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.
Jacobsen (1991) described the importance of filtration and the removal of particulates that move from the food into the oil. He recommended that operators filter regularly “to remove charred batter and breading materials because these materials can darken oil, contribute bitter flavors to foods, impede heat transfer, and ruin the appearance of fried food.” Particles remaining in the oil continuously leach their components into the oil, chemically degrading the frying medium.
Blumenthal (1987) has observed that “food particles in an oil act as reactive sites for oil degradation.” A simple analogy to particulates in oil may be a starter crystal in a sugar solution in the production of rock candy. Without the starter, the desired crystallization will not occur. The same idea holds true with the chemistry of the frying oil: Remove the particulates, and reaction rates are slowed. Again, this may be seen in Figure 1.
The Pros and Cons of Oil Filtration
In foodservice or restaurant operations, oil life extension is one of the most important elements to ensure profitability. There are other potential benefits, but people also need to understand that oil filtration or treatment is not a panacea. Table 1 lists both potential benefits and concerns.
Let’s take a look at some of these benefits of and concerns with oil filtration and treatment. Some of these issues are closely interrelated. For example, extending oil life, which, as noted, is of paramount importance not only to restaurant and foodservice operators, but also to industrial operators, will also result in reduced oil usage and improved food-to-oil ratios. The food-to-oil ratio is a calculation that shows the amount of food fried to pounds of oil used. This can result in significant savings for the operator and also means that the amount of oil that has to be discarded (waste oil) is reduced. Simply extending fry life from two to three days in a 50-pound fryer will save almost 3,000 pounds of oil per year. Extending the oil life to six days will save 5,800 pounds per year, as may be seen in Table 2. This also means less oil handling, fewer concerns with discard issues, and reduced operating costs.
Food quality is another potential benefit. Food quality is a characteristic that depends upon the operator. It could be improved shelf life, enhanced flavor, or improved overall appearance of the product. One industrial processor adopted an active treatment system, which improved the shelf life, flavor profile, and overall appearance of a fried pepper product. The product fried in oil that was subjected to the oil treatment had improved pepper flavor and aroma when evaluated by an expert sensory panel.
When evaluating any kind of system in a food processing environment, operators must look at both potential benefits and concerns. As an example, Europe discourages the use of powders for oil treatments, whereas they are allowed in the United States. There have also been issues when it comes to handling powders. People should wear the appropriate personal protective equipment (PPE) when handling powders, which should include gloves and masks. This can be more difficult to manage and enforce at the restaurant level than in industrial operations. Lastly, there are products on the market that may seem beneficial but are, in reality, more damaging to the oil. There are treatment products on the market that will reduce free fatty acids in the oil by converting them to alkaline soaps. The soaps are very damaging to frying oil, enhancing the formation of free fatty acids and catalyzing oxidation reactions that will produce off flavors in fried food, in addition to significantly reducing oil life.
Perhaps the biggest concern with any kind of filtration system is blinding of the filter. Industrial operations often use indexing paper filters to continuously remove particulates from oil. A stationary system could blind very quickly, especially in an operation producing battered products, hence the indexing or moving paper. The type of product being fried has a direct influence on the potential for blinding a filter. By-products of frying that are slimy, pasty, or sticky will have a greater potential for blinding a filter. Examples of these products are meats, fish, and kettle-style potato chips. One might ask, “Why kettle style?” They are not rinsed prior to frying, so the potato starch ends up in the oil. Items like fried corn products, such as tortilla chips and breaded products, yield grainy by-products that are less likely to blind a filter and, in fact, can even build a filter cake on the filter medium.
Selecting an Oil Filtration or Treatment System
There are many different products available to the fryer operator at both the foodservice/restaurant and industrial levels. Each and every fryer operator will probably utilize a passive filtration system of some sort, but the question is, “Would installing an active system be beneficial or not?” In industrial frying, there are many different systems available to remove particulates, some of which are not filters at all. There are drag bars that remove particulates that settle to the bottom of the fryer and are then dragged from the fryer by the bar. Some processors use centrifugal separators. Systems that filter oil include catch boxes, stainless steel screens, rotary drums, stainless steel baskets, continuous belt filters, bag filters, and filter presses. Heat and Control is one of the main producers of fryers and frying systems.
There are also active systems currently in use by industrial frying operations. Most of these are designed to treat oil at the end of the day’s production. One example is a system where the used oil is mixed with an active treatment powder and allowed to react with the oil in a mix tank. The treated oil is then filtered to remove the powder and transferred to a holding tank or back into the fryer to be used in the future. The Dallas Group provides powders and the treatment system. Other systems, such as those from Filtercorp, utilize filter pads impregnated with active ingredients. The oil from the fryer is slipstreamed from the fryer and pumped to a filter vessel and through the filter pads on a continuous, real-time basis, before it is returned to the fryer or a holding tank. Filtering/treating oil continuously would require a pre-filter to remove suspended solids to prevent blinding of the system.
Today, a significant percentage of the fryers used in restaurant or foodservice operations are manufactured with a built-in filter apparatus but may also have working relationships with one or more suppliers of filtration products and services. There are also fryers that do not come with a built-in filter. In these fryers, an operator must utilize a portable filter that can be hooked up to the fryer at the end of the work day. Years ago, some operators used cone filters. The user would place a filter in the metal cone, and the fryer operator had to ladle oil into the cone so it could flow though the filter via gravity. These units posed a significant risk to workers handling the hot oil, so it is good that they have been phased out.
Both active and passive systems are used in foodservice and restaurant operations. The most common passive system is filter paper, which simply removes suspended solids from oil. Active systems include impregnated pads or papers, paper and powder, and powder. With paper and powder systems, the active treatment product is sprinkled on paper, and the oil is filtered over the powder and through the paper. In some cases, the powder is added to the oil and filtered out, but this type of system is losing favor as there are concerns about adulterating the oil and potentially the food.
If an industrial processor or foodservice/restaurant operator wishes to adopt a filtration system of any sort or make changes to what they are currently doing, they should conduct the necessary frying studies. These studies will not only allow them to gather baseline data on what they are currently doing but will provide them with information to properly evaluate the benefits, if any, of the new system. How to conduct frying studies and why these studies are so important was addressed in the last issue of Food Quality & Safety magazine in Part 2 of this series. As part of the decision process, a fryer operator should:
- Understand the chemistry of their oil;
- Understand what impurities they wish to remove from the oil;
- Understand the basic steps for treatment required in a given operation;
- Understand the limitations of the treatments being reviewed; and
- Understand their operations so that the benefits of the post-treatment will produce good results.
The frying study is the best tool to gather this necessary information for the decision-making process. It is also essential that all fryers who wish to evaluate any filtration or treatment system clearly establish goals and the indices that are to be used to determine endpoints; that is, will it be a chemical index of oil quality, a physical indicator, or a food quality attribute? All studies really should include the latter, since producing consistently high quality food is why people fry and why people enjoy fried foods so much.
Table 1: Oil Filtration Pros and Cons
|Pros of Filtration
· Reduced energy usage
| Cons of Filtration
Table 2: Oil Usage and Costs
|Day of Oil Use||
(days in year/oil usage)
(cycles x oil used
Oil = $1.00/lb
*Assuming a 50-pound fryer.