Thermal Food Engineering Operations. NITIN KUMAR

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СКАЧАТЬ and S.N. Naik

       Centre for Rural Development and Technology, IIT Delhi, Delhi, India

       Abstract

      Heat utilization in food processing is a century-old tradition utilized for the preservation, extraction, cleaning of different food products. Food quality and safety are the two major concerns in the industrial sector besides targeting different microorganisms found in the food. Techniques for food preservation are always improving at an increasing pace to avoid degradation of the food product, and novel methods to improvise microbial inactivation are also trending. With the rising customer demands for fresh products, industries are giving special attention to the heat resistance of spores from sensitive cold growing spores and finding ways for minimal processing with an amalgamation of facilities for cold storage and improved shelf life. Intensive studies were investigated for developing new alternative conventional techniques which look after better preservation of food, maintaining the color, texture, nutritional quality, and flavor of the food product. In the accumulation of environmental and food safety concerns, the techniques which have evolved as an innovative thermal preservative method in agricultural and food applications are ohmic heating, radiofrequency heating, microwave heating, infrared heating, and instant control pressure drop technology.

      Keywords: Food quality, minimal processing, ohmic heating, food safety

2.1 Introduction

Food quality and safety have a long history and are continuing as the major concerns for preservation and targeting microorganisms. The chief cause for any type of deterioration of food products is microorganisms, and therefore inhibition of these are targeted using different methods by the food industries [1]. Varied techniques of food preservation have been utilized for centuries to avert poisoning and degradation of the food. With the passing phase, the new models and techniques are evolved promising to avert the degradation by inactivating microorganisms. But then again slowing down or terminating the growth of microorganisms will not ensure the proper safety of the food until and unless the environment is taken care of, like maintaining a cold chain throughout. Processing the food by application of heat has extended antiquity for the inactivation of microorganisms and is still considered as the utmost significant method of preservation [2]. Louis Pasteur introduced pasteurization in the late 19th century to eradicate spoilage-causing organisms and with the passing of time, new science-based heat processing emerged.

      In the old heating process, the stability of the microbes was often quite high but the knowledge of the kinetics for different microbes was relevant. Traditional canning methods aim at the destruction of all spores (sterilization) or of all spores that can grow in the container below 40°C (commercial sterilization or appreciation) [3]. With the rising customer request for fresh products, industries are giving special attention to the heat resistance of spores from sensitive cold growing spores and finding ways for minimal processing with an amalgamation of facilities for cold storage and improved shelf life [4]. Quite a few sterilization methods have already been utilized by most of the food industry, the objective of which is to inactivate the microorganism from which the most common form of destruction is targeting the DNA of the microbial cell and destroying it in both liquid and solid food products through denaturation [5] did a study and concluded that the denaturation process requires high heat for 15 s at 71.06 °C accompanied by drying with air inlet temperature extending from 135 to 205 °C for 5–6 s.

Several works have been carried out during the past decades aiming to inhibit the growth of the foodborne pathogens in varied products. However, some of the traditional thermal treatments, al though they carefully inactivate all the microbes and extend the shelf life of the food, are still not favored as they cause degrading effects to the nutritional properties of the food product such as deterioration of the antioxidant compounds, flavor, proteins vitamins and volatile oils [6]. Therefore, intensive studies were investigated for developing new alternative conventional techniques which offer better preservation of food, maintaining the color, texture, nutritional quality, and flavor of the food product. In the accumulation of environmental and food safety concerns techniques which evolved as an innovative thermal preservative method in agricultural and food applications are ohmic heating, radiofrequency heating, microwave heating, infrared heating, and instant control pressure drop technology [7–9]. These physical factors can cause the inactivation of microorganisms at ambient or sublethal temperatures.

      The major objective is to gather the knowledge of all the techniques, mechanisms of inactivation, and factors affecting the physical and orogenetic parameters of the particular food products. Furthermore, predicting kinetics and microbial nature during the application of varieties of different techniques through mathematical modeling is also considered the most vital tool in the food matrix. Therefore, the chief goal of this chapter is to provide a record that will provide data of the food degrading organisms, its quantitative behavior portrayal for different food matrix with varied environmental setup and besides with the pre-assumed mechanism of different alternative thermal techniques involved.

      The final goal, however, is to answer the question of whether the microorganisms are inactivated after nonthermal processing. Although mathematical descriptions of heat inactivation date already from the early 20s, the development of new software during the last decades enables a better description of inactivation kinetics under different conditions. Combined with better control of temperature in each container a milder heat process can be designed without compromising microbial safety and stability [10]. The principles of modeling death kinetics will be addressed within the framework of quantitative microbiological risk assessment. The effect of environmental conditions and their extrapolation to real food situations will also be discussed.

2.2 Innovate Thermal Techniques for Food Reservation

      Pasteurization and sterilization are the leading and finest processing techniques in the industrial sector. These techniques stop the rapid multiplication of spoilage and microbial organism present in the food and eliminate it by the application of heat in an acidic medium [11]. Temperature also plays a vital role in the thermal processing method. The higher the temperature of the food product while entering the process, the more speedily it helps in killing the microbes present in the food [12].

Though high-temperature traditional processing confirms the microbial safety of the food products by reducing it into 3 log cycle as per the 21 Code of Federal Regulation Part 114 [13], the limitations which arise during the application in terms of organoleptic and nutritional properties compels to limit the usage of these traditional techniques. These limitations also extend to non-uniform heating, degradation of the final quality of product, and low heat transfer. But currently, potential heating systems have come up as an alternative to replace the exciting conventional system, some of which are microwave heating, radiofrequency heating, ohmic heating, infrared heating, and instant control pressure drop technology (DIC) [14]. These alternative techniques of heat processing provide the advantages of volumetric and uniform heating while maintaining the quality of the final product and reducing the surplus treatment time by heating directly the food sample [15].

      Few bacteria display an advanced heat resistance, subsequently being exposed to temperatures which only stress them [16, 17]. The growth medium of the organism, the growth temperature, and the phase of growth are significant issues concerning their aptitude to withstand heat. It is a very common phenomenon that the bacteria surviving different stress conditions have quite a high tolerance level to the different environmental situations and heat [18]. Inactivation does not mean that all the microbial cells are destroyed but it depends upon the number of cells present in the food sample. The maximum number of cells indicates the increased consumption of time for inactivation of the microorganisms in the food sample. The design of the thermal inactivation process for a given food depends on (i) the heat resistance of the most resistant microorganism (in the product); (ii) the food products dimensions; and (iii) thus the rate of heat penetration within the food matrix [19].

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