FOOD ENGINEERING APPLICATIONS FST 318 BY SANNI L

FOOD ENGINEERING APPLICATIONS FST 318 BY SANNI, L. O. (Prof)/KAJIHAUSA, O. E Department of Food Science & Technology, University of Agriculture, PMB 2240, Abeokuta, Nigeria.

Course Objectives • To expose the students the relevance of thermo physical properties to food processing. • To teach them the importance of thermal processing and its application in the food industry. • To teach the students theory of food dehydration and train them on the use of different drying equipments. • To expose them to the application of fluid flow to food processing.

Grading • Continuous Assessment Test – CAT - 20% • Examination - 70% • Attendance - 10% • Total - 100%

COURSE OUTLINE • Lecture 1 - Thermophysical Properties • Lecture 2 - Thermal Processing • Lecture 3 - Types of Thermal Treatments • Lecture 4 - Reaction Kinetics During Thermal Processing • Lecture 5 - Food Dehydration Theory and Applications • Lecture 6 - Application of Fluid Flow Theory

LECTURE ONE Thermophysical Properties Thermophysical properties can be simply defined as material properties that vary with temperature without altering the material’s chemical identity. These properties will include thermal conductivity and diffusivity, heat capacity, thermal expansion and thermal relative properties as well as viscosity and mass and thermal diffusion coefficients, speed of sound, surface and interfacial tension in fluids

Relevance to food processing • They have important application in unit operations which involve conduction of heat through food to remove water e. g. drying, frying, freeze- drying. • They are important in the design of manufacturing equipments.

LECTURE TWO THERMAL PROCESSING Thermal processing involves heating food, either in a sealed container or by passing it through a heat exchanger, followed by packaging. Reasons for Heating Foods: • to inactivate pathogenic or spoilage microorganisms. • to inactivate enzymes. • to induce physical changes and chemical reactions, such as starch gelatinization protein denaturation or browning.

Safety and Quality Issues • The two most important issues connected with thermal processing are food safety and food quality. • Quality issues revolve around minimizing chemical reactions and loss of nutrients and ensuring that sensory characteristics (appearance, colour, flavour and texture) are acceptable to the consumer.

• There may also be conflicts between safety and quality issues. For example, microbial inactivation and food safety is increased by more severe heating conditions, but product quality in general deteriorates. To summarise, it is important to understand reaction kinetics and how they relate to: Ø microbial inactivation Ø chemical damage Ø enzyme inactivation Ø physical changes

Product Range The products of thermal processing include those which can be filled into containers and subsequently sealed and heat-treated and those which can be processed by passing them through a continuous heat exchanger.

LECTURE THREE Different types of Thermal Treatments There are three different categories of thermal treatments that have been developed to obtain optimum quality products. Ø In-container processing Ø Aseptic processing Ø HIST (High Temperature short time) processing.

1. In-container (In-can) processing – This is the most conventional concept for thermal processing. It involves placing a product in a hermetic container and then thermally processing the container and product. The thermal process may be: (a) Batch using a resort to provide the heating holding and cooling phases of thermal investment. (b) Continuous, with products rolling on conveyors into a tunnel with three sections (heating, holding, cooling).

In in-can processing, retorting (heat processing) can be achieved through the following ways: v By saturated steam – latent heat is transferred to food when saturated steam condenses on the outside of the container. v By hot water: Food are processed in glass containers or flexible pouches bender hot water with an over pressure of air. v By flame – at atmospheric pressure using direct flame heating of spinning cans.

2. Aseptic Processing In aseptic processing, the product is packed only after processing. It must therefore be transported through equipment where it will be heated, hold at the required temperature for the required time, and then cooled, and it must then be packed in an aseptic environment, into sterilized packages. The advantage stems from the resistance to heat transfer that food products themselves exhibit.

3. HTST (High Temperature Short Time) process Whether the food is processed in-container or aseptically, a HTST (High Temperature Short Time) process would result or significant quality gains and also minimizes energy costs and maximizes productivity. If we assume a constant temperature for treatment, we could use any temperature high enough to kill microbes. The higher the temperature, the shorter the time required. Therefore, if we use a higher temperature we can achieve a microbial safety target in shorter time – the higher the better.

LECTURE FOUR REACTION KINECTICS • When heat inactivation studies are carried out at constant temperature, it is often observed that microbial inactivation follows first order reaction kinetics i. e. the rate of inactivation is directly proportional to the population. • The heat resistance of an organism is characterized by its decimal reduction time (D), which is defined as the time required in reducing the population by 90% or by one order of magnitude or one log cycle, i. e. from 104 to 103, at a constant temperature, T.

• Temperature Dependence Food scientists use a parameter known as the z value, to describe temperature dependence. This is based on the observation that, over a limited temperature range, there is a linear relationship between the log of the decimal reduction time and the temperature. This is used to define the z value of inactivation of that particular microorganism as follows: the z value is the temperature change which results in a tenfold change in the decimal reduction time.

LECTURE FIVE FOOD DEHYDRATION Food drying also called dehydration is the process of removing water from a product in order to reduce considerably the reactions which lead to the product’s deterioration. Removing water from the food product inhibits the growth of microorganisms (enzymes) and bacteria. The water is eliminated by evaporation into the surrounding air.

Drying properly requires mastering three fundamental properties: (i) The added thermal energy which heats the products and set water migrating towards its surface and turning into water vapour. (ii) The capacity of the surrounding air to absorb the water vapour given off by the product. This capacity depends on the percentage of moisture already present in the air before it enters the dryer and on the air temperature. (iii) The velocity of the air going over the products surface, which must be high (up to a certain limit) especially at the beginning of the drying process, in order to take the moisture away rapidly.

Heat and Mass transfer Whatever method of drying employed, food dehydration involves getting heat into the product and getting moisture out. These two processes – heat transfer and mass (water) transfer out are not always favoured by the operating conditions. The following considerations are important in this regard. § § § § Surface area Temperature Air velocity Humidity Atmospheric pressure and vacuum Evaporation and temperature Time and temperature

The Three Phases in a Drying Process • This first phase (in which drying velocity increases) is short, to non-existent, and corresponds to the rise in temperature of the product until it reaches an equilibrium when the product receives as much as heat from the air as it needs to give to the water to vapourise. • Constant-rate period This is known as the constant-rate period and continues until a certain critical moisture content is reached. • Falling-rate period When the moisture content of the food falls below the critical moisture content, the rate of drying slowly decreases until it approaches zero at the equilibrium moisture content (that is the food comes into equilibrium with the drying air). This is known as the fallingrate period.

Drying methods and Equipment. Some of the more common drying methods include drum drying, spray drying, vacuum shelf drying, vacuum belt drying, atmospheric belt drying, freezedrying, fluidized-bed drying, rotary drying, cabinet drying, land drying, tunnel drying and others. Some of these methods are particularly suited to liquids foods and cannot handle solid food process, others one suitable for solid foods or mixtures containing food pieces. One useful division of drier types separates them into air converting driers, drum or roller driers, and vacuum driers, drum or roller driers, and vacuum driers.

LECTURE SIX APPLICATION OF FLUID FLOW THEORY • Basic Fluid Properties. The transport of a fluid (especially liquid) food in a transport system is directly related to liquid properties, primary viscosity and density. These properties will influence the power required for liquid transport as well as the flow characteristics within the pipeline

LIQUID TRANSPORT SYSTEM A typical liquid transport system will consist of four basic components. The liquids products will be contained in some vessels is the conductor pipeline for liquid flow. Unless flow can be achieved by gravity, the third primary component is the pump, where mechanical energy is used to enhance product transport. The fourth components of the system is the valve used to control or direct flow. The vessels used in these types of system may be of any size and configuration.

Pipelines for Processing Plants The pipelines used for liquids foods components have numerous unique features. Probably the most evident feature is the use of stainless steel for construction. This metal provides smoothness, clean ability and corrosion prevention. The corrosion resistance of stainless steel is attributed to “passivity” the formation of a surface film on the metal surface when exposed to air.

Types of Pumps Within the exception of situations where gravity can be used to move liquid product or product components, some type of mechanical energy must be introduced to overcome the forces opposing transport. The two most popular types of pumps in the food industry are centrifugal and positive displacement. There are variations of each type, but the concept of operation for each type is the same.

1 centrifugal pumps Ø volute type Ø turbine type 2 positive displacement pumps Ø reciprocating- I piston pump II diaphragm Ø rotary pumps- I lobe pump II gear III screw IV peristaltic