Research On The Serviceability Limit Of Frying Oil

Oil after a long-term repeated high-temperature cooking, frying and oxygen in the air and water contact,it will occurs hydrolysis, thermal oxidation, thermal polymerization, pyrolysis of a series of complex chemical reactions, resulting in a number of volatile saturated and unsaturated aldehydes, ketones, lactones and other substances harmful to human health. At present, it has many various indicators of quality to evaluate frying oils and fats, relevant literature have reported is the proportion, density, color, dielectric constant, viscosity, specific surface tension and other physical indicators and acid value, saponification value, iodine value, peroxide value, carbonyl base, polar compounds content, MDA content, such as chemical indicators, but it is the lack of a reasonable uniform methods and systems evaluation.Monitoring means of physical and chemical properties of oils and fats are not healthy, making some fried foods and catering enterprises can not be used for the timely replacement of oil and seriously endanger the health of consumers. In this paper, mainly from the sensory analysis, viscosity, permeability, acid value, peroxide value, carbonyl base price, the total polar material, thin chromatography analysis and GC-MS for qualitative and quantitative analysis of the limit use of frying oil.1. As the frying temperature and time, oil from the lemon-yellow color gradually deepened to black or dark brown and very sticky, odors from strong sweet-smelling to strong burnt blurred-smelling; taste rancidity from normal to even suffering piquant; transparency has been reduced, and finally become a serious sticky; smoke also frying temperature and time as the increase in gradually serious, even seriously irritates the eyes to tears.2. As the frying temperature and frying time increased the degree of polymerization of the oils and fats increased, the viscosity was very significant upward trend in temperature, the relationship between the viscosity and frying time in index is visible; As in the different temperature, the frying temperature increased, the viscosity value of the upward trend was evident. The oil viscosity can be used as an determine indicator of quality change.3. As the frying time, the changes of acid value is ups and downs is not a linear relationship, but on the whole is increasing. In the frying temperature increased, in particular time acid value is not linear, but on the whole there is still increased as the temperature trend, Frying conditions at 160℃, when the frying time of 20h, the frying oil has exceeded health standards; fried at 180℃under the conditions, when the frying time of 16h, it will be to exceed the health standards. Fried at 200℃under the conditions, when the frying time of 14h, the frying oil has over the course of health standards.4. In the same temperature as the frying time longer transmittance is decreased, and decrease is large, and at different frying temperatures with the temperature increasing, the transmittance is also reduced gradually. Thus a gradual increase in the frying temperature and frying time extended the transmittance of frying oils and fats significantly reduced.5. Peroxide value of oil samples in the early frying process is growing, but throughout the frying process it is fluctuated and has not discipline, with the frying time and frying temperature improve it has not the law, however, in the early stage of frying with the frying temperature and time increased, peroxide value is increase, frying time and temperature is higher the more likely to exceed the national standards. 6. As the frying time and frying temperature increased, the total polar material was the evident upward trend, and has a linear relationship. Frying conditions at 160℃, when the frying time for 8h, the frying oil is about to over the course of the value of health standards; 180℃under the conditions, for 6h, it more than to exceed hygiene standards; 200℃under the conditions, 6h at has exceeded health standards. Therefore, total content of polar substances could be used as an important indicator of quality of frying oil from the gradual growth from the linear relationship.7. At the same temperature, with the frying time extended,the carbonyl value was significantly increased and is the linear relationship, in the same time, as the temperature increased, the carbonyl value is also clearly on the rise. At 160℃, when 6h carbonyl value has exceeded the national standards of edible vegetable oils, in the 10h it close to the frying oil national standard. At 180℃, 4h carbonyl value has exceeded the national standard of edible vegetable oil, carbonyl value is next to the national standard for frying oil on 8h. At 200℃, when the 2h, the carbonyl value has exceeded the national standard of edible vegetable oil, carbonyl value is next to the national standard for frying oil on 6h. Overall, the carbonyl value of the frying oil was a significant linear increasing trend with the frying time and frying temperature increased.8. Frying oil through thin-layer chromatography analysis of experimental results, in fresh cooking oil and fried at different temperatures and different time periods under the R_f value, we can see that the fresh oil for frying oil more than the obvious tailing spot, and with the frying time and temperature increased, the color of their tail spot gradually deepened. These tailing spot the difference between frying oil is an important feature of edible vegetable oils.9. The experimental results show that myristic acid, palmitic acid, linoleic acid, linolenic acid, antioleic acid, oleic acid, stearic acid, arachidic acid, Eutrema acid are fried in fresh oil and frying oil samples ,Its contents is different with its peak area with the frying time, and there was no significant linear relationship. In the ion chromatogram can be found, with the frying time and temperature increased there is a number of small peaks and a gradual increase in these small peaks , because many frying oil containing hydrocarbon types, vinyl, aldehydes and ketones, and with frying time increase in the level of these substances are no obvious regularity. This indicates that repeated high-temperature frying process, resulting in a number of volatile saturated and unsaturated aldehydes, ketones, lactones and other substances harmful to human health. Therefore GC-MS can be used as an important way to determine the quality of frying oil in the qualitative and quantitative changes.