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FS532 > Unit operation > Flashcards

Unit operation Flashcards

1
Q

Unit Operations

A

Categories of common
operating steps practiced in the food industry

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2
Q

Engineering Principles

A

Thermodynamic equilibrium, Balances, Transport phenomena, Reaction engineering

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3
Q

Rate

A

Driving Force
S Resistances

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4
Q

Handling options

A

– Solid
– Powder
– Fluid

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5
Q

Materials handling

A

– Pumps
– Conveyors
– Flumes
– Pneumatic conveying
– Elevators
– Cranes and hoists
– Trucks/forklifts

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6
Q

Powder Handling

A

Pneumatic conveying

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7
Q

Pneumatic Conveying

A

– Head loss due to elevation change
– Solids acceleration
– Gas friction loss
– Solids friction loss
– Bend/elbow/fitting loss

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8
Q

Issues with Pneumatic Conveying

A
  • 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
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9
Q

Pumps

A

centrifugal pumps and postiive displacement pumps

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10
Q

Centrifugal Pumps

A

More efficient with low-viscosity liquids such as milk and fruit
juices, where flow rates are high, and pressure requirements are
moderate

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11
Q

Positive Displacement Pumps

A

ü Flow rates are accurately
controlled
ü Good for liquids with
high and low viscosities

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12
Q

CP what fluids

A

Best for thin fluids (milk, juice, etc.)

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13
Q

CP characteristics

A

– 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

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14
Q

PD pumps fluids

A

– Thick fluids, but not particulates

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15
Q

PD characteristics

A

– 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

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16
Q

Mixing/Blending

A

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.

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17
Q

Types Solid – Solid

A

– Dry blends

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18
Q

Type Solid – liquid

A

– Reconstitution
– Slurries
– Batters, pastes, and doughs

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19
Q

Liquid – liquid

A

– Oil/water blends

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20
Q

Gas – liquid

A

– Fermentation
– Meringues
– Soufflés
– Marshmallow
– Ice cream

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21
Q

Quantifying Mixing

A

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

22
Q

Kinetics

A

study of chemical reaction
rates and mechanisms

23
Q

Zero-orders

A
  • 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
24
Q

Zero order reaction

A

A=A0+KT

25
Q

First-order reactions

A

ln(A/A0)=kt

26
Q

First order

A
  • 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
27
Q

Second-order

A
  • 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
28
Q

Second-order reactions

A

1/A-1/A0=kt

29
Q

Residence Time and
Residence Time Distribution

A
  • 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.
30
Q

Types of reactors

A

batch, stirred tank, plug flow

31
Q

Batch

A
  • Residence time is the
    duration of the batch
    cycle
  • The same for every
    portion of the material
32
Q

Continuous Stirred Tank Reactor

A
  • 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
33
Q

Residence Time

A

taux = V/Q (V: active volume (capacity) of the reactor, m3
Q: volumetric flow rate, m3 s-1)

34
Q

residence time distribution

A

Pulse of tracer (dye) injected into reactor
* Some begins to exit immediately
* The rest washes out over time

35
Q

Plug Flow Reactor

A
  • 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
36
Q

Roles of Fermentation in Food Processing

A

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

37
Q

Classification of Food Fermentations

A
  1. Natural – Utilizes existing microorganisms.
    How all food fermentations began.
  2. 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
38
Q

Batch Fermentation

A
  • 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
39
Q

Microbial growth curve

A

Lag, log, stationnary

40
Q

Lag phase

A

The microorganisms adapt to the new environment, available
nutrients, pH, Temperature, etc.
No increase in biomass

41
Q

Log phase

A

ü Active growth and multiplication of microorganisms
ü Biomass increase
ü Growth rate of microbes in log phase is dependent on
substrate (nutrient supply)

42
Q

Stationnary phase

A

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

43
Q

Monod Equation

A

u=umax*CS/Ks+Cs

44
Q

Rate of accumulation

A

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))

45
Q

Yield Factor (Yx/s)

A

Mass of cells formed/Mass of substrate consumed

46
Q

Continuous Fermentation

A
  • 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
47
Q

Dilution rate

A

F/Vr

48
Q

Critical dilution rate

A

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.

49
Q

Adv Batch

A

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

50
Q

Adv Continuous

A

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

51
Q

Disadv batch

A

High labor cost: skilled labor is
required
Much idle time: Sterilization,
growth of inoculum, cleaning after
the fermentation
Safety problems: when filling,
emptying, cleaning

52
Q

Disadv continuous

A

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

FS532 (6 decks)

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