Unit operation Flashcards
Unit Operations
Categories of common
operating steps practiced in the food industry
Engineering Principles
Thermodynamic equilibrium, Balances, Transport phenomena, Reaction engineering
Rate
Driving Force
S Resistances
Handling options
– Solid
– Powder
– Fluid
Materials handling
– Pumps
– Conveyors
– Flumes
– Pneumatic conveying
– Elevators
– Cranes and hoists
– Trucks/forklifts
Powder Handling
Pneumatic conveying
Pneumatic Conveying
– Head loss due to elevation change
– Solids acceleration
– Gas friction loss
– Solids friction loss
– Bend/elbow/fitting loss
Issues with Pneumatic Conveying
- Particle segregation
– Small ones round corners faster than large ones - Particle abrasion
– Creates fines - Energy use
– Typically, higher HP required than mechanical - Requires particle separation from air prior to venting
– EPA requirement for particulate emissions - Size limitations
– Small and light enough to fluidize the particles
Pumps
centrifugal pumps and postiive displacement pumps
Centrifugal Pumps
More efficient with low-viscosity liquids such as milk and fruit
juices, where flow rates are high, and pressure requirements are
moderate
Positive Displacement Pumps
ü Flow rates are accurately
controlled
ü Good for liquids with
high and low viscosities
CP what fluids
Best for thin fluids (milk, juice, etc.)
CP characteristics
– Must be primed to initiate flow
– Flow rate related to (rpm)
– Pressure developed related to (rpm)2
– Power requirement related to (rpm)3
– Maximum head (height to which a fluid can be moved)
* Decreases as flow rate increases
* Zero flow at max head
* Decreases sharply as viscosity increases
– If downstream pipe is blocked or shut off, fluid just spins
within housing, building up heat
PD pumps fluids
– Thick fluids, but not particulates
PD characteristics
– Self priming
– Delivers specific amount of fluid per rotation regardless
of downstream conditions
* Required for pasteurization/aseptic systems
– Can pump against very high pressures, with constant flow
rate regardless of pressure
– If outlet is blocked, will continue to build pressure until the
pump housing (or something else) bursts
* Often used with a blow-out valve
Mixing/Blending
Mixing is the dispersion of components,
one throughout the other(s) such that the
frequency of each component in a sample of
the mixture is proportional to the fractions of
these components in the whole batch.
Types Solid – Solid
– Dry blends
Type Solid – liquid
– Reconstitution
– Slurries
– Batters, pastes, and doughs
Liquid – liquid
– Oil/water blends
Gas – liquid
– Fermentation
– Meringues
– Soufflés
– Marshmallow
– Ice cream
Quantifying Mixing
Complete mixing is the case where the components in
all the sub-mixtures are in the same proportion as the
original mixture
* Minimal variation from the target in all samples
* Measured by target components, depending on type of
mixing
– Composition
– Color/appearance
– Minor constituents
Kinetics
study of chemical reaction
rates and mechanisms
Zero-orders
- Frequently reported for changes in foods
- Reactions where the amount of product formed is only a
small fraction of the amount of precursors present - Decomposition reactions where only a small amount of
product is formed from a reactant - The reactant is in such a large excess that its concentration
remains effectively constant - The rate appears to be independent of the concentration
Zero order reaction
A=A0+KT
First-order reactions
ln(A/A0)=kt
First order
- A first -order reaction is characterized by a logarithmic change in
the concentration of a reactant with time. - Many of the reactions involved in the processing of foods can be
modeled as first-order reactions
Second-order
- Second-order kinetics is not frequently reported in
the food science literature - Reported for changes of amino acids involved in the
Maillard reaction - Loss of lysine (bound in proteins) in sterilized milk
due to the Maillard reaction - The actual mechanism of lysine loss is much more
complicated than a relatively simple bimolecular
reaction
Second-order reactions
1/A-1/A0=kt
Residence Time and
Residence Time Distribution
- Length of time during which a food or portions of
it have been subjected to a process - The time the food has “resided” in the reactor
- In food processing a reactor is: a fermenter, oven,
extruder, drying tunnel, a barrel for wine aging, a
box of cookies, etc.
Types of reactors
batch, stirred tank, plug flow
Batch
- Residence time is the
duration of the batch
cycle - The same for every
portion of the material
Continuous Stirred Tank Reactor
- Perfectly agitated vessel
- Continuous feeding and
discharge - The composition and all other
factors are uniform at all
points within the reactor - The composition of the
discharge is identical to the
fluid bulk in the reactor at the
same time
Residence Time
taux = V/Q (V: active volume (capacity) of the reactor, m3
Q: volumetric flow rate, m3 s-1)
residence time distribution
Pulse of tracer (dye) injected into reactor
* Some begins to exit immediately
* The rest washes out over time
Plug Flow Reactor
- Material flow as a block (plug)
- Each part of the fluid has the
same velocity - No mixing within the liquid
- Residence time is equal for
every portion of the fluid - Residence Time Distribution is
flat
Roles of Fermentation in Food Processing
1) Development of flavors, aromas and textures
2) Preservation through lactic acid, alcoholic, acetic
acid, alkaline fermentations and high salt fermentations
3) Enrichment of food substrates with vitamins, protein,
essential amino acids and essential fatty acids
4) Detoxification during food fermentation processing
5) Decrease in cooking times
Classification of Food Fermentations
- Natural – Utilizes existing microorganisms.
How all food fermentations began. - Controlled – Add the desired microorganism.
Many previously natural food fermentations are
now controlled
* Submerged Culture:
Microorganisms are incubated in
a liquid medium. Sometime
subjected to continuous, vigorous
agitation
* Solid State (SSF):
Microorganisms are grown on a
solid support in absence (or near
absence) of free water
Batch Fermentation
- A closed system
- Sterile nutrient culture medium in the
bioreactor is inoculated with
microorganisms - Fermentation is carried out under
optimal physiological conditions. - Add acid or alkali to maintain pH
- Antifoam agents to minimize foam
Microbial growth curve
Lag, log, stationnary
Lag phase
The microorganisms adapt to the new environment, available
nutrients, pH, Temperature, etc.
No increase in biomass
Log phase
ü Active growth and multiplication of microorganisms
ü Biomass increase
ü Growth rate of microbes in log phase is dependent on
substrate (nutrient supply)
Stationnary phase
ü The substrate in the growth medium gets depleted
ü The metabolic end products that are formed may inhibit the
growth
ü Microbial growth may either slow down or completely stop
ü The biomass remains constant during this phase
Monod Equation
u=umax*CS/Ks+Cs
Rate of accumulation
d/dt (VRCi) =VRrfi (VR: Culture Volume
Ci: moles of i / unit culture volume
Rfi: moles of I formed / (unit culture volume x unit time))
Yield Factor (Yx/s)
Mass of cells formed/Mass of substrate consumed
Continuous Fermentation
- Open system to maintain cells
in a state of balanced growth - Continuous addition of fresh
medium and removal of
culture media at same rate - Chemostat, continuous steady
state growth
Dilution rate
F/Vr
Critical dilution rate
ü The dilution rate has the critical value, Dcrit » μmax.
ü It is slightly lower than μmax.
ü If D > Dcrit, cell is washed out, CX = 0.
ü Then, the substrate is not consumed, CS = Cso.
Adv Batch
Versatile: can be used for
different reactions every day.
Safe: can be properly
sterilized.
Little risk of infection or strain
mutation
Complete conversion of
substrate is possible
Adv Continuous
Works all the time: low labor cost,
good utilization of reactor
Often efficient: due to the
autocatalytic nature of
microbial reactions, the
productivity can be high.
Automation may be very appealing
Constant product quality
Disadv batch
High labor cost: skilled labor is
required
Much idle time: Sterilization,
growth of inoculum, cleaning after
the fermentation
Safety problems: when filling,
emptying, cleaning
Disadv continuous
Continuous production fails
due to a) infection, b)
spontaneous mutation of
microorganisms to non
producing strain
Inflexible: can rarely be
used for other productions
without substantial
retrofitting
