Study on Differences in Quality Indices of Peanut Oil Obtained by Different Oil-making Processes

Zhijun LIU Pingyang SHAN Wenchao XU Di WANG Jidong XU Xiaolong LI Lei WANG

Abstract    [Objectives]  The differences in indices of different peanut oil were found out by analyzing the quality of peanut oil obtained by different oil-making processes.

[Methods]  The effects of different oil-making processes on the quality of peanut oil were analyzed and compared through the determination of the main physicochemical indices， VE contents and fatty acid compositions of peanut oil obtained by cold pressing， hot pressing and leaching and refining.

[Results]  The leached refined peanut oil had the lightest color and the lightest fragrance; the hot-pressed peanut oil had the highest VE content; the refractive index， relative density and main fatty acid compositions of the three kinds of peanut oil had no significant differences; and there were trans-fatty acids in the leached refined peanut oil， and the trans-fatty acid contents in the leached refined fresh and aged peanut oil were 1.90% and 4.39%， respectively. The trans-fatty acid content is expected to be a characteristic index to distinguish pressed peanut oil from leached refined peanut oil.

[Conclusions]  This study can provide a theoretical basis for distinguishing and identifying peanut oil obtained from different oil-making processes.

Key words   Hot pressing; Cold pressing; Leaching; Refining; Peanut oil; Quality

Peanut oil is rich in nutrients and fragrant， and is a kind of high-end edible oil favored by consumers. China’s national standard for peanut oil GB 1534-2003 divides finished edible peanut oil into pressed finished peanut oil and leached finished peanut oil according to different oil-making processes， and specifies the definitions， quality indices and characteristic indices of these two kinds of peanut oil. However， the standard does not distinguish whether the peanut oil is pressed at high temperature or at low temperature. At present， most of the peanut oil sold in the market is the strong-flavor peanut oil produced by the pressing process after high-temperature frying and steaming  [1-3] . This kind of peanut oil has a strong fragrance， but a deep color and high content of fat-soluble impurities as well. It is not suitable as a first-class oil， and high-temperature pressing seriously denatures and destroys peanut protein， and peanut cakes can only be used as a cheap feed  [4-6] . Low-temperature pressing and low-temperature cold filtration process retains the characteristics of the original ecological oil-making process to the greatest extent， and the obtained peanut oil has a light color and a delicate flavor， and is gradually accepted and respected by consumers. However， the leached crude oil obtained from solvent leaching of high-temperature-pressed peanut cakes undergoes multiple refining processes such as hydration and degumming， alkali refining and deacidification， adsorption decolorization， and steam distillation and deodorization， and it is  difficult  to distinguish it from cold-pressed peanut oil in terms of  sensory  and quality indicators， which gives the opportunity to unscrupulous operators to confuse the peanut oil product market with shoddy products  [7] . In this study， the differences in indices of different peanut oil were found out by analyzing the quality of peanut oil obtained by different oil-making processes， so as to distinguish and identify peanut oil products obtained by different oil-making processes.

Materials and Methods 

Materials and instruments

Fresh cold-pressed refined peanut oil， hot-pressed refined peanut oil， leached refined peanut oil （National Standard Grade 1）， and aged cold-pressed refined peanut oil， hot-pressed refined peanut oil and leached refined peanut oil were selected as samples. The fresh peanut oil samples were taken from a peanut oil processing plant in Hebei Province， and aged peanut oil samples were taken from a peanut oil processing plant in Hebei Province and were sealed and stored at room temperature in the laboratory for more than one year. All peanut oil samples were free of antioxidants， and the raw materials were peanuts of the same origin and variety. The used 95% ethanol， cyclohexane， iodine monochloride， acetone， bromophenol blue， petroleum ether， potassium biphthalate， chloroform， sodium methoxide and methanol were all analytically pure; and the used n-hexane and isopropyl ether were chromatographically pure.

SHZ-D circulating water vacuum pump; AL104 analytical balance; 101 type electric heating blast drying oven; HH-1 type constant-temperature water bath; RE52-86A rotary evaporator; DT-500A electronic balance; constant-temperature air bath  shaking  box; density meter （range  0.8-0.9）;  GC-2010 gas chromatograph （Shimadzu， Japan）; 10-AT series high performance liquid chromatograph （Shimadzu， Japan）.

Experimental methods

Determination of VE content in peanut oil

With reference to GB/T 5009.82-2003， the content of VE in peanut oil was determined by high performance liquid chromatography. The chromatographic conditions were as follows： RF-10AXL fluorescence detector， chromatographic column， Agilent SIO column （250 mm × 4.6 mm， 5 μm）， n-hexane-isopropyl ether （volume ratio  90∶ 10）  as the mobile phase at a flow rate of 1.5 ml/min， column temperature 40 ℃， excitation wavelength 298 nm， emission wavelength 325 nm， and injection volume 5 μl.

Analysis of fatty acid composition of peanut oil

Gas chromatography analysis conditions： chromatographic column， SGE fatty acid column （0.25 mm×120 mm）; detector， flame ionization detector （FID）; column temperature 180 ℃; inlet temperature 210 ℃; detector temperature 230 ℃; hydrogen flow 47.0 ml/min; air flow 400 ml/min; injection volume 2 μl.

Evaluation of sensory quality of peanut oil

A small amount of sample was poured into a beaker and heated to 50 ℃， and stirred with a glass rod to smell the aroma. The aroma score was divided into following types： not fragrant （1-2 points）， slightly fragrant  （3-4 points） ， fragrant （5 points）， very fragrant （6-7 points），  extremely fragrant （8-9 points）. The evaluators consisted of 5-6 persons familiar with the aroma of peanut oil， and each sample was randomly assigned.

Results and Discussion 

Analysis of main physical and chemical indices of peanut oil obtained by different oil making processes 

Sensory quality of different peanut oils

It can be seen from Table 1 that the color of leaches refined peanut oil was the lightest， followed by cold-pressed peanut oil， and the hot-pressed peanut oil had the darkest color. After more than 1 year of storage， the leached refined peanut oil had obvious color recovery. The aroma scores of the three peanut oil samples showed that the order of aroma from strong to weak was： hot-pressed peanut oil， cold-pressed peanut oil， and leached refined peanut oil. After more than 1 year of storage， the leached refined peanut oil showed the most serious loss of aroma， and had very poor aroma and taste， and hot-pressed peanut oil and cold-pressed peanut oil also lost a lot of aroma， and showed a taste becoming worse.

Analysis of VE contents in peanut oil obtained by different oil making processes

The analysis results of VE contents in peanut oil obtained by different oil making processes are shown in Table 2. It can be seen from Table 2 that the VE contents of peanut oil obtained by different oil making processes showed great differences. Regardless of fresh peanut oil or aged peanut oil， the hot-pressed peanut oil had the highest VE content， and the leached refined peanut oil had the lowest VE content. Due to the multiple steps of refining （especially steam distillation and deodorization process） of leached peanut oil， natural antioxidants such as VE in it will be lost to a large  extent  [8] ，  while comparing peanut cold pressing and hot pressing， the solubility of VE in oil is higher under the action of high temperature. After a certain period of storage， the VE content in the leached refined peanut oil was significantly reduced， and the VE content in the aged peanut oil was less than 1/2 of the VE content in the fresh oil， while the VE content in the pressed peanut oil did not change much. The VE contents in the aged hot-pressed peanut oil， cold-pressed peanut oil， and leached refined peanut oil were 95.43%， 92.86%， and 46.98% of those in fresh peanut oil， respectively.

Analysis of fatty acid compositions of peanut oil obtained by different oil making processes

The main fatty acid compositions of peanut oil obtained by different oil-making processes were determined， and the results are shown in Table 3. Comparing Table 3 with the national standard for peanut oil， the main fatty acid compositions were all within the national standard.  Comparing the peanut oil obtained by three different oil-making processes， the contents of linoleic acid and linolenic acid in the cold-pressed peanut oil were higher than those of other two kinds of peanut oils， and the linoleic acid content in leached refined peanut oil was significantly lower than that in pressed peanut oil.

Zhijun LIU  et al.  Study on Differences in Quality Indices of Peanut Oil  Obtained  by Different Oil-making Processes

Conclusions and Discussion

With the different peanut oil-making processes， the quality indicators （color， flavor） and VE content of peanut oil showed differences. The color of leached refined peanut oil was better than that of pressed peanut oil， but the fragrance was the least; and the VE content of hot-pressed peanut oil was significantly better than that of leached refined peanut oil. The main fatty acid compositions of peanut oil obtained by different oil making processes were slightly different， but the contents of linoleic acid and linolenic acid in cold-pressed peanut oil was higher than those of other two peanut oils， and the content of linoleic acid in leached refined peanut oil was significantly lower than that in pressed peanut oil.

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 Editor： Yingzhi GUANG   Proofreader： Xinxiu ZHU