Food Technology

Food Technology Food Science – sciences study the

of food

Application of basic and engineering to fundamental physical, chemical and biological nature of foods and engineering principals processing.

 Food Technology –

wholesome foods.

use of information generated by food science in selection, processing, preservation, packaging, storage and distribution as it affects the consumption of safe, nutritious and

 Both of above subjects are multidisciplinary subjects  Under food engineering, engineering concepts of food and food processing are studied  Food engineers deal with conversion of raw agricultural products into edible foods.  Much of the materials shown in the syllabus are related to food engineering e.g. basic fluid flow dynamics, centrifugation, homogenization, mixing, emulsification, size reduction, refrigeration, freezing, sterilization etc.

Unit Operations in the Food Industry  When engineering operations are categorized according to their unifying theory each category resulted is called a unit operation. Some of Basic Physical Properties and Concepts of Food Materials  Density (ρ) m (Kg) ρ= V (m3) standard units = Kg m-3

 Bulk Density Bulk density =

mass of the bulk volume of the bulk

Porosity – fraction of the volume taken up by air Porosity (Є)=

Volume of air (Va) Volume of the bulk (Vb)

 Specific Gravity (SG) SG =

mass of a liquid

mass of equal amount of pure water at the same temperature  Density of gases depends on temperature and pressure  The relationship between temperature and pressure of gases is given by the ideal gas equation. PV = nRT P = Pressure V = volume n = no. of moles R = universal gas constant T = temperature (0K)

Viscosity  Internal resistance of a fluid to flow.  Mothfeel of foods depends on the viscosity  Arises due to intermolecular forces of a liquid e.g. tomato ketchup.  Viscosity changes with temperature  When a fluid is flowing over a surface the fluid flows as layers till its velocity reaches to certain higher level.  Uppermost layer flows at the highest velocity.  Each layer drags the lower layer at a velocity lower than it.

 Velocity gradient (shear rate) can be observed from top layer to the surface – the layer above the surface does not move – boundary film.  Shear force or shear stress  The graph shear force against shear rate indicates that there is linear relationship for some simple liquids – Newtonian fluids.  Some fluids behave as Newtonian fluids at lower concentrations and non-Newtonian at higher concentrations.  Non- Newtonian fluids can be divided into several sub classes.

Pseudoplastic fluid – viscosity shear rate Dilatant fluid - viscosity shear rate Bingham fluid – no flow until critical shear stress is reached- viscosity and shear rate change linearly . Casson plastic fluid – as above but viscosity and shear rate change non-linearly. Thixotropic fluid – structure brakes down and viscosity decreases

 Thickness of the boundary film depends on velocity, viscosity and temperature  A fluid flow as a series of layers without mixing – streamline flow or laminar flow .

 If the velocity of a streamline fluid is increased above certain level layers mix together – turbulent flow

 Flow rate

thickness of the boundary film

 Reynolds showed that the fluid flow through a pipe can be characterized by a dimensionless number – Reynolds Number (Re)

Re =

Dvρ µ

D = diameter (m), v = velocity (ms-1), µ = viscosity Re Number

Nature of the flow

Re < 2100

streamline or laminar flow

2100 < Re > 4000

transitional flow

Re > 4000

turbulent flow

Re > 10 000

fully developed turbulent flow

Material Transfer – Mass Transfer  Transfer of matter is an important aspect in many food manufacturing processes. e.g. solvent extraction, distillation, loss of nutrients in washing and cleaning.  Rate of material transfer is influenced by two factors. 1. moving force 2. resistance from the medium

Mass Balance/Material Balance  Based on the law of mass conservation (LMC).  Mass balance equation is an account written for a selected mass of material based on the LMC.  General mass balance equation for a system would take the form of total mass of RM = P + SM + L RM = raw materials P = products SM = stored materials L = loss  A mass balance equation can be written for a complete system or a part.

Pearson Square  Used for calculating the relative masses of two components to prepare a mixer of known composition.

Fluid Flow  In the food industry many fluids are handledraw materials, finished products, cleaning solutions and wastes etc.  In engineering the term fluid is applied for any thing that can be made to flow without disintegration by applying pressure – liquids, gases (compressible fluids), powders and particulate foods.  Studied under – Fluid statics and Fluid Dynamics

Pressure (P)  Force (F) exerted on a unit area (A). P = F/A F = mg m = mass g = acceleration due to gravity ρ = m/V m= ρV F= ρVg

 Pressure above the selected plane A V = hA m = hAρ F = hAρg  The total force must include any additional force on the surface of the liquid.  If the force act on a unit area of the surface is Ps F = A Ps + hAρg F/A = P P = Ps + hρg

 Pressure at a given point of a liquid depends on the height of the liquid column above.  Usually pressure of liquids is expressed with respect to atmospheric pressure or zero.

Fluid Dynamics  Fluids in motion are studied.  In liquid transport systems used in the food industry, four basic components can be seen. Tanks Pumps Valves Pipelines  Lows of mass and energy conservation are used.

 Figure

 If the velocity of the fluid in the section (1) is v1, area of the cross section of the pipe is A1 and the density of the fluid is ρ1 and corresponding values of section (2) are v2,A2 and ρ2. ρ1 A1 v1 = ρ2A2 v2  If the fluid is incompressible ρ1 = ρ2 A1 v1 = A2 v2

continuity equation

According to the low of energy conservation all types of energies involved during the process have to be taken into account for writing an energy balance equation. During this process -potential energy -kinetic energy -pressure energy -energies exchange with the surrounding energy lost due to friction energy input from the pump heat energy exchanged

 Energy is measured relative to a convenient reference point.  Energy change of unit mass of a liquid  Potential energy change ΔPE ΔPE = g(Z2-Z1)  Kinetic energy change ΔKE 2

ΔKE =

V 2

–

2 v 1

2α for a laminar flow α = 0.5 for a turbulent flow α = 1

 Pressure energy change due to pressure change ΔP/ρ ΔP/ρ = P2-P1/ρ  Frictional energy loss ΔPf / ρ = Ef For a straight pipe Fanning equation (D’Arcy or Fanning D’Arcy) equation Ef = 2fv2L/D f = friction factor For a sudden constriction in the cross section 2

Ef = Kfv2 / 2

v = velocity downstream

 If

2

2

D2 / D1 < 0.715 2

2

Kf = 0.4 (1.25 –D2/D1 )  If 2

2

D2 / D1 > 0.715 2

2

Kf = 0.75(1–D2/D1 )  For a sudden increase in the cross section v = velocity upstream

 Energy loss due to friction by different parts of pipelines is expressed using Le/D Le = equivalent length Fitting

Le/D

Elbow 900 square Elbow 900 standard Elbow medium sweep Elbow long sweep Elbow 450 standard Gate valve open Gate valve ½ open

60 32 26 20 15 7 200

 According to the low of energy conservation all the energies mentioned above must be provided by the pump (ΔEp). ΔEp = ΔPE + ΔKE + ΔP/ρ + Ef 2

2

ΔEp = g(Z2-Z1) + V2 – v1/ 2α + P2-P1/ρ + Ef 2

2

gZ2 + V2 /2α + P2/ρ + Ef = gZ1 + V1 /2α + P2/ρ + ΔEp

 Friction factor (f) for a fluid depends on Re and

 Friction factor can be found out with the help of the Moody graph.

 ε/D roughness ratio  ε = roughness factor  Friction factor (f) can be predicted for a streamline flow using the Hagen-Poiseuille equation. f = 16/Re Applicable in the region of 02100  Blasius equation 0.136(Re) -0.25 f= 4

Material

Roughness factor (ε)

Material

Roughness factor (ε)

Riveted steel

0.001-0.01

Galvanized iron

0.0002

Concrete

0.0003 -0.003

Asphalted cast iron

0.001

Wood staves

0.0002-0.003

Commercial steel 0.00005

Cast iron

0.000

Drawn tubing

Smooth

Fluid Flow Applications  Measuring pressure and velocity  Making fluid to flow Measuring pressure and velocity Pressure  Using piezometer -U-tube  Bourdon – tube pressure gauge  Bellow gauge

Manometer Tube

Velocity  Pitot tube Appling Bernouilli’s equation when the liquid column is stabilized 2

Z+Z’ = v1/2g + p/ρ1g  Pitot-static tube z = v21/2g  Venturi and orifice meter v1 = C√[2(p1-P2)/ρ] x A22/(A12-A22) In a properly designed Venturi meter C

-The orifice meter operates on the same principal and the only difference is using a plate with a hole instead of a tapered tube. -Orifice discharge coefficient is smaller. Propeller meters Impact meters Rotameters Making fluid to flow Pumps and fans convert mechanical energy from some other source into pressure energy and/or velocity energy

Positive Displacement Pump • The fluid is drawn in and then forced through the outlet. High pressure head but can not be throttled. Reciprocating piston pump Gear pump Rotary pump Jet Pump • A high velocity-jet is produced in a Venturi nozzel. • Used in places where mechanical handling is undesirable and in vacuum pump.

• Not much efficient, no mechanical part therefore low cost. • Low head Air-lift Pumps • Flow of liquid or gas are introduced to impart the energy. • Flow of air or liquid comes from the external source or within the system. Propeller • Propellers are used for imparting energy. • Used for mixing and conveying fluids

• Head is low Centrifugal Pumps and Fans • Converts rotational energy to velocity and pressure energy.