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FDFTEC4004AApply basic process engineering principles to food processing

PARTICIPANT SELF LEARNING RESOURC

ContentsIntroduction and Unit Details ………….………………………………………...... 4

Element 1 Map a production process …………………………………………. 7

Map a production process - flow chart……….………………………….……….… 9

Creating a flow chart …….………………………………………………….……….. 12

Element 2 Calculate yields and efficiencies of a production process …. 15

Calculate yields and efficiencies of a production process ………….……….… 16

Element 3 Apply principles of fluid flow to a production process ……. 21

Flow of fluids ……………………………………………………………………… 22

Principles of fluid flow …………………………………………………………… 24

Properties of fluids ……………………………………………………………….. 28

Pumping ……………….…………………….………………………….……….… 31

Pumping systems ………………………………………………………….……….. 38

Element 4 Apply principles of heat transfer to a production process …. 45

Principles of heat transfer …………………………………………………………. 46

Conduction calculations ………………………………………………………….. 47

Convection calculations ………………………………………………………….. 50

Sources of heat & methods of application to foods ………………...……….… 58

Effect of heat on micro-organisms ……………………………………………….. 63

Effect of heat on the nutritional and sensory properties of food …………….. 72

Moisture content and water activity ………………………………………………. 76

Properties of heat and steam ………………………………………………………. 81

Retorting …………………………………………………………………………….. 82

UHT …………………………………………………………………………………… 87

Chilling ………………………………………………………………………………. 93

Effects of chilling ………………………………………………………………..…. 94

Chilling equipment …………………………………………………………………. 96

Chilling processes …………………………………………………………………. 98

Freezing ……………………………………………………………………………. 100

Effects of freezing ……..………………………………………………………….. 105

Freezing equipment ………………………………………………………………. 107

Freezing equipment efficiencies ………………………………………………… 111

© Food-Wise Training SolutionsVersion 1.1 of 172

FDFTEC4004AApply basic process engineering principles to food processingPARTICIPANT SELF LEARNING RESOURCE

Element 5 Apply principles of evaporation to a production process …. 112

Principles of evaporation ……………………………………………………..…… 113

Heat and mass transfer calculations ………………………..…………………… 115

Evaporation equipment …………………….……………………..………….….… 124

Effect of evaporation on food …………………………………..…………………. 131

Determination of Moisture content and total solids …………………………….. 132

Element 6 Apply principles of drying to a production process ……....…. 134

Principles of dehydration ………………………………………………….……… 135

Mechanism of drying ………………………………………………………………. 136

Psychrometrics ………………………….……………………………….……….… 138

Calculation of drying rate ………………………………………………………….. 141

Drying equipment ……………………………………………………….…………. 147

Effect of drying on food ……………………………………………….…………… 155

Element 7 Apply principles of process control to management of production processes …………………………………………………………………………. 158

Process control systems …………………….………………………….……….… 159

Appendix - Formulae …………………………………………………….……….. 163

Activity Answers……………………………………………….……………………. 164

Image Credits……………………………………………………………………….. 171

© Food-Wise Training Solutions of 172 Version 1.1



Map a production process - flow chartA flowchart allows you to identify the actual flow or sequence of events in a process that any product or service follows. Flowcharts can be applied to anything from the travels of an invoice or the flow of materials. It is a diagrammatic way of picturing or mapping a process

A flowchart shows the process’ complexity, problem areas, redundancy, unnecessary loops and where simplification and standardization may be possible. It compares and contrasts the actual versus the ideal flow of a process and so helps identify improvement opportunities. It allows a team to come to agreement on the steps of a process and to examine which activities may impact the process performance. A process flowchart can serve as a training aid to understand the complete process.

Flow Chart SymbolsBelow there are two different systems for the symbols used in flow charts. The first is primarily used by engineers; the second, used in HACCP plans.

Engineering Symbols


A

An oval is used to show the materials, information or action (inputs) to start the process or to show the results at the end (output) of the process.

A box or rectangle is used to show a task or activity performed in the process. Although multiple arrows may come into each box, usually only one output or arrow leaves each activity box. A diamond shows those points in the process where a yes/

no question is being asked or a decision is required.A circle with either a letter or number identifies a break in the flow chart and that it is continued elsewhere on the same page or another page.

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Calculate yields and efficiencies of a production process

Image

The law of conversion of mass states, that the mass of a material entering a process equals the mass of material leaving. This has applications in, for example in mixing, fermentation, evaporation and dehydration. In general a mass balance for a process takes the form:

MASS OF RAW MATERIALS IN = MASS OF PRODUCTS

AND WASTES OUT + MASS OF STORED MATERIALS + LOSSES

Similarly an energy balance states that the amount of heat or mechanical energy entering a process equals the total energy leaving with the products and wastes + stored energy + energy lost to surroundings. If heat losses are minimized then energy losses to the surroundings may be ignored for approximate solutions to calculations of, for example, the quantity of steam, hot air or refrigerant required. For more accurate solutions compensations should be made for heat losses.

Efficiency FormulaEfficiency generally refers to how far we are getting the particular outcome for the given input with as less waste as possible.

Efficiency is defined as the ratio of useful work done to the heat or energy absorbed by the particular system. It is denoted by η, (the Greek small letter eta).1

Efficiency formula can be used to calculate the efficiency for any given input. It is unit-less (i.e no meters, kilograms, Joules etc) and is expressed as a percentage.

However the term ‘efficiency’ is frequently used in the context of energy and energy usage and formula in the terms of energy is given by:

η= energy outputenergy input

× 100 %

1 (Pearson, 2014)


Boundary filmPipe walls

Boundary filmPipe walls

Laminar Flow

Turbulent Flow



Properties of FluidsImage

In any system where fluids flow, there exists a boundary film (or surface film) of fluid next to the surface over which the fluid flows. The thickness of the boundary film is influence by a number of factors including velocity, viscosity density and temperature of the fluid. Fluids which have a low flow rate or high viscosity may be thought of as a series of layers which mover over one another without mixing. This produces movement of the fluid in a single stream which is termed laminar (or streamline) flow. Ina pipe the velocity of the fluid is highest at the centre and zero at the pipe wall. Above a certain flow rate which is determined by the nature of fluid and the pipe, the layers of liquid mix together and turbulent flow is established in the bulk of the fluid (the flow remains laminar in the boundary film.

The boundary film tends to be thicker in a laminar flow situation than a turbulent flow.



Activity 6What pressure must be generated at the discharge of a pump that delivers 100 L/minute of a fluid having a

specific gravity of 1.02 (ie density of 1020 kg/m3) and a viscosity of 100 centipoise. 2

Additional information: Pipe internal diameter = 0.0356m Length of pipe = 50m The pipe is straight and level Discharge end of the pipe is at atmospheric pressure There are 0.001Pa.s per centipoise

Hints: To determine the appropriate equation for calculating the pressure drop, it is necessary to calculate the Reynolds number to establish if the flow is laminar or turbulent.

2 (Toledo, 1980)




Gear pumps are the simplest type of rotary pumps, consisting of two gears laid out side-by-side with their teeth enmeshed. The gears turn away from each other, creating a current that traps fluid between the teeth on the gears and the outer casing, eventually releasing the fluid on the discharge side of the pump as the teeth mesh and go around again. Many small teeth maintain a constant flow of fluid, while fewer, larger teeth create a tendency for the pump to discharge fluids in short, pulsing gushes.

Images &

Screw pumps are a more complicated type of rotary pumps, featuring two screws with opposing thread —- that is, one screw turns clockwise, and the other counterclockwise. The screws are each mounted on shafts that run parallel to each other; the shafts also have gears on them that mesh with each other in order to turn the shafts together and keep everything in place. The turning of the screws, and consequently the shafts to which they are mounted, draws the fluid through the pump. As with other forms of rotary pumps, the clearance between moving parts and the pump's casing is minimal.

Moving vane pumps are the third type of rotary pumps, consisting of a cylindrical rotor encased in a similarly shaped housing. As the rotor turns, the vanes trap fluid between the rotor and the casing, drawing the fluid through the pump.

Image



Principles of Heat TransferImage

Instead of considering foods on a commodity basis, food processing can be rationalised and the foods more conveniently dealt with according to groups in which there is a common, physical, chemical or conversion activity. The unit process of ‘heat processing’ embraces baking, boiling, frying, grilling, blanching and other conversion activities where the application of heat is used primarily to effect chemical changes in the food. 3

Many unit operations in food processing involve the transfer of heat into or out of a food. There are three ways in which heat may be transferred: by radiation, by conduction and by convection.

1. Radiation is the transfer of heat by electromagnetic waves: for example in an electric grill.

2. Conduction is the transfer of heat by direct transfer of molecular energy within solids (for example through metal containers or solid foods)

3. Convection is the transfer of heat by groups of molecules that move as a result of differences in density (for example in heated air) or as a result of agitation (for example stirred liquids.

In the majority of applications all three types of heat transfer occur simultaneously but one type may be more important than others.

Steady-state heat transfer takes place when there is a constant temperature difference between two materials. The amount of heat entering a material equals the amount of heat leaving, and there is no change in temperature of the material. This occurs for example when heat is transferred through the wall of a cold store if the store temperature and ambient temperature are constant and in continuous processes once operating conditions have stabilised. However in the majority of food processing applications the temperature of the food and/or the heating or cooling medium are constantly changing and unsteady-state heat transfer is more commonly found. Calculation of heat transfer under these conditions are extremely complicated but are simplified by making a number of assumptions and in some cases using prepared charts to give approximate solutions. 4

3 (Brennan, Butters, Cowell, & Lilly, 1981)4 (Fellows P. J., 1997)


21oC

Air0.8 mm

Boundary films

Glass panes each 1.6 mm thick

-15oC

R1 R

2

R4 R

5

R3



ExampleCalculate the rate of heat transfer across glass panes which consists of two 1.6mm (0.0016m) thick glass separated by 0.8 mm (0.0008m) layer of air. The heat transfer coefficient on one side which is at 21 oC is 2.84 W/m2.K andOn the opposite side which is at -15 oC is 11.4 W/m2.KThe thermal conductivity of the glass is 0.52 W/m.K andThe thermal conductivity of air is 0.031 W/m.K.

Solution:

Using the diagram (right) there are 5 resistances to heat transfer:

R1 = the convective resistance to one surface exposed to air.

R2 = the conductive resistance of one layer of glass

R3 = the conductive resistance of the air layer between the glass

R4 = the conductive resistance of the other layer of glass

R5 = the convective resistance of the opposite surface exposed to air.

1U

= 1h1

+x2

k2+

x3

k3+

x4

k4+ 1

h5

1U

= 12.84

+ 0.00160.52

+ 0.00080.031

+ 0.00160.52

+ 111.4

1U

=0.352+0.0031+0.0258+0.0031+0.0877

1U

=0.4718

U =2.12W /m2 . K

As we don’t know the size (square meterage (A)) of this double glazing then the

formulation is transposed to: QA

=U (θa−θb )So the rate of heat transfer is:



QA

=2.12 (21−(−15 ) )=76.31W /m2




Properties of heat and steamImage

When a liquid evaporates its go through a process where: the liquid heats up to the evaporation temperature then; the liquid evaporates at the evaporation temperature by changing state from fluid to

gas then; the vapor heats above the evaporation temperature - superheating

The heat transferred to a substance when the temperature changes is often referred to as sensible heat. The heat required for changing state (or phase), such as evaporation is referred to as latent heat.

Sensible heat is energy that is related to a change in temperature. For example when apple sauce is heated (or cooled) from 20oC to 70oC. The term is used in opposition to latent heat.

Latent heat refers to the amount of energy released or absorbed by a chemical substance during a change of state that occurs without changing its temperature, meaning a phase transition such as the melting of ice or the boiling of water. For example the energy released by a phase change, such as the condensation of water vapour, with little or no change in temperature.

Energy needs to be absorbed (endothermic) for the following phase changes:Solid to Liquid to Gas

Energy is released (exothermic) for the following phase changes:Gas to Liquid to Solid

It is this (exothermic) release of energy as to why you are more likely to be burnt from the steam of a boiling kettle than putting your hand in a (dry) oven, both of which are at 100oC.

The names used to describe the flow of energy are:Phase Change Common term Thermodynamic term

Solid to liquid Melting Latent heat of fusionLiquid to gas Boiling / evaporation Latent heat of vaporisationGas to liquid Condensation Latent heat of condensationLiquid to solid Freezing Latent heat of sublimation



ChillingImage

Chilling is an operation in which the temperature of a food is reduced to between -1oC and 8oC. It is used to reduce the rate of biochemical and microbiological changes and to extend the shelf life of fresh and processed foods.

The relationship frequently observed between the reaction rate and temperature is similar to that described in connection with the thermal destruction of bacterial spores by moist heat. That is, the logarithm of the reaction rate is a linear function of temperature. In chilling the reaction rate is more frequently measured in terms of Q10 (the ratio of the rate at one temperature to that at the temperature 10oC lower) rather than by the z value. The Q10 for many chemical reactions is about 2, that is the reaction rate approximately doubles for each 10oC temperature rise. However the rate of respiration of harvested fruit and vegetables is about Q10 = 2.5, while the quality deterioration of frozen strawberries is about 10 times greater than this.5

Chilling is often used in combination with other unit operations (e.g. fermentation, baking and pasteurisation) and preservation systems (e.g. packaging and modifying the composition of the storage atmosphere) to extend the shelf life of mildly processed foods. Synergies can be achieved and less severe unit operations used when combined with chilling. This enhanced preservation effect is also found with the following systems:

Controlled atmosphere storage Modified atmosphere storage Modified atmosphere packaging Cook-chill (especially those involving vacuum packaging)

The successful supply of chilled foods to the consumer is heavily dependent on sophisticated distribution systems which involve chill stores, refrigerated transport and chilled retail display cabinets. In particular, low-acid chilled foods, which are susceptible to contamination by pathogenic bacteria (e.g. fresh and pre-cooked meats and unbaked dough) must be prepared and packaged under strictly controlled hygienic conditions. 6

The factors that control the shelf-life of fresh crops in chiller storage include:1. the type of food and variety2. the part of the crop selected (the fastest growing parts have the highest metabolic

rates and the shortest storage lives)3. the condition of the food at harvest (damage, contamination & degree of maturity)4. temperature of distribution and retail display5. the relative humidity of the storage atmosphere (which influences dehydration

losses) 7

5 (Brennan, Butters, Cowell, & Lilly, 1981)6 (Fellows P. J., 1997)7 (Fellows P. J., 1997)




Heat and mass transfer calculations

Below is a schematic diagram of steady state operation of an evaporator:

Where:mf (kg/s) mass transfer rate of feed liquormp (kg/s) mass transfer rate of productXf solids fraction of feedXp solids fractions of productmv (kg/s) mass transfer rate of vapour producedms (kg/s) mass transfer rate of steam usedΘf (oC) initial feed temperatureΘb (oC) boiling temperature of foodΘs (oC) temperature of steam


Condensate

Steam ms

Temperature θs

Product liquor m p Solids fraction xp

Liquid feed mf

Solids fraction xf

Temperature θf

Vapour out mv

Temperature θb


Evaporation Equipment1. Natural Circulation Evaporators

Open or closed pan evaporators. Consist of a hemispherical pan heated directly by gas or electrical resistance wires or heated indirectly by steam passed through internal tubes or an external jacket. They are fitted with a lid for vacuum operation and stirrer or paddle to increase the rate of heat transfer and to prevent food from burning onto the pan. They have relatively low rates of heat transfer and low energy efficiencies and they cause damage to heat-sensitive foods. However they have low capital costs, are easy to construct and maintain and are flexible for application where frequent changes of product are likely or when used for relatively low or variable production rates. 8

Short tube evaporators. This type of equipment is an example of a tube and shell heat exchanger. It is also used for pasteurisation, heat sterilisation and freezing. It consists of a vessel (or shell) which contains a bundle of tubes (vertical or less commonly). The feed liquor is heated by steam condensing on the outside of the times. Liquor rises through the tubes, boils and recirculates through a central downcomer. These evaporators have low construction and maintenance costs, high flexibility and higher rates of heat transfer than open or closed bans, when used with relatively low-viscosity liquids. They are generally unsuited to high-viscosity liquors as there is poor circulation of liquor and a high risk that the food burns onto the tube walls, with consequent problems of heat damage, low rates of heat transfer and difficulty in cleaning. Widely used for concentrating syrups, salt and fruit juices.

Image

External calandria evaporators. These are tube and shell heat exchangers which are, fitted with an external pipe for recirculation of product. This way, convection currents are established to produce relatively high rates of heat transfer. The calandria is easily accessible for cleaning. They are suitable for concentrating heat-sensitive foods, including dairy products and meat extracts, when operated under partial vacuum.9

8 (Fellows P. J., 1997)9 (Fellows P. J., 1997)


Multiple effect vertical short tube evaporator



Mechanism of dryingWhen hot air is blown over wet food, heat is transferred to the surface and latent heat of vaporisation causes water to evaporate. Water vapour diffuses through a boundary film of air and is carried away with by the moving air.

This creates a region of lower water vapour pressure at the surface of the food and a water vapour pressure gradient is established from the moist interior of the food to the dry air. This gradient provides the ‘driving force’ for water removal from the food. Water moves to the surface by the following mechanisms:

1. liquid movement by capillary forces

2. diffusion of liquids caused by differences in the concentration of solutes in different regions of the food

3. diffusion of liquids which are adsorbed in layers at the surfaces of solid components of the food

4. water vapour diffusion in air spaces within the food caused by vapour pressure gradients.10

Consider an inert solid, wetted with pure water is being dried in a current of heated air flowing parallel to the drying surface. Assume the temperature and humidity of the air above the drying surface remain constant throughout the drying cycle and all the necessary heat is supplied to the material by convection. If the change in moisture content of the material is recorded throughout the drying process, the data can be presented in the form of curves. A study of this curve shows that the drying cycle can be considered to consist of a number of stages 11

A-B Represents a ‘settling down’ period during which the solid surface conditions come into equilibrium with the drying air. This is often a negligible proportion of the overall drying cycle. 12

B-C Known as the constant rate period of drying. During this period the surface of the solids remains saturated with liquid water by the movement of water within the solids to the surface taking place at a rate as great as the rate of evaporation from the surface. Drying takes place by movement of water vapour from the saturated surface through a stagnant air (boundary) film into the main stream of drying air. The rate of drying is dependent on the rate of heat transfer to the drying surface. The rate of mass transfer is balanced with the rate of heat transfer and so the temperature of the drying surface remains constant. The surface of the solid can be compared to the wick of a wet-bulb thermometer. 13

10 (Fellows P. J., 1997)11 (Brennan, Butters, Cowell, & Lilly, 1981)12 (Brennan, Butters, Cowell, & Lilly, 1981)13 (Brennan, Butters, Cowell, & Lilly, 1981)



Activity 16Diced potato is dried by air at 80oC with a relative humidity of 10% and blown perpendicularly through the mesh rack at

0.5m/s. The potato is a cube 10mm x 10mm x 10mm.

Calculate the drying rate for the constant period (using equation 6.3)

The diced potato is in a single layer covering 90% of the mesh rack/s. There are 10 racks with each rack 60cm x 85cm. Latent heat of evaporation is 2300 000 J/kg




Effects of drying on foodTexture / Shrinkage / Case hardeningChanges to the texture of solid foods are an important cause of quality deterioration. The nature and extent of pre-treatments (e.g. the addition of calcium chloride to blancher water), the type and extent of size reduction and peeling, each affect the texture of rehydrated fruits and vegetables. Foods that are adequately blanched, loss of texture is caused by gelatinisation of starch, crystallisation of cellulose, and localised variations in the moisture content during dehydration, which set up internal stresses. These rupture, compress and permanently distort the relatively rigid cells to give the food a shrunken shrivelled appearance. On rehydration the product absorbs water more slowly and does not regain the firm texture associated with the fresh material.14

Animal and vegetable tissue undergo some degree of shrinkage during drying by all the drying methods, with the possible exception of freeze-drying. The bulk density and porosity of dried vegetable pieces depends largely on the drying conditions. At high initial drying rates the outer layers of the pieces become rigid and their final volume is fixed early in the drying. As drying proceeds, the tissues split and rupture internally forming an open structure. The product in this case has a low bulk density and good rehydration characteristics. At low initial drying rates the pieces will shrink inwards to give a product a high bulk density. Shrinkage of foodstuffs during drying may influence their drying rates because of the changes in drying surface area. 15

The rate and temperature of drying have a substantial effect on the texture of foods. Generally, rapid drying and high temperatures cause greater changes than do moderate rates of drying and lower temperatures. As water is removed during dehydration, solutes move from the interior of the food to the surface. The mechanism and rate of movement is specific for each solute and dependant on the type of food and the drying conditions. As the solutes are being concentrated on the surface; high air temperatures (particularly with fruits, fish and meats) cause complex chemical and physical changes to the surface and the formation of a hard impermeable skin - termed case hardening. It reduces the drying rate and produces a food with a dry surface and a moist interior. It is minimised by controlling the drying conditions to prevent overly high moisture gradients between the interior and the surface of the food. 16

In powders the textural characteristics are related to bulk density and the ease with which they are rehydrated. These properties are determined by the composition of the food, method of drying and the particle size of the product. Low-fat foods (e.g. potato, fruit juices and coffee) are more easily formed into free-flowing powders than whole milk or meat extracts. Powders are ‘instantised’ by treating individual particles so that they form free-flowing agglomerates or aggregates. The surface of each particle is easily wetted when the powder is rehydrated, and particles sink below the surface to disperse rapidly through the liquid. These characteristics are respectively termed wet-ability, sink-ability, dispersibility and solubility. 17

14 (Fellows P. J., 1997)15 (Brennan, Butters, Cowell, & Lilly, 1981)16 (Fellows P. J., 1997)17 (Fellows P. J., 1997)



Process Control SystemsProcess control and automation has changed manufacturing in almost every industrial sector and have increased efficiency and product consistency. However, until recently, food manufacturing has been an exception to this trend. There are many reasons for this, but one of the most often cited is that, unlike most products, food products by their very nature differ significantly in consistency and shape. This presents a considerable challenge to automated processing procedures.18

Food manufacturing and assembly is mainly performed manually, and this is particularly so in small-to-medium-size enterprises (SMEs). Currently, some of the larger manufacturers use state-of-the-art automation; this is not as prevalent in SMEs, where both the technical infrastructure and spending on engineering research and development are lower.19

As production demand increases, companies have seen the benefit of automation. Automation in food production is frequently introduced at the end-of-line packaging and palletising with the automation moving upstream and effective pick and place robots undertaking operations with actual food products on production lines. However, these systems are currently only generally installed on the high-volume, long-life, single-product lines. Smaller to medium enterprises, which constitute over 99 percent20 of Australian food manufacturers, have been much slower to incorporate automation. The reasons for this include limited low-cost labor and expertise, market volatility, a belief that automation is unsuitable for the assembly of soft, variable, fragile, slippery/sticky natural products and the predominance of short-term orders, which has discouraged capital investments in automation. 21

For products that are high volume, long life or a single product, hard or fixed automation solutions are appropriate. These can use either robots or combinations of simpler electromechanical devices. If products are regular in shape and well located, simple solutions will often suffice. However, if product localization is poor or product shapes vary considerably, the automation solution will usually be much more complex. But that’s changing. In recent years, a combination of robotics and electromechanical systems have shown they can automate processing of most food products.

Process control and automation improves product uniformity and production efficiency and reduces processing cost. Some important elements of process control are:

Detailed production planning and supervision Scheduling of materials and resources Tracking the flow of product through the process Management of orders, formulations and batches Evaluation of process and product data

18 (Davis, 2014)19 (Davis, 2014)20 (Australian Government - Australian Workforce and Productivity Agency, 2013)21 (Davis, 2014)


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