Frying
• Mass Transfer
Water in a frying food migrates from the center to the surface. As
water is removed at the surface due to heating, water is 'pumped' to
the surface. The rate of water loss and its ease of migration through
the product are important to the final characteristics of the food.
• Heat Transfer
Water evaporation from the surface of a frying food also removes
heat from the surface and inhibits charring or burning at the surface.
The heat of vaporization of water to steam removes much of the
heat at the food/oil surface.
• Heat Removal
As long as water is being removed at a sufficient rate, the surface of
the food will not char. Subsurface water in the food will also conduct
heat away from the surface and towards the center of the product.
Frying
• Interior Cooking
The transfer of heat to the interior of the product by water will result
in cooking of the interior of the food. Want enough heat to 'cook' the
product, but not enough to cause damage - example -French fry
• Oil - Food Interactions
Ideally the food products should have similar dimensions and thus,
similar surface to volume ratios. Once an equilibrium is established
all processes should be the same unless there are changes in
equipment function or in oil composition.
• Oil
The properties of oil change with frying. New oil has a high heat
capacity that diminishes with use. Other factors such as viscosity
may change dramatically with use
Food
Oil
Steam
Food
Frying
• For a given product, there is an optimum amount of fat
absorption, determined as the "best tasting product.“
• Manufacturers of fried foods establish a desired value (or
range) for absorbed fat (part of the finished product
specifications), using some combination of tradition,
recommendations, and sensory panel testing.
• The three main determinants of degree of fat absorption
are:
– · Frying temperature
– · Concentration of surface-active agents
– · Water binders in the food.
Frying
• Temperature.
– If the fat temperature in the fryer is too low, frying time
is often extended to achieve the desired depth of
crust coloration.
– However, this also allows more time for fat to
penetrate into the interior of the product.
– Also, the pressure of the steam being generated is
less than optimum, which also makes it possible for
more fat to penetrate.
– We have all undoubtedly had the unhappy experience
of being served pale, greasy french fries. This
situation invariably means that the cook is not
keeping good watch on the fryer thermostat (or that
the fryer is being overloaded).
Frying
• Surface-active agents.
– Emulsifiers promote breakdown of the steam barrier,
and this allows the oil to wet the solid food surfaces.
– Both of these effects increase the absorption of fat.
– In some cases (e.g., cake doughnuts), a small
amount of a wetting agent such as lecithin is added to
the batter to help achieve the desired level of fat
pickup.
– Also, free fatty acids generated by fat hydrolysis and
oxidized fat components (see below) are surface
active and promote fat absorption.
Frying
• Water binders.
– Soluble proteins, gelatinized starch, gums and other
fibers, and similar water-binding ingredients decrease
fat absorption by keeping more water in the system.
– This property is used in formulating batters and
breadings for coating foods that are to be deep-fried.
– Studies have shown, for example, that the inclusion of
3% cellulose fiber in a doughnut formulation markedly
decreases fat absorption.
Frying
% oil
Raw
Fried
Ocean perch
1.2
13.1
Potato chips
0.1
39.8
French fries
0.1
13.2
Cake doughnuts
5.2
21.9
Frying - Stages of oil
Oil Stage
Description
Tri
Glycerides
Polar fat
Poly
mers
Free
Fatty
Acids
(%)
(%)
(%)
(%)
New fat
No surface-active
materials
>98
<2
0.5
0.03
Break-in
Some surface-active
materials
90
10
2
0.5
Fresh
Best for cake
doughnuts
85
15
5
1
Optimum
Best for meats, french
fries, vegetables, etc.
80
20
12
3
Degraded
Much surface-active
material; is easily
absorbed
75
25
17
5
Runaway
Very degraded
65
35
25
8
Frying - Stages of oil
• Break-in oil.
White product, raw, ungelatinatized starch at center of fry; no
cooked odors, no crisping of the surface, little oil pickup by the
food.
• Fresh Oil
Slight browning at edges of fry; partially cooked
(gelatinization) centers; crisping of the surface; slightly more
oil absorption.
• Optimum Oil
Golden brown color; crisp, rigid surface; delicious potato and
oil odors; fully cooked centers (rigid, ringing gel); optimal oil
absorption.
• Degrading Oil
Darkened and/or spotty surfaces; excess oil pickup; product
moving towards limpness; case hardened surfaces.
• Runaway Oil
Dark, case hardened surfaces; excessively oily product;
surfaces collapsing inward; centers not fully cooked; off-odor
and flavors (burned).
Frying - Quality of oil
• Indicators of frying oil quality:
– Total polar compounds
– Conjugated dienes
– FFA
– Dielectric constant
– Color
– pH
– Sensory
– Smoke point, Fire point, Flash point
Frying - Quality of oil
• Flavor Reversion
– Special type of oxidative deterioration
characterized by an objectionable flavor prior
to the onset of true rancidity
– May develop during exposure of fat to
ultraviolet or visible light or heat
– Reverted soybean oil described as “painty”,
“beany”, “haylike” or “grassy” and in final
stages as “fishy”
– Linolenic acid most common source
Emulsifiers
• Hydrophillic-Lipophillic Balance (HLB)
• This is a concept for choosing emulsifiers. The
value of HLB ranges from 1-20.
• Low HLB emulsifiers are soluble in oil while high
HLB emulsifiers are soluble in water.
• Bancroft's rule tells us that the type of emulsion
(i.e. oil in water or water in oil) is dictated by the
emulsifier and that the emulsifier should be
soluble in the continuous phase.
• Low HLB emulsifier's are soluble in oil and give
rise to water in oil emulsions.
Emulsifiers – HLB level
Solubility
HLB Range
No dispersability in water
1-4
Poor Dispersion in water
3-6
Milky appearance
6-8
Stable milky appearance
8-10
Translucent to clear dispersion
10-13
Clear solution
13+
HLB values of some food emulsifiers
EMULSIFIER
HLB VALUE
Oleic acid
1.0
Acetylated monoglycerides
1.5
Sorbitan trioleate
1.8
Glycerol dioleate
1.8
Sorbitan tristearate
2.1
Propylene glycol monostearate
3.4
Glycerol Monoleate
3.4
Glycerol monostearate
3.8
Acetylated monoglycerides (stearate)
3.8
Sorbitan monooleate
4.3
Propylene glycol monolaurate
4.5
Sorbitan monostearate
4.7
Calcium stearoxyl-2-lactylate
5.1
Glycerol monolaurate
5.2
Sorbitan monopalmitate
6.7
HLB values of some food emulsifiers
EMULSIFIER
HLB VALUE
Sorbitan monopalmitate
6.7
Soy lecithin
8.0
Diacetylated tartaric acid esters of monoglycerides
8.0
Sodium Stearoyl lactylate
8.3
Sorbitan monolaurate )
8.6
Polyoxyethylene (20) sorbitan tristearate
10.5
Polyoxyethylene (20) sorbitan trioleate
11.0
Polyoxyethylene (20) sorbitan monostearate
14.9
Sucrose monolaurate
15.0
Polyoxyethylene (20) sorbitan monooleate
15.0
Polyoxyethylene (20) sorbitan monopalmitate
15.6
If you decrease the size of fat globules then you
need more emulsifier to fully coat the surface.
Stokes' Law
• Creaming or sedimentation is proportional to:
– 1. Diameter of the particle squared
– 2. Difference in density between the particle and the
continuous phase
• And inversely proportional to:
– 3. Viscosity of the continuous phase
Rate = [Diameter squared x density difference x g] / [16 x viscosity]
• How can we change 1?
2?
3?
Use of emulsifiers
• For Emulsions, If You Don't Have A Clue, Use At 5% Of The Fat.
• Use Unsaturated Emulsifiers With Unsaturated Fats.
• Mixtures Work Better Than A Single Emulsifier When Stabilizing
Foams And Emulsions.
• Bancroft's Rule
– Emulsion Stability Is Favored By Solubility In The Continuous Phase
– i.e. High HLB----> oil/water
– Low HLB-----> water/oil
• HLB and most other rules go out the window when protein and
(sometimes) polysaccharides enter the system.
• Only saturated monoglycerides complex with starch.
• Emulsifier forms affect functionality.
– flakes vs powder vs hydrates vs gels
Use of emulsifiers
• Many functions are due to affects on polymorphism.
• Emulsifier preparations frequently contain unsaturation
and may be an important contributor to off-flavors.
• Emulsifier preparations are seldom pure and thus
variation from manufacturer to manufacturer may be
substantial.
• When you find a non-obvious usage of emulsifiers, the
function is often related to interaction with starch or
protein.
• Order of addition may be very important.
• Processing steps like homogenization may substantially
change the function of emulsifiers.
Fat replacers
•
•
•
•
•
•
Functions of Fats in Foods
Plasticizer
Mouthfeel
Appearance
Carrier for flavors
Carrier for
– Fat Soluble Vitamins
– Carotenoids
– Essential Fatty acids
• Heat Transfer
Fat replacers
•
•
•
•
•
Water
Carbohydrate
Protein
Modified fats
Synthetic fats
• Reference ADA.1998.Position of the American Dietetic Association: Fat
Replacers. J. Amer. Dietetics Assoc. 98(4):463-8
Fat replacers
• Water
– Cheap
– Label friendly
– Usually free from regulatory constraints
– Doesn’t effect calories from fat
Fat replacers
• Carbohydrates
– Microcrystalline cellulose
– Starch
– Modified
• Physical
• Chemical
– Maltodextrins
Fat replacers
• Gums
– Sodium alginate
– Propyleneglycol alginate
– Xanthan gum
– Guar or Locust bean gum
– Pectin
– Carrageenan
– Agar
– Carboxymethyl cellulose
– Beta glucan
Fat replacers
• Protein
– Simplesse
– Whey or whey/egg
– Other "microparticulated" proteins
– Protein gels
– Gelatin
– Whey
Fat replacers
• Modified fats
– Mono and Di-glycerides
– Medium chain triglycerides
– Caprenin
– Salatrim — "Benefat"
Fat replacers
• Caprenin
– Triglyceride containing:
– Behenic (22:0)
– Capric (10:0)
– Caprylic (8:0)
• Used in the failed "new" Snickers
• Not successful because of a surprised
increase in LDL
Fat replacers
• Salatrim aka Benefat
– Short And Long Acyl Triglyceride Molecule
• Triglyceride containing
–
–
–
–
Stearic (18:0) and
Acetic (2:0) and/or
Propionic (3:0) and/or
Butyric (4:0)
• Salatrim
–
–
–
–
4.5 - 6 calories per gram
Reduction based on:
Short chain fatty acids
~50% absorption of stearate
Fat replacers
• Olestra
– Sucrose polyester
– 6-8 fatty acids
– Lipase sterically inhibited
• Olestra problems
– Anal leakage
– Gastric distress
– Stripping of fat soluble vitamins
• Olestra Advantages
– No calories
– Can be used as frying medium
– Now approved for snack foods
Fat replacers
• Use of fat replacers
– Conclusion
• There are lots of tools
• Only sensory equivalent products will survive
• Potential to reduce calories, but it is likely that
calories in the diet will be unaffected