Food Engineering

Question 1

What are unit operations

Answer 1

Unit operations refers to any processing step that involves a physical or chemical transformation of the material

Question 2

What is a continuous sequence

Answer 2

A continuous process, raw materials are constantly inputted while products are constantly removed

Question 3

What is a batch sequence

Answer 3

A batch is when a fixed set of raw materials is input through the sequence of unit operations producing a fix set of products

Question 4

What factors are kept constant in a continuous process

Answer 4

All factors

Question 5

What is the difference between batch and continuous processing

Answer 5

Batch processing is easier to control as it has simple process controls, cheaper equipment and less throughput
Continuous processing has expensive equipment, complicated process controls and higher throughput. There is no downtime between batches for continuous process

Question 6

What is w/w

Answer 6

mass of solute/mass of solution or food

Question 7

What is v/v

Answer 7

volume of solute/volume of solution or food

Question 8

What is w/v

Answer 8

Mass of solute/Volume of solution

Question 9

What is 1ppm

Answer 9

1g of solute in 1 million ml of solution assuming it is water, 1g solution in 1 million g of solution, 1g of solute in 1 million g of food, 1 mg per liter or 1 mg per kg

Question 10

What is 1 ppb

Answer 10

1 g of solute in 1 billion ml of solution or 1μ per l or 1μ per kg

Question 11

What is reaction heat

Answer 11

Reaction Heat is energy produced or absorbed during a chemical reaction or change of phases

Question 12

What happens in a steady state of heat transfer

Answer 12

Heat in + Reaction Heat = Heat Out + Heat loss

Question 13

What is Q_s=mC_p ΔT

Answer 13

Qs is the amount of energy needed to increase the temperature by change in temperature given a certain mass
The specific heat capacity of food decreases as the water content decreases

Question 14

What does q ̇_x= −k dT/dx mean

Answer 14

Where q ̇_x is the heat flux per surface area in J.s-1.m-2 and the heat is flowing in the x direction
dT is the difference in temperature between two points in the body in units of K
dx is the distance between the two points in the body in units of m
k is the thermal conductivity in units of J.s-1.m-1.K-1

Question 15

When the material is thin what is the fastest method of heat transfer

Answer 15

conduction

Question 16

What is Q ̇=hAΔT

Answer 16

Where Q ̇ is the rate of convective heat transfer in J.s-1
h is the convective heat transfer coefficient in units of J.s-1.m-2.K-1
A is the surface area in m2
ΔT is the temperature difference between the fluid and the solid interface

Question 17

What does the transfer coefficient depend on

Answer 17

The transfer coefficient is dependent on whether it is a natural or forced convection & the property of the food such as its viscosity

Question 18

What are some mass transfer processes

Answer 18

Diffusion of salt or nutrients in food products
Migration of aromatic volatiles or moisture through a package
Evaporation of water during dehydration of concentration
Separation of components in a distillation column or during membrane filtration

Question 19

What is (m_B ) ̇/A=−D dC/dx

Answer 19

Where (m_B ) ̇ is the mass flux of fluid B across fluid A in units of kg.s-1
A is the surface area in units of m2
D is the diffusivity in units of m2.s-1
C is the concentration in units of kg.m-3
x is the distance across the concentration gradient in units of m.
(m_B ) ̇ is the flux or the movement of B across A. In our case, the water vapour will be B and the surrounding air will be A. The rate by which diffusion occurs is dependent on several factors, of which the most significant is the surface area. Therefore the flux of fluid B is always expressed as kg/m2.s.

Question 20

What drives diffusion

Answer 20

Diffusion is driven by difference in concentration, the larger the concentration, the faster the rate of diffusion, rate of diffusion is also affected by the inherent properties in the system. Fluids with higher viscosity will have lower rate of diffusion

Question 21

What are the names of the molecules that pass through the semi permeable membrane and those that do not

Answer 21

The molecules that pass through are known as the permeate while larger molecules are known as the retentate

Question 22

What is σ ̇=σ ̇_0+K〖(γ ̇)〗^n

Answer 22

Where σ ̇ is the shear stress in units of pascal (Pa)
σ ̇_0 is the yield shear stress in units of pascal (Pa)
γ ̇ is the shear rate in units of s-1
n is the flow behaviour index which is dimensionless
k is the consistency coefficient in units of Pa.sn

Question 23

What is shear stress

Answer 23

The shear stress σ ̇ is the force per unit area applied to cause the fluid to flow.

Question 24

What is shear rate

Answer 24

The shear rate γ ̇ is a measure of how fast the fluid deforms or flows when shear stress is applied. The more shear stress you apply, the faster the fluid deforms, hence the faster it flows.

Question 25

What are newtonian fluids

Answer 25

Newtonian fluids where one does not have yield shear stress and their shear rate increases proportionately with their shear stress

Question 26

What are shear thinning fluids

Answer 26

Shear thinning/ pseudoplastic fluids have a flow behaviour index higher than 0 but less than 1. Most liquid foods are shear thinning

Question 27

What are shear thickening fluids

Answer 27

Shear thickening or dilatant fluid is when the more shear stress is applied, the more it resist deformation. They have a flow behaviour index more than 1

Question 28

What is Bingham pseudoplastic or Herschel-Bulkley plastic

Answer 28

A fluid with positive yield stress and the shear stress & shear rate are not linearly proportional

Question 29

What are bingham plastic

Answer 29

Fluids with positive yield stress but with flows like Newtonian fluids are known as Bingham plastics

Question 30

What is laminar flow

Answer 30

Laminar flow is when fluids are flowing on multiple parallel layers with no mixing between each layer

Question 31

What is turbulent flow

Answer 31

In turbulent flow, layers are moving in chaotic directions resulting in mixing between layers

Question 32

What is Re= Dνρ/μ

Answer 32

Where Re is the Reynolds number
D is the diameter of the pipe in units of m
ν is the fluid velocity in units of m.s-1
ρ is the density of the fluid in units of kg.m-3
μ is the viscosity of the fluid in units of Pa.s (kg.m-1.s-1)

Question 33

What happens when the Reynolds number is below 2100

Answer 33

it is in laminar flow

Question 34

What happens when the Reynolds number is higher than 4000

Answer 34

It is in turbulent flow

Question 35

What are Fluids with Reynolds number between 2100 and 4000

Answer 35

It is in transition where their flow patterns are in-between that of laminar and turbulent flows.