Editor’s note: This is the first in a series of three articles on frying. Subsequent articles will be published in the August/September and October/November issues of FQ&S.
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Fried foods are enjoyed the world over. Each country has fried items that may be acknowledged as favorites or comfort foods. There are churros in Mexico, schnitzels in Austria, fish and chips in England, and dumplings in China. Yet according to Dr. Walter Clark, “The popularity of fried food persists in spite of public concern about calories, cholesterol, and saturated fat and that fat intake should be moderated as part of a balanced diet.” This sound like a recent statement? You would be wrong. This is from a 1991 paper in Food Technology.
So, why do frying and fried food persist? There are two reasons. The first has already been alluded to: Fried foods taste good. Properly prepared fried foods have wonderful flavors, textures, smells, and mouthfeel. People simply enjoy good food. The second reason is more practical. Frying is a very efficient means for preparing foods, which not only will fully cook the food but will also help to ensure its overall microbiological safety. To fully cook a piece of chicken in an oven might take 30 to 35 minutes, whereas cooking that same piece of chicken in a deep-fat fryer might take 4 to 6 minutes. For a restaurant or food service operator, time is literally money. The phrase “properly prepared fried food” is used intentionally. The food industry at both the industrial level and for food service and restaurant operators relies on repeat sales, so food quality is an essential element for success. So, quality management in frying is an essential business tool. In fact, the importance of food quality, especially the sensory parameters, was underscored by the first of the eight recommendations that came from the 3rd International Symposium on Deep-Fat Frying in 2000 (see “Recommendations of the 3rd International Symposium on Deep-Fat Frying,” below).
“Principle quality index for deep-fat frying should be sensory parameters of the food being fried.”
At the 10th International Symposium on Deep-Fat Frying, held in Hagen, Germany in March 2020, participants emphasized the importance of food quality by citing the 2000 symposium and making the same statement the symposium’s first recommendation.
Michael Blumenthal, PhD, was one of the first to look at frying using a systems approach. He described frying as a dehydration process and proposed that the process be defined using a five-phase frying quality curve (see Figure 1). The five phases are break-in, fresh, optimum, degrading, and runaway. To ensure the best quality food, processors and food service/restaurant operators should strive to maintain their oil in the optimum phase for as long as possible.
Elements of Frying Quality Management
So, how do fryer operators best maintain food quality? The key is properly managing the frying oil. There is a very simple equation: Bad oil equals bad food. Oil chemists have tended to focus on the degrading oil, but as Dr. Blumenthal’s work emphasizes, the key is a systems approach. Understand how to manage the different elements making up fryer operations. Canadian scientist Dr. C.J. Robertson (1967, 1968) described six areas to ensure the quality of fried foods. These recommendations, seen below, are more than 50 years old but still apply to frying operations:
- Proper design, construction, and maintenance of equipment;
- Proper operation of equipment;
- Proper cleaning of equipment;
- Minimized exposure to UV light;
- Salt and other metals sources kept away from oil; and
- Regularly filtered oil.
In 1993, Stier and Blumenthal proposed that a seventh principle be added. They suggested that oil be tested regularly. This recommendation was reiterated in the 2000 recommendations shown in “Recommendations of the 3rd International Symposium on Deep-Fat Frying,” below. Of course, fryer operators must select the proper frying oil for their operation and ensure that the oil that is delivered to them meets the established specifications. Selecting the proper oil is a topic unto itself.
Let’s examine these different elements and learn a bit more about quality frying.
Design, Construction, and Maintenance of Equipment
Buying equipment that is easy to operate, clean, and maintain makes everyone’s life easier. This is especially important in foodservice and restaurant operations since the persons who will be operating those units are not highly skilled. In industrial operations, fryers must be properly designed and sized for the product and scheduled volumes of food being fried. Operating an industrial fryer at less than capacity will damage oil, and, hence, food quality. If an industrial operator is not sure of volumes, it would be better to buy a small fryer and operate at full capacity.
And, when purchasing equipment, potential buyers must look at basic principles of sanitary design to ensure safe and hygienic operations. The American Meat Institute has defined 10 basic principles for sanitary design of equipment. Any decisions to purchase equipment for industrial frying operations should incorporate these principles into the decision-making process.
Proper Equipment Operation
Each fryer, whether purchased for industrial operations or a restaurant, will come with a detailed operating manual. This must be followed to ensure the best quality food and efficient operation. One of the key elements when operating fryers is managing temperature. Ideally, fryers should be operated at temperatures as low as possible to ensure the production of high-quality foods. Raising temperatures by 10 degrees C will double the reaction rates and, therefore, the rate of oil degradation. So, increasing operating temperatures is never the answer to an operational issue. Another key element for maintaining oil quality is how down time is managed. It is best to drop the temperature in the fryer during lunch breaks or slack times in restaurants.
Properly Clean Equipment
It is imperative that fryers be properly cleaned when needed. The product mix, type of product, equipment operation, and frying processes all have an effect on how dirty a fryer might get. One of the major concerns is polymer formation on the surface, which means that aggressive cleaning chemicals such as sodium hydroxide-based cleaners must be employed. Most cleaning compounds contain materials such as sodium or calcium salts. The cleaning process must not only remove the soil, but also make sure that cleaning chemical residues be removed. The progression of cleaning, therefore, would be as follows: empty the fryer, rinse to remove gross soil, implement caustic cleaning, drain the fryer, rinse to remove cleaner residue, and, finally, use an acid rinse to neutralize and remove any residual cleaner. The last step should be a water rinse. It is imperative that the system be properly drained. If any water remains in the fryer, there is a potential safety issue. If there is a significant amount of polymer on the surfaces of the fryer, it must be removed by scrubbing. If an operator fails to remove soap residues and moisture, that residue will react with the cooking oil and water to form soaps. Soaps will act as prooxidants with the potential to damage the oil and reduce its useable life.
Minimize Exposure to UV Light
UV light will catalyze oxidation of fats and oils at the double-bonds in the unsaturated triglycerides. The end result of these reactions may be compounds that will act as prooxidants that will further damage the oil. The answer to this issue is simple—select lighting that does not generate ultraviolet light. Avoid fluorescent lighting.
Keep Salt and Other Minerals from Oil
Metals will also catalyze oxidation reactions. When looking at the reactivity of metals, the progression of reactivity is:
Copper > Brass > Iron > Zinc > Stainless Steel > Magnesium > Calcium > Sodium
Equipment design and maintenance will contribute to the potential for exposure to metals. As an example, on an industrial frying line producing potato chips, the seasoning reel will be located far enough from the fryer so that seasoning will not get into the oil. In a restaurant, one should never season or salt fried foods while they are draining after being removed from the fryer.
All fryers, including those that are used industrially and in food service and restaurants, should be constructed from stainless steel. This must include all plumbing. If valves or fittings have to be replaced, replacements must be stainless. A sure way to destroy frying oil and the product being fried is to use a brass or bronze fitting instead of stainless steel.
It has already been mentioned that metals in cleaning compounds will react with oils in the presence of water to form alkaline soaps, which are significant prooxidants.
Today, most fryers include a built-in system to filter oil. Filtration will remove charred materials, breading, and other materials that come from the food being fried. These materials will darken oil and may re-deposit on finished, fried foods, compromising their appearance. Removing particulate from frying oil will help extend oil life. Food particles often act as the focus for degradation reactions, which will damage oil and compromise food quality.
There are two kinds of filtration systems used in frying operations:
- Passive Filtration—These systems simply remove particulate from the oil through sieving. Some also call passive filtration simply filtration.
- Active Filtration—Active systems are much more complex. These systems not only remove particulates but will also remove oil soluble components. Active systems are also referred to as treatments.
A well-designed filtration or treatment system can significantly enhance oil quality and extend oil life. The frying oil quality curve in Figure 1 also shows how the use of an oil treatment product called Supersorb by Filtercorp extends the optimum frying period, thereby extending oil life and helping maintain food quality for a longer period.
Test Oil Regularly
As noted, testing oil regularly was suggested by Stier and Blumenthal as a tool for quality management. This is where the “rubber hits the road,” so to speak. Dr. Robertson’s six principles of quality will help maintain oil quality and, therefore, allow the fryer operator to produce high quality food, but the question is, “At what point does oil quality change to the point where food is now unacceptable?” Unfortunately, there is no such thing as one size fits all, since each and every fryer operator’s perception of quality differs. It is up to each operator to conduct frying studies to establish the relationship between oil and food quality.
There are many chemical markers of frying oil degradation (see “Chemical Markers of Oil Degradation,” below). There are also a number of rapid tests available that can be used in lieu of conducting a chemical test. It is up to the fryer operator to determine which chemical marker or markers correlate with optimum food quality and the point at which the oil degrades to the point that food quality is unacceptable. Chemical testing of the progressively degrading oil must be accompanied by sensory testing of the food. The processor or restaurant operator must be involved with the sensory work as they know what is acceptable better than anyone else.
There are challenges when it comes to oil testing. One is determining the end point, the second is determining which test or tests to use, and the final is actually doing the test. It is much easier to test degrading frying oil in an industrial operation than it is in a restaurant. Restaurant operators must use a simple, quick test, as the people working in restaurant operations are generally not very sophisticated. Most foodservice and restaurant operators do no testing at all, but instead discard oil based on schedules.
The ultimate goal of frying is to produce good-tasting food. This goal is universal; that is, it applies to both industrial frying and foodservice or restaurant operations. The key to producing high quality food is maintaining the frying oil in the best condition for as long as possible—Dr. Blumenthal’s optimum state. There are a number of means available to fryer operators to maintain and monitor oil quality. These include purchasing good equipment, operating that equipment properly, cleaning the fryers, filtering oil, and minimizing the potential for oxidizing the frying oil by keeping ultraviolet light and metals away from fryers. Unfortunately, there is no single end point for all fryer operators. Each operator will need to determine an end point for their own operation since everyone’s concept of food quality is a little different.
Recommendations of the 3rd International Symposium on Deep-Fat Frying
- Principle quality index for deep-fat frying should be sensory parameters of the food being fried.
- Analysis of suspect fats and oils should utilize two tests to confirm abuse. Recommended analyses should be:
- Total polar materials (<24%)
- Polymeric triglycerides (<12%)
- The use of rapid tests for monitoring oil quality is recommended:
- Correlate with internationally recognized standard methods;
- Provide an objective index;
- Be easy to use;
- Be safe for use in food processing/preparation area;
- Quantify oil degradation; and
- Be field rugged.
- Affirming previous work: There are no health concerns associated with consumption of frying fats and oils that have not been abused at normal frying conditions.
- Encourage development of new and improved methods that provide fats and oils chemists and the food industry with tools to conduct work more quickly and easily. Work should strive to develop methods that are environmentally friendly, and using lower quantities of less hazardous solvent systems.
- Encourage and support basic research focused on understanding the dynamics of deep-fat frying and the frying process. Research should be cross-discipline, encompassing oil chemistry, food engineering, sensory science, food chemistry, and nutritional sciences.
- One of the basic tools to ensure food and oil quality is the use of filtration. Filter materials should be used to maintain oil quality as needed.
- Used, but not abused, oils may be topped up or diluted with fresh oil with no adverse effects on quality. Abused fats and oils were defined in the first two recommendations developed during this program.
This event took place in Hagen, Germany in 2000.
Chemical Markers of Oil Degradation
- Total polar materials (TPM)
- Free fatty acids (FFA)
- Alkaline soaps
- Oil color
- Peroxide value (PV)
- Anisidine value (AV)
- Polymeric triglycerides