Editors’ note: This is the third in a series of three articles on frying. Part 1, “How to Ensure Quality in Fried Foods,” was published in the June/July issue of FQ&S and Part 2, “Frying Studies,” was published in the August/September issue.
In part 1 of this series on frying, we referenced the work of Dr. C.J. Robertson, who cited six elements for quality frying:
- Proper design, construction, and maintenance of equipment;
- Proper operation of equipment;
- Proper cleaning of equipment;
- Minimal exposure to ultraviolet (UV) light;
- No salt and other metals sources in oil; and
- Regular oil filtration.
Fryer operators at foodservice/restaurant or industrial operations should follow these criteria to better maintain and manage their frying oil and help ensure the production of quality fried foods. This means producing good tasting, high quality fried foods. Of these six criteria, perhaps the most effective and the one that can potentially yield the most benefits is the last: Filter oil regularly.
The following quote by Yates in 1996 supports this statement: “After the selection of the equipment itself, the most important aspect of frying oil filtration is the choice of the filter medium.”
The efficacy of filtration is a function of many things, one of which is obviously the equipment. Oil filtration varies in complexity depending upon the system and materials that are utilized. More than 30 years ago, Michael Blumenthal, PhD, attempted to simplify the basics of oil filtration media or systems by defining two basic types of filtration:
Passive Filtration Systems: These systems simply remove particulate from the oil through sieving. Passive filtration has also been called simply “filtration” by some. Examples of passive systems are filter paper, diatomaceous earth, bag filters, and steel screens. McLeod expanded on this type of system, describing passive filtration as the mechanical removal of solids by screening or fine filtration (2015). This is considered to be anything from 2 mm to 4 mm screens down to typically
50 microns. Other types of mechanical filtration, such as cake or depth filter pads, can go down to as low as 1 um.
Active Filtration: Active systems are much more complex. These systems not only remove particulates but will remove oil-soluble components from the frying oil. Active systems are also referred to as “treatments.” These include powders, impregnated paper or pads, and paper with active powders. Gupta (1992) further stated that active filters are those where the oil impurities are reduced via physical as well as chemical reactions, in addition to the removal of the suspended materials in the oil.
Figure 1 shows how oil life may be extended through the use of a passive filter and an active system or treatment. Fryer operators must understand that once frying is initiated, the damage to the frying oil cannot be reversed. It can, however, be slowed, which is one of the principle benefits of oil filtration or treatment.
Chow and Gupta (1994) provide support for this statement. They have observed that, “in reality, it is never possible to take any kind of used oil, reprocess it, and turn it into a product as good as the original.” They further note that “it is, however, possible to treat used oil in a specific manner to retard its degradation, and thereby prolong its useful life and reduce overall cost of the oil.”
Figure 2 is an example of how an active depth filter system or treatment works. Looking at the figure, you can see sieving or filtering to remove particulates, plus entrapment, adsorption, and absorption—reactions with the soluble components of the oil—which means that the oil is actually treating the oil. The figure also provides additional detail on how sieving, entrapment, adsorption, and absorption function.