The detection of gutter oil has always been a "world problem". Due to the complexity of the composition of the gutter oil, many research units after painstaking research, it is still difficult to find a reliable and effective detection method.

Last December, the Food Safety Risk Assessment Center of the Ministry of Health for the second time to the country to collect gutter oil detection methods. Recently, the Ministry of Health disclosed that it had initially finalized seven testing methods, and was evaluating and assessing the authenticity and reliability of these seven testing methods, but they have not yet been announced.

Gutter oil detection methods are still not unveiled. Why is gutter oil detection so difficult? Can we really find a reliable and effective detection method? Detection methods can really guard the gutter oil back to the table of the last line of defense?

This issue of Technology Visibility will help you solve the mystery of gutter oil detection.

Dressed in an invisible suit

Gutter oil is from a complex source, mixed with different ingredients, and by washing, distillation, decolorization and other processing, or mixed with edible vegetable oils, it is difficult to distinguish between sensory analysis and some physical and chemical indicators, the conventional detection indicators are basically ineffective

In 2011, the Ministry of Public Security uncovered a large gutter oil across many provinces of the case of cooking oil production and sale of cooking oil, the police seized a large amount of gutter oil in Ninghai, Zhejiang Province. In 2011, the Ministry of Public Security cracked a large-scale case of cooking oil production and sale across multiple provinces, and the police seized a large amount of gutter oil in Ninghai, Zhejiang Province, but only two of the 10 samples sent for testing were found to be substandard.

By the end of 2011, Chongqing police cracked the first major case of gutter oil production and sale in southwest China. However, the gutter oil in the case, which was already recognized by police as having been made from kitchen waste, was tested according to China's main testing indicators for cooking oil testing, but almost all of them passed.

This is the embarrassment that China's gutter oil testing methods are currently experiencing.

Because of the complexity of the source of gutter oil, mixed with inconsistent composition, and after washing, distillation, decolorization and other processing, or mixed with edible vegetable oil, has been very difficult to distinguish through sensory analysis and some physical and chemical indicators.

According to the analytical methods of the national hygienic standard for edible vegetable oils (GB/T5009.37-2003), these tests mainly determine the organoleptic, moisture content, acid value, peroxide value, carbonyl value and iodine value of the gutter oil.

In a May 2011 paper in Occupation and Health, Liu Bo, an engineer at the Center for Disease Control and Prevention in Taizhou City, Jiangsu Province, pointed out that gutter oil can be made to meet the national health standards for edible oils by the refining process of alkali refining, dewatering, decolorization and deodorization, which can result in the indicators of acidity, moisture and sensory content. As for the indicator of peroxide value, because peroxide is easy to be decomposed by heat, the peroxide value of grease after heating is lower than that before heating, therefore, the conventional testing indicators can only determine the quality of grease, but not to determine whether it is gutter oil.

Wang Zhutian, an expert at the National Center for Food Safety Risk Assessment, also pointed out that the degree of refining of gutter oil has been very high, and that the gutter oil that is imagined to have some contamination is no longer the same thing as the gutter oil that is now highly refined.

He said in an interview with the media, "For example, some pollutants, it can be completely removed by refining, so it is simply not possible to measure again, that is, why according to some of our current testing methods, such as hygiene indicators, quality indicators, as well as possible pollutant indicators can not be detected."

Wu Yongning, director of the chemical contamination monitoring room at the Chinese Center for Disease Control and Prevention's Institute of Nutrition and Food Safety, went so far as to say that once the government announced the testing indicator, rivals were likely to quickly and silently erase it from the gutter oil, rendering the test ineffective.

Liu Zhihong, a professor at Wuhan University's School of Chemical and Molecular Sciences, said in an interview with Nanfang Daily that the biggest problem with gutter oil is the carcinogenic aflatoxin. "Although current technology can detect aflatoxin, not every kind of gutter oil has excessive aflatoxin in it."

This is the same dilemma that every testing program currently encounters.

September 18, 2011, the Ministry of Health issued a news release, all-out efforts to organize scientific research to study the identification of gutter oil test methods. However, the five gutter oil testing methods developed by seven technical organizations that were recruited all failed because "experts found that these methods were not very specific.

This includes the Beijing Food Safety Monitoring Center's "Beijing Solution," which had high hopes. The specificity indexes of gutter oil selected for this program include four categories: "polycyclic aromatic hydrocarbons (PAHs), cholesterol, electrical conductivity, and specific genes". Among them, carcinogenic PAHs are considered to be the most hazardous component of gutter oil that has been proved so far.

This is the Beijing food safety monitoring center of the test staff spent nearly three months, the comprehensive use of chromatographic analysis, spectral analysis, physical and chemical analysis and gene identification technology and other modern analytical testing methods, has more than 80 technical indicators for a full range of screening to determine.

But after the expert group organized by the Ministry of Health validated it, it was still not approved. In the actual test, the experts found that the detection of polycyclic aromatic hydrocarbons as the focus of the "Beijing program", but actually on some of the gutter oil samples can not do anything, because "after man-made special treatment, not all gutter oil contain polycyclic aromatic hydrocarbons".

Faced with the full force of researchers, the cunning gutter oil is like a cloak of invisibility.

Difficult to find specificity

Existing more than 350 kinds of detection methods, can be called "all the methods are effective, but all the methods are not suitable for all the gutter oil", it is difficult to achieve "neither wrong blame the good oil, and do not let go of bad oil "

In the new call for methods, the National Center for Food Risk Assessment (NCFRA) put forward three screening principles for the detection of gutter oil: first, normal vegetable oil samples should not be misclassified; second, gutter oil samples should have a high rate of correct detection; and, third, gutter oil samples should be able to be detected in the order of a gradient from the highest to the lowest.

For the detection method of gutter oil, in fact, there has long been a lot of domestic research, through experiments, proposed a lot of gutter oil specific indicators.

The "cholesterol, conductivity" and other two indicators in the "Beijing program" have long been discussed by many researchers, and are considered to be an important and effective basis for identifying gutter oil.

Gutter oil in contact with salt, monosodium glutamate (MSG), underground metal pipelines, and used iron drums, metal ions exceeded the standard, especially sodium and iron ions. In addition, the waste grease of the catering industry will also produce some small molecules of polar substances in the process of rancidity, which will affect the electrical conductivity of the grease together with various metal ions.

Some research results show that qualified edible vegetable oils have low electrical conductivity, while gutter oils have larger electrical conductivity, which is three times that of canola oil, 11 times that of soybean salad oil, and 28 times that of lard. Accordingly, the researchers concluded that gutter oil can be tested against cooking oil by its electrical conductivity.

Gutter oil has a complex composition and is inevitably mixed with animal fats during recycling. Animal fats generally contain a large amount of cholesterol, while in vegetable oils generally do not contain cholesterol. Some studies have shown that the cholesterol content of soybean oil and canola oil is 0.031 mg/g, while the cholesterol content of pure gutter oil is 0.429 mg/g.

But all of these indicators may in fact be "open" to gutter oil. If a batch of gutter oil has only been used to fry French fries or doughnuts, it is entirely possible that it contains no electrolytes and has a low conductivity, according to Dr. Yun Wu Xin, a member of the Scientific Squirrel Society and a doctor of food engineering.

The same is true for cholesterol measurements: gutter oil, which is made up mostly of vegetable oils, can pass muster. This, coupled with blending with qualified cooking oil, can further dilute the cholesterol content within the gutter oil.

The researchers looked for other breakthroughs. Sodium chloride and monosodium glutamate, the most commonly used flavoring ingredients in food cooking, can be retained with food residues within waste oils and fats, such as frying waste oil and driving rain oil, making a significant difference between the sodium chloride and monosodium glutamate content of regular oil and waste oil. In a 2010 paper in Modern Scientific Instruments, researchers detected average sodium ion levels in gutter oil that were much higher than in qualified cooking oil.

Other researchers have concluded that certified cooking oil does not contain the synthetic chemical sodium dodecylbenzene sulfonate, whereas gutter oil, which is collected from restaurant dishwashing systems and comes into contact with underground domestic sewage, contains high levels of the detergent sodium alkylbenzene sulfonate.

A researcher measured the volatile components in gutter oil and found that the sample oil contained 16 volatile harmful components, 15 of which were aliphatic hydrocarbons and one was hexanal. Hexanal is a secondary product of oxidative deterioration of fats and oils, and can be used as an important basis for identifying gutter oil.

Some researchers through the thin-layer chromatography found that driving oil and frying old oil thin-layer chromatography has obvious trailing spots, while the edible vegetable oil is not. Separated by column chromatography and infrared analysis of trailing spot composition, found that driving oil, frying old oil trailing component is qualified cooking oil does not contain aldehydes, ketones.

There are also studies pointing out that the determination of fatty acid composition of each cooking oil has its characteristic fatty acid profile, the relative content of fatty acids must be. Gutter oil is a mixed oil system containing a variety of animal and vegetable fats and oils. For cooking oil systems adulterated with gutter oil, the relative fatty acid composition of such cooking oil is disrupted, and by comparing it with its normal fatty acid profile, it is possible to determine whether it is adulterated or not.

But Liu Zhihong analyzed that these methods are more or less problematic, and it is difficult to achieve the ideal effect of "neither blame the good oil, but also do not let go of the bad oil. What are the components of the gutter oil puzzling. Dalian Product Quality Supervision and Inspection Institute researcher Pan Wei frankly, the existing more than 350 kinds of testing methods, can be called "all the methods are effective, but all the methods are not suitable for all the gutter oil.