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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

25
First-order reactions
ln(A/A0)=kt
26
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
27
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
28
Second-order reactions
1/A-1/A0=kt
29
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.
30
Types of reactors
batch, stirred tank, plug flow
31
Batch
* Residence time is the duration of the batch cycle * The same for every portion of the material
32
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
33
Residence Time
taux = V/Q (V: active volume (capacity) of the reactor, m3 Q: volumetric flow rate, m3 s-1)
34
residence time distribution
Pulse of tracer (dye) injected into reactor * Some begins to exit immediately * The rest washes out over time
35
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
36
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
37
Classification of Food Fermentations
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
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
39
Microbial growth curve
Lag, log, stationnary
40
Lag phase
The microorganisms adapt to the new environment, available nutrients, pH, Temperature, etc. No increase in biomass
41
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)
42
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
43
Monod Equation
u=umax*CS/Ks+Cs
44
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))
45
Yield Factor (Yx/s)
Mass of cells formed/Mass of substrate consumed
46
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
47
Dilution rate
F/Vr
48
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.
49
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
50
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
51
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
52
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

FS532 (6 decks)

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