrients than water blanching. There is a slight improvement in ascorbic acid retention with IQB (individual quick blanch) as compared to conventional steam blanching. In conclusion, blanching can significantly reduce the nutrient 30 Processing by Heat Application content of foods. The extent of losses of nutrient is dependent on the blanching method and the product. Effect of pasteurization on nutrients The foods having a low pH (mostly below 4.5) are generally pasteurized. Some food products, which receive pasteurization, are summarized in the box given below: Food Products preserved by Pasteurization treatments Sl. Product Pasteurized Temperature (oC) No. 1. Carbonated Beverages 60-65 2. Non Alcoholic fruit drinks 65-70 3. Dill Pickles, Carbonated fruit juices 70-75 4. Apple juice (Holding process) grape juice, 75-80 raspberries, strawberries in syrups in cans or jars 5. Processed and pickled vegetables, wine (U.S.), 80-85 desiccated coconut 6. Apple juice (flash process), canned olives, 85-90 citrus juices, peeled tomatoes (pH 4.1) 7. Tomato puree, citrus juices (flash process) 90-95 tomato juice, peeled tomatoes (pH 4.5), Jam 8. Wine (flash process), fruit puree, fruit juices, 95-100