Hydrogenation

Treatment of an oil with hydrogen and a suitable catalyst to decrease the number of double bonds and increase the degree of saturation.

Rate is determined by:

Nature of substrate
Type and concentration of catalyst
Pressure (Concentration of hydrogen)
Temperature
Agitation

Stages in Hydrogenation

Transfer and/or diffusion
Adsorption
Hydrogenation/Isomerization
Desorption
Transfer

Transfer and adsorption are critical steps in controlling the degree of isomerization and selectivity of the reaction. Transfer of reactants and products to and from the bulk liquid oil phase and the surface of the catalyst.

Diffusion
Diffusion of reactants into pores on the catalyst surface. Diffusion of products out of the catalyst surface pores.

Selectivity
Define selectivity as the ratio of the rate of hydrogenation of linoleic acid to that of oleic acid. Commonly observed selectivities range for 4 to 50. Desire highly selective catalysts. Why?

Characteristics of some food lipids

Rate of oxidation of fatty acids, their esters and triglycerides.

Effects of Hydrogenation

During hydrogenation double bond migration can occur. When migration occurs, there are 2 trans for every cis bond formed. Catalysts can be selective. Selectivity is defined as the ratio of the rate of reaction of C18:3 to C18:2. During hydrogenation there must be absorption of reactants to the catalyst surface ( hydrogen and lipid ). Because the catalysts does not dissolve in the reaction mixture, this is an example of heterogeneous catalysis.

Gunstone half hydrogenation cycle:

If hydrogen is removed from the following case, then:

Or if removed from this case:

To be selective C18:3 must be bound tighter than C18:2. To achieve this, must limit hydrogen so that odds favor a molecule with 3 double bonds reacting before a molecule with two double bonds. The conditions which favor selective reactions also favor double bond migration. The movement of double bonds creates molecules that can not be called essential fatty acids and that have been suggested to cause possible health hazards. The following table summarizes the effects of hydrogenation conditions on selectivity, reaction rate and double bond migration:

The effects of processing conditions on hydrogenation

The effects of hydrogenation include:

Method

Oil is heated with catalyst (Ni), heated to the desired temperature (140-225°C), then exposed to hydrogen at pressures of up to 60 psig and agitated. An example of heterogeneous catalysis.

Conditions

Starting oil must be:

Refined
Bleached
Low in soap
Dry

The catalysts must be:

Dry
Free of CO2 and NH4

Isomerization

An equilibrium will be established between positional and geometric isomers in the mixture. Double bonds that are reformed tend to have a trans/cis ration of 2:1. All trans would be expected if there were no steric considerations.

Purposes

Convert liquid fats to plastic fats
Improve oxidative stability
Covert soft fats to firmer fats

Heterogeneous Catalysts

Most commonly utilized
Catalysts and reactants exists in different physical states
Hydrogenation reaction takes place on surface of catalyst
Nickel containing catalysts are most frequently utilized

Nickel Catalysts

Typical Ni catalyst is usually reduced Ni dispersed in the absence of air into hardened fat to stabilize it. In such systems, the support plays an essential role in determining the specific reactivity of the catalyst.

Advantages of Nickel

Availability
Low Cost
Inert nature of metal to the oil

Hydrogenation Limitations

Selectivity is never absolute
Little preference for C18:3 over C18:2
Important amounts of trans acids are formed
Selectivity and isomerization are linked

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.

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.

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).

Water and Oil
contact with the surface of the food product. The removal of heat from the food surface as steam prevents good contact between the oil and the food. As cooking progress, compounds are formed that allow the oil and food to interact.

Surfactants

Frying is basically a dehydration process
The heat transfer medium is a non aqueous material and food is almost all water. Oil and water are immiscible.
For frying to occur, heat must be transferred from the non-aqueous medium into the mostly aqueous food.

Any changes in heat transfer must result from degradation products formed as a result of breakdown or interaction of the oil.

The food materials leaching into the oil, breakdown of the oil itself and oxygen absorption at the oil-food interface all contribute to change the oil from a medium that is almost pure triglyceride to a mixture of hundreds of compounds.

Those materials which affect the heat transfer at the oil-food interface must act to reduce the surface tension between the two immiscible materials. These materials act as wetting agents and are regarded as surfactants.

As the oil degrades, more surfactants are formed, causing increased contact between food and oil. This causes excessive oil absorption and an increased rate of heat transfer to the surface of the food. Eventually, excessive darkening and drying of the surface occur, while conduction to the interior is constant.

Oil Quality

Indicators of frying oil quality:

Total polar compounds
Conjugated dienes
FFA
Dielectric constant
Color
pH

Smoke Point of Cottonseed Oil