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.

Engineering Preservation of Food Products by Refrigeration and Freezing

As heating plays important role in preservation of food products, refrigeration and freezing are also important methods for preserving different food products. Hence in heating addition of salt to process water increases its boiling point similarly in freezing addition of salt to water and ice mixture lowers its freezing point. This is basic principle used in food freezing from 1850s. Due to development in this technique value of ice production was increased great extent and researchers started mechanical production of ice. They concluded that evaporation of ether and other volatile liquids showed that ice would form on surface of containers as liquid evaporated.

Ferdinand Carre patented technique of refrigeration with ammonia. This method was finally refined by Dr. Carl Linde in Germany and David Boyle in U.S. in late 19^th century. This safe steady and reliable ammonia compression system adapted all over the world for production of ice, storage of refrigerated and frozen foods and cooling of beer from 1880s. At the same frozen meat shipments started from Australia and New Zealand. During the same period of 19^th century quantitative understanding of laws of thermodynamics was being developed. Those laws of thermodynamics provided a sound scientific basis for understanding the flow of energy fluids and work in refrigeration machinery.

As a food engineer Birdseye developed consumer size packages for the distribution of frozen foods and commercial equipment for rapidly freezing this food material. Meanwhile Birdseye developed plate freezer and belt freezer.  Due to all this development sine 1950 frozen food became a staple in the market place. Food engineers worked on different thermodynamic properties of various freezing methods including cryogenic freezing by liquid nitrogen and solid carbon dioxide.

Engineering Preservation of Food Products by Heat

Food engineering is known to world from past several years but it is improving its impact from last 100 years. Preservation of food by using heat treatment is considered as first invented technique for preserving food or for extending shelf life of food product. In early years this method was constrained by misunderstanding that air caused food spoilage. But in 19^th century several researchers started use of steam i.e. temperature above boiling point of water is used for heat treatment of food products. Further they started use of salt or calcium chloride in process water to increase boiling point solution and to improve heat treatment. That solution of water and salt or calcium chloride is called as brine solution. As calcium chloride gave higher boiling point than common salt and microbial growth in the acidic food is higher than basic so in next few year researchers increased use of calcium chloride instead of common salt.  Hence in food industry for heat preservation of bottled and canned foods use of calcium chloride is increased.

The more useful study of ‘commercial sterilization of low acid foods stored in different containers’ was conducted in last decade of 19^th century in Boston. This study was conducted by Prof. Samuel C. Prescott with Willam Underwood and they concluded that time at temperature needed to obtain commercial sterilization of food products packed in different containers. This study gave proper set up for the preservation of food by heat treatment. This study helped food engineers to develop graphical models for the time and temperature study to obtain commercial sterilization of canned food. During 1925 C. Olin Ball first time used computer based model for heat inactivation kinetics of Clostridium Botulinum. In next decade after this model Stumbo worked on this kinetics of microbial inactivation. The mathematical models developed by these both C. Olin Ball and Stumbo are basis of heat preservation methods used in food processing. 

Food Engineering a Different Branch

Food engineering is a mathematical or engineering based branch of food technology which relates with design, fabrication, modification maintenance and operations of food process engineering equipments. It could be a dryer, an extractor, an evaporator, a condenser or a pasteurizer. This branch depends on other major branches of engineering such as a chemical engineering and mechanical engineering. Hence with all this aspect we can define food engineering is study engineering operations on agricultural products, in which agricultural product or food material is used a raw material to develop a consumable product or edible product. It is a branch related with different aspect of food packaging methods or food preservation methods to enhance shelf life of food.

This is branch which mostly includes following studies;

1.  Removal of moisture or water from fruit or vegetable to enhance its shelf life.
2. Evaporation of water from fruit juices.
3. Extraction of oil from different oil seeds by several methods.
4. Processing of milk and milk products to reduce their microbial load and increase shelf life.

But these operations could be done by chemical engineer also then why food engineering is different than that braches reasons are as given below;

1. Food material contains different components such as proteins, vitamins, fats, carbohydrates, enzymes, minerals and micro –organisms and management of all this components at each time in one process is skill of food engineer.
2. Food material changes their properties at each and every stage of processing such as moisture content reduced at each stage of drying. Hence food engineer have to concentrate on heat transfer as well as mass transfer in same process at same time so it is different from others.
3. Same raw material has different property so food engineer have to consider all properties of same material.
4. One operation in food processing can be done in several ways like drying can be done in tray dryer, tunnel dryer, freeze dryer, rotary dryer or vacuum dryer so to achieve proper conditions good food engineering skills are required.

Hence food engineer have to concentrate on several factors or combinations of different factors at a same time so it is considered as food engineering is a different or distinct branch.

Importance of food process engineering

Food is one of the most important needs of the human being. As world's population is increasing day by day and production of food products is decreasing in comparison with same. Hence there is need of .food processing in this era. After harvesting food products losses are higher due to some major physical, chemical and environmental factors. To reduce these post-harvest losses there is need of food processing. As per different studies and statistical analysis post-harvest losses in the world are about 65 million tonnes. This overall huge amount is worth $12500 million per year. This is one of the major challenges in the world. Hence to minimize these losses is important task to complete requirement of world’s population.

Firstly in order to minimize these losses several operations such as drying, sterilization and high pressure processing etc. should be designed in proper way and this is duty of food process engineer. Second important work of food engineer is to develop a plant design for different raw products to convert it into consumable.

Therefore overall purpose of food process engineer is to design operation process for reduction in post-harvest losses and also to develop operation process for conversion of raw food material into a edible food product as per need. This can be done by using different techniques which may be thermal or non- thermal.