Transcript Thermal Processing - Rutgers University

Engineering and thermal
processing
Engineers are different from scientists
• What do scientists do?
• What do engineers do?
• The importance of simplifying assumptions
– examples
Engineers deal with
• Heat transfer, always runs from hot to cold so
refrigerators or freezers need pumps (energy
input)
• Mass transfer
• Mass balance
– Consider a simple drier
nd
2
The
Law of Thermodynamics:
everything goes to maximum
randomness (entropy)
• Unless you put in energy
– Electric, microwave, pressure, thermal, ionizing
– Types of Heat Transfer (demo)
• Conduction
• Convection
• Radiation
Most food processing involves
the addition or removal of energy.
Most types of food spoilage have
engineering solutions.
Thermal Processing and Refrigeration
Heating and Cooling:
• Heat transfer (heating or cooling) is driven by ΔT (the
temperature difference)
ΔT (°F)
Boiling H2O
152
Oven 350°F
292
Broiler @ 2000°F
1940
Refrigerator 40°F
20
Household Freezer 10°F
50
Commercial Freezer -40°F
100
Liquid Nitrogen -380°F
440
Assume a Room
Temperature of
60°F.
Pasteurization:
• 7 log kill for Salmonella in Milk = 99.99999%
Temperature:
145°F (63C)
161°F (72C)
212°F (100C)
280°F (131C)
Time:
30 min
15 sec
0.01sec
6 sec
• Pasteurization of eggs:
3 log reduction = 99.9%
145°F for 3 to 4 mins.
• Fruit juices are pasteurized to reduce microbial
count and inactivate enzymes; they were thought
not to carry pathogens.
Log Numbers
Time (min)
1
2
3
4
5
6
7
8
9
10
# survivors
1,000,000
100,000
10,000
1,000
100
10
1
0.1
0.01
0.001
Log # survivors
6
5
4
3
2
1
0
-1
-2
-3
Log vs linear
Pasteurizers (Ohio State University)
80 C for thirty seconds
60 C for thirty minutes
Canning:
• Invented by Appert in 1790s.
• Commercial sterility.
• Destroys pathogens and spoilage organisms
that can grow.
• Requirements for high acid foods (pH < 4.6)
are not very stringent. Why?
• Can be done in a boiling water bath.
Home canned tomatoes, OK
Home canned beans, No
Requirements for low acid
(pH >4.6) foods are very strict
• Low acid foods pH > 4.6
• Target is C. botulinum 12
log reduction =
99.9999999999%
• 2 to 4 min @ 250°F
• Requires certified retort
operators, approved
process
Flow Chart: Canning of Corn (FAO)
Blanching!
Flow Chart: Canning of Peas (FAO)
Flow Chart for the pea canning process
Blanching!
Mild vs. Severe Heat Treatment:
Mild Heat Treatment):
Severe Heat Treatment:
•Aims:
•Kill pathogens
•Reduces bacterial load (Food
is not sterile)
•Inactivate enzymes
•Advantages:
•Minimal damage to flavor,
texture, and nutritional quality.
•Disadvantages:
•Short shelf life
•Another preservation method
must be used, such as
refrigeration or freezing
•Examples:
•Pasteurization
•Blanching
•Aims:
•Kills all bacteria
•Food will be commercially
sterile
•Advantages:
•Long shelf life
•No other preservation method
is necessary
•Disadvantages:
•Food is over-cooked
•Major changes in texture,
flavor, and quality
•Examples:
•Canning
Refrigeration and Freezing:
(85 % of food is frozen at some point)
Refrigeration:
Freezing:
Micro growth
Slow
Stops
Chemical reactions
Slow
Even Slower
Quality
“Fresh”
“Frozen”
Shelf life
Short
Long
Margin of error
Small (spoilage, chill injury, cross
Large
contamination)
Freezing:
Slowly:
•Water migrates
•Freeze “pure”
•Solutes concentrate
•Large crystals
Rapidly:
•Freezes in place
•No solute concentrate
•Small crystals
•Better structures
How refrigeration works
Simplified
Energy
Input
Condense
Expand
Freezing:
• 3 Types:
– Plate Freeze
• Contact with very cold surface
– Blast Freeze
• Very cold air
• Very high velocity
– Immersion
• N2
• Flash Frozen = “IQF”
Processing can create multiple
products– Apple Juice
Bottle – “hot pack”
Clear glass
Brown plastic
Concentrate
Liquid
Frozen
Metal can
Aseptic – Brix pack
(“Sipps”)
“Nonthermal” processing
next lecture…
•
•
•
•
•
•
High pressure
Irradiation
UV light
Pulse light
High intensity light
Pulsed electric field
How pumpkin pies are made…
Happy Thanksgiving!