Fats and Oils

AarhusKarshamn

The only difference between a fat and an edible oil is that at ambient temperature a fat is semisolid, and appears more or less firm to the touch, and an oil is liquid. They are both of similar general chemical composition. Edible oils are completely different in chemical form from mineral oils that are used as fuels or lubricants. Most mineral oils are hydrocarbons and are hazardous to human health so should not be even a small component of food. Edible oils are esters of fatty acids and glycerol, they are called glycerides. Typically there is a great range of different glycerides in a particular fat or oil. Each glyceride has a specific melting point so the mixture which makes up a fat has a long melting range. When the "melting point of a fat" is referred to it normally means the slip melting point. This is a specific laboratory test and indicates the temperature when approximately only 4% of fat solids are present.

In addition to butter, which is a well known fat used in baking, there is a great range of vegetable and animal (including fish) fats used in biscuit manufacturing. In all cases the fat will have been refined and deodorised. This process removes colours, acids and other materials that give unpleasant flavours that occur naturally in fats extracted from various sources and which develop during storage.

In many cases the fat refiner can modify the fats by processing so that more or less any physical or chemical form of fat is theoretically possible. These changes can be expensive so the modified fats that are commonly encountered by biscuit manufacturers are limited to blends with particular characters or fats which have been merely "hardened" to raise their melting temperatures.

Oils are glycerides which have significant amounts of unsaturated fatty acids in their composition. Fats have less unsaturated and more saturated fatty acids. The unsaturated bonds make the oil or fat more susceptible to oxidative rancidity. The "hardening" process breaks the unsaturated bonds in the fatty acids and adds hydrogen atoms. This increases the stability of the fat to rancidity and also raises the melting point.

Fat rancidity

Fats, more than any of the other ingredients used to make biscuits, can cause quality problems. The main problem is rancidity which is manifest as very unpleasant flavours. Oxidative rancidity, which is the most common type, happens when fats are exposed to the air. Certain chemical bonds in the fats change to produce new substances which even when present at very low concentrations render the fats unsuitable for use. These changes are more likely in fats which are liquid at ambient temperatures.

There is another type of rancidity known as hydrolytic rancidity which involves the formation of soaps (saponification) from the fat. The conditions for this chemical change to occur require damp alkaline conditions but this happens very rarely in biscuit fat handling.

Bulk handling of fats

It is common to handle fats in bulk as warm liquids, oils. The freshly refined oil is delivered in road tankers and will be around 50°C. This is pumped into silos at the factory and must be held at about 40-45°C to ensure that there is no crystallisation during storage. The silo is kept warm usually by a water jacket, as electric heaters can produce very hot areas that may cause unwanted chemical changes in the fat.

As required, the oil is pumped from the silo and is ideally cooled before use in a dough. Cooling is achieved in special machines known as scraped surface chillers and the process usually also involves plasticising to give small crystals.

There are certain precautions that should be taken in bulk handling oils.

  • before accepting a delivery of oil, sampling should ascertain that the material is both of the type expected and that its quality is satisfactory in terms of taste and certain other chemical characters.
  • delivery should be filled into an empty and clean silo. In this way contamination with an old oil is avoided.
  • there must be a filter in the pipe line between the tanker and the silo and this filter must clean before use and examined after the delivery is complete. It is not unknown for some very unpleasant matter to have become included with the fat and it is important to know about this before the oil is used for making food.
  • the process of oil refining involves the use of fine materials that may not have been completely removed. These materials are harmless but as an oil is held in a silo there will be sedimentation. It is therefore necessary to design the silo with a sloping floor and to draw off the oil from the high end of the floor. In this way sedimented matter does not pass out of the tank.
  • even by observing all the precautions, rancidity cannot be avoided. It is therefore important to use bulk oil as fresh as possible and in no case to hold it for more than two or three weeks.

Plasticised and boxed fat, shortenings

It is normally ill advised to use liquid oil, either warm or cold, in a biscuit dough mix because the structure of the dough is affected by the rapid coating of the flour by liquid oil. Also it seems that the presence of very small fat crystals, as occurs during the cooling and plasticising of fats, has a highly beneficial effect on the structure during baking. Therefore bulk handled oil should be cooled before use.

Fats for use in puff pastry

Puff pastry fat must have a high SFI at the dough temperature yet be sufficiently plastic to permit rolling to very thin continuous films between layers of dough. To achieve this extreme degree of plasticity it is normally necessary to allow a well plasticised fat to stabilise for a number of hours before re-plasticising it at low temperature. One needs a much longer plastic range than cream fats (shallower melting curve) and to achieve this it is necessary to compromise by having some high melting glycerides present. Any glycerides that melt above blood temperature will leave a waxing feel in the mouth so should be used with caution. Some commercial pastry margarines used for puff pastry have a lot of higher melting glycerides which give very unpleasant tails in the mouth.

It is probable that the inclusion of 13-17% water and some simple emulsifier may aid the preparation of sufficiently plastic fat for use in puff doughs.

Nutritional aspect of fats

There are health concern claims about fats rich in saturated fatty acids or fats with structures such as trans fatty acids that can develop during the hardening processes.

For ethnic and vegetarian reasons most biscuit manufacturers tend to use fats that do not have lard, beef or fish oils in their composition.