Emerging Food Processing Technologies 1

Microwave Heating:

Microwaves, which are a part of the electromagnetic spectrum and have a frequency range between 300 MHz and 300 GHz, have been extensively employed to various food processing such as drying, sterilization, tempering, and cooking because MW heat foods in a rapid and direct manner. Frictional energy resulting from molecular dipole moments and conductive migration of charged ions in the alternating electromagnetic field can instantly generate heating inside food material.

Dielectric properties play a key role in understanding the interaction between electromagnetic fields and the components of foods. Dielectric properties of food products are mainly affected by frequency, temperature, and moisture contents. The domestic microwave oven has become the most useful home appliance owing to simple operation and rapid heating or thawing of “ready to eat” foods.

Radio Frequency Heating:

Similar to Microwave heating, radio frequency heating, which can be classified as dielectric heating, which rapidly heats up solid phase or semisolid phase food products. The distinct differences between Microwave Wave and Radio Frequency include different regions of the electromagnetic spectrum and frequency ranges. Radio Frequency energy can be generated by passing alternating or direct current thorough food samples located between two electrodes without direct or mechanical contact as shown in figure below, because the electromagnetic field converted from the high electric field is able to stimulate the migration of ions within food products .

Ohmic Heating:

On the contrary to Radio Frequency heating, internal heat dissipation during ohmic heating can be generated by applying an alternating current through food products with direct contact to two electrodes. Ohmic Heating has made considerable contributions to thermal uniformity improvements in single-phase foods. The energy conversion efficiency during OH process is remarkably high as compared with other emerging thermal processing methods. The rate of ohmic heating can be determined by the square of the applied electric field strength and the electrical conductivity of the food.

Importance of Microorganisms in Sterilization and Pasteurization

The main aim behind sterilization and pasteurization process is to inactivate microorganisms and enzymes that cause spoilage and particularly which causes food poisoning. 

Thus the principal reason for characterizing the heat resistance of microorganisms is in order to design a safe sterilization step. 

The goal is to determine the required operating conditions (time/temperature) to achieve sterilization criterion.

One of the major factors that affects a microorganism’s heat resistance is pH. It is possible to classify food products into three groups according to pH:

• Low - acid products: pH ≥ 4.6
• Medium - acid products: 3.7 ≤ pH ≤ 4.6
• Acid products: pH ≤ 3.7

 This is important because the heat resistance of microorganisms is greater at this pH ( ≥ 4.6). On the other hand, fruits,juices and most soups are medium - acid or acid products and require a much softer heat treatment to achieve the sterilization criterion.

1. Vegetative cells 10 min at 80 °C
2. Yeast ascospores 5 min at 60 °C
3. Fungi 30–60 min at 88 °C
4. Bacillus stearothermophilus 4 min at 121.1 °C
5. Clostridium thermosaccharolyticum 3–4min at 121.1 °C
6. Clostridium botulinum spores 3min at 121.1 °C
7. Clostridium botulinum toxins types A and B 0.1–1min at 121.1 °C
8. Clostridium sporogenes 1.5 min at 121.1 °C
9. Bacillus subtilis 0.6 min at 121.1 °C

Need of Non Thermal Processing

Nowadays, most of the liquid foods are processed commercially by Ultra High Temperature (UHT) or High Temperature Short Time (HTST) processes.Though heating liquid foods at high temperature  inactivates enzymes and microorganisms, the organoleptic as well as nutritional properties of the food affect due to denaturation of protein with the loss of vitamins. Hence there is a demand for a non-thermal method processing which is economical, compact, energy efficient, safe, socially and environmentally acceptable and which does not adversely affect nutrition, texture and flavour of the treated food. Consumers are also eagerly claiming high quality, minimally processed foods.

The term ‘non-thermal processing’ is more important for novel non-thermal technologies such as high-pressure processing (HPP), pulsed electric field (PEF), high-intensity ultrasound, ultraviolet light, pulsed light, ionizing radiation and oscillating magnetic fields, intended for microbial inactivation. 

Mass Transfer

Mass transfer is one of the fundamental phenomenons that occur during processing of food products or storage of agricultural products. Mass transfer mostly pertain to migration of moisture in porous food material due to an external stimulus such as convective drying, infrared heating, microwave heating, water absorption by food materials etc. Unsteady state denotes a condition where the movement of moisture occurs due to either convection or diffusion changes with time. Mass transfer can occur in the multicomponent form for different components in food materials or can occur simultaneously with heat transfer.

Liquid transfer are not only due to volumetric liquid concentration gradient but also due to temperature gradient additionally; when liquid is present in vapor form an additional transfer is also possible. Mathematical formulation of mass transfer has been attempted with number of different physical models. Unsteady state coupled transfer problems described by non linear partial differential equations, for mass transfer and heat transfer are more relevant for lower density and high moisture foods.

Mass transfer contributes to a change in mechanical conformation which in turn affects mechanical properties. Temperature induced mass transfer also affects chemical reaction such as gelatinization, enzyme kinetics, reaction kinetics etc. Coupling the physical process of heat and mass transfer with mechanical changes in relation to chemical interactions makes this study complex and necessitates several assumptions for a practical implementation. 

Heat Transfer

Heat is energy transferred between materials due to temperature differences. There are three mechanisms which are responsible for thermal energy transfer conduction, convection and radiation. These mechanisms occur individually or in combination.

1. Conduction:-

                Conduction is process of heat transfer by molecular transport and microscopic interactions. Conduction can occur in solids, liquids and gases but usually is only a major contributor in heat transfer through solids. Conduction through material is explained by Fourier’s  Law.

2. Convection:-

                Convection is the process of heat transfer by macroscopic movement of molecules. There are two mechanisms responsible for convection

                1. Molecular Diffusion:-

                                It involves random motion of molecules due to their internal energy content.

                2. Macroscopic Motion:-

                                It occurs because of either forced movement of fluids or natural convection that results                       from density changes due to temperature difference.

3. Radiation:-

                In most systems radiation is seldom a significant contributor to the total heat flux; however when vacuum is present or extremely large temperature difference greater than 100°C exist,  radiation effects should be considered. There are many systems where radiation from sun is the only contributor such as solar drying of grass, fruit or lumber.  

                Radiation is the energy transfer by electromagnetic waves which can include both visible as well as invisible. Radiation does not require a medium such as fluid or solid to transfer energy. Radiation can be reflected, absorbed or transmitted between surfaces. A black body is defined as a material that can absorbs 100% of incident radiation and reflects none. Emissivity of any body is important factor used during radiation. Emissivity is defined as ratio of emitted energy by a material to that of a black body at same temperature. For most non-metallic materials emissivity ranges from 0.90-0.95 and for metallic materials emissivity varies from 0.02-0.9 depending on surface finish and material composition.

Types of Blanching

1. Water Blanching:

                It is method performed in hot water at a temperature ranging from 70 to 100°C. But in this type combination of low temperature long time and high temperature short time have also been used. Water blanching usually results in a more uniform treatment than others. For automatic processing different water blanchers are developed in that basically screw or chain conveyor used to transport product inside tank where hot water is filled. Water is heated indirectly with steam in a heat exchanger so steam quality does not need to be a “food grade”. Main disadvantage of this method is it takes longer processing time so it results in increased leaching of minerals and nutrient such as vitamins and also produces large amount of effluent with large BOD.

2. Steam Blanching:

                In this type product is transported by chain or screw conveyor through chamber where a “food grade” steam is directly injected at 100°C. Steam blanching is usually used for cut and small products and it requires less time than water blanching and losses are also less. Due to temperature gradients between surface and center of food products uneven blanching takes place and which results in “overblanched” near surface and “underblanched” in center. To increase efficiency forced convection blancher has been developed. This technology allows higher product bed depths and higher product throughputs. To reduce product non-uniformities another technology is developed which is called as individual quick blanching.

3. Microwave Blanching:

                It is developed because of important findings were retention of ascorbic acid and carotene and require very short processing time compare to others. To overcome some drawbacks on disadvantages it has been combined with water blanching.

4. Gas Blanching:

                Hot gas blanching using combustion of flue gases with addition of steam to increase humidity and product dehydration has been studied. This type of blanching has the advantage of reducing waste production and retention of nutrients. But major disadvantage of this is it results in product weight loss. This approach not currently used in industry and needs further research.

Blanching: An Important Unit Operation

Introduction:-

Blanching is an important unit operation in a food processing and done as pretreatment before freezing, canning or drying. This technique is done for the purpose of inactivation of enzymes, modification of texture, removal of trapped air and preservation of color, flavor and nutritional value. Hot water blanching and steam blanching are most commonly used methods in industry but nowadays microwave and hot gas blanching have also been used. Different hot water and steam blanchers are developed to improve its quality efficiency and to improve processing technique with different thermal properties as well as geometries.

Principles and Equipment:-

Studies on effects of blanching as a pretreatment prior to freezing have been reviewed in late in late 1920s and early 1930s. Most vegetables blanched prior to freezing to in activate enzymes that cause developments of off flavors and off colors during freezing. Some exceptions include onions, peppers and leaf because they lose color and flavor during blanching. Gas removal is an important purpose of blanching before canning because it allows easier can fill, reduces strain on can during heating and reduces can corrosion. Fruits are usually not blanched or blanched under mild conditions prior to freezing because blanching produces undesirable texture changes.

Before drying fruits and vegetables are sometimes blanched. After blanching vegetables are quickly chilled by spraying cold water or by conveying them through a flume of cold water. Blowing dry air has also been used to take advantage of evaporative cooling using water adhered to the surface of product.