Food Engineering Flashcards
What are unit operations
Unit operations refers to any processing step that involves a physical or chemical transformation of the material
What is a continuous sequence
A continuous process, raw materials are constantly inputted while products are constantly removed
What is a batch sequence
A batch is when a fixed set of raw materials is input through the sequence of unit operations producing a fix set of products
What factors are kept constant in a continuous process
All factors
What is the difference between batch and continuous processing
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
What is w/w
mass of solute/mass of solution or food
What is v/v
volume of solute/volume of solution or food
What is w/v
Mass of solute/Volume of solution
What is 1ppm
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
What is 1 ppb
1 g of solute in 1 billion ml of solution or 1μ per l or 1μ per kg
What is reaction heat
Reaction Heat is energy produced or absorbed during a chemical reaction or change of phases
What happens in a steady state of heat transfer
Heat in + Reaction Heat = Heat Out + Heat loss
What is Q_s=mC_p ΔT
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
What does q ̇_x= −k dT/dx mean
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
When the material is thin what is the fastest method of heat transfer
conduction
What is Q ̇=hAΔT
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
What does the transfer coefficient depend on
The transfer coefficient is dependent on whether it is a natural or forced convection & the property of the food such as its viscosity
What are some mass transfer processes
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
What is (m_B ) ̇/A=−D dC/dx
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.
What drives diffusion
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
What are the names of the molecules that pass through the semi permeable membrane and those that do not
The molecules that pass through are known as the permeate while larger molecules are known as the retentate
What is σ ̇=σ ̇_0+K〖(γ ̇)〗^n
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
What is shear stress
The shear stress σ ̇ is the force per unit area applied to cause the fluid to flow.
What is shear rate
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.
What are newtonian fluids
Newtonian fluids where one does not have yield shear stress and their shear rate increases proportionately with their shear stress
What are shear thinning fluids
Shear thinning/ pseudoplastic fluids have a flow behaviour index higher than 0 but less than 1. Most liquid foods are shear thinning
What are shear thickening fluids
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
What is Bingham pseudoplastic or Herschel-Bulkley plastic
A fluid with positive yield stress and the shear stress & shear rate are not linearly proportional
What are bingham plastic
Fluids with positive yield stress but with flows like Newtonian fluids are known as Bingham plastics
What is laminar flow
Laminar flow is when fluids are flowing on multiple parallel layers with no mixing between each layer
What is turbulent flow
In turbulent flow, layers are moving in chaotic directions resulting in mixing between layers
What is Re= Dνρ/μ
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)
What happens when the Reynolds number is below 2100
it is in laminar flow
What happens when the Reynolds number is higher than 4000
It is in turbulent flow
What are Fluids with Reynolds number between 2100 and 4000
It is in transition where their flow patterns are in-between that of laminar and turbulent flows.
