lnfluence of two-stage harvesting on the properties of cold-pressed rapeseed (Brassica napus L.) oils

NlNG Ning ,HU Bing ,BAl Chen-yang ,Ll Xiao-hua ,KUAl Jie ,HE Han-zi ,REN Yi-lin,WANG Bo,JlA Cai-hua,ZHOU Guang-sheng,ZHAO Si-ming

1 Key Laboratory of Crop Ecophysiology and Farming System in the Middle Reaches of the Yangtze River (Ministry of Agriculture and Rural Affairs),College of Plant Science and Technology,Huazhong Agricultural University,Wuhan 430070,P.R.China

2 Key Laboratory of Environment Correlative Dietology (Ministry of Education),College of Food Science and Technology,Huazhong Agricultural University,Wuhan 430070,P.R.China

3 School of Life Science and Technology,Wuhan Polytechnic University,Wuhan 430023,P.R.China

4 College of Engineering,Huazhong Agricultural University,Wuhan 430070,P.R.China

Abstract Rapeseed (Brassica napus L.) harvesting method is critical since it significantly determines the seed yield,oil quality,and industrial efficiency.This study investigated the influences of harvesting methods on the quality of cold-pressed rapeseed oil of two varieties.Oil color,peroxide value (POV),tocopherol content,fatty acid composition,and polarity of total polyphenols (PTP) contents of two rapeseed varieties in Huanggang and Xiangyang were compared through artificially simulated combined harvesting and two-stage harvesting.Results showed significant differences in the quality of rapeseed oil between the two harvesting methods.The red value (R-value),POV,total tocopherol contents,linoleic and linolenic acid content,and PTP content of the pressed rapeseed oil prepared by the combined harvesting method were about 27.6,5.7,15.8,2.0,0.5,and 28.6% lower than those of the oil produced from the two-stage harvesting method,respectively.Xiangyang and Huayouza62 performed better in the two regions and two varieties,respectively.To sum up,the rapeseed oil obtained 41–44 days after final flowering of combined harvesting,35 days after final flowering,and six days of post-ripening of the two-stage harvesting had the best quality.

Keywords: rapeseed,two-stage harvesting,combined harvesting,oil quality

1.lntroduction

Rapeseed is the most important oil crop in China.Its average annual production and import in recent years amounted to 13.4 and 4.31 million tons,respectively,providing about 6.73 million tons of rapeseed oil,accounting for 24.4% of the world’s total rapeseed oil production and 47.0% of domestic vegetable oil production (Wanget al.2020).Over recent years,with the improvement of living standards,the quality of cooking oil has attracted considerable public attention.

The yield and quality of rapeseed oil are mainly related to their genotype,maturity stage,climate change,cultivation parameters (plant density and row spacing)(Nishizawaet al.2010;Wertekeret al.2010;Liuet al.2011),and rapeseed growth status (Khanet al.2018).Previously reported,the appropriate harvesting period and methods are key factors in avoiding harvesting loss and improving rapeseed oil quality (Jinet al.2020);besides,differences in fatty acids contents,acid value,and peroxide value (POV) under different harvesting methods(Huanget al.2019).In a previous study,rapeseed varieties significantly impacted protein content,whereas harvesting methods and periods affected the moisture content,unthreshed rate,grain loss rate,and straw moisture content (Zhanget al.2019).Furthermore,the rapeseed yield of mechanical two-stage harvesting was greater than combined harvesting with a lower loss rate,while the combined harvesting had a higher economic benefit (Wuet al.2014).

In recent decades,mechanical harvesting technology has been significantly improved,making rapeseed production an important part with lower cost and higher efficiency than traditional manual harvesting (Shiet al.2009).Rapeseed mechanical harvesting includes combined harvesting (uses combine harvesters to complete cutting,detaching,and sorting one time when the rape horn fruits are fully ripe) and two-stage harvesting (uses cutting and drying machinery to cut down the plant when the rape horn fruits are not fully ripe),followed by picking and threshing machines.Both harvesting methods have been implemented in rapeseed production in China (Shiet al.2015;Baiet al.2021).

The oil color is the basis of appearance when consumers buy,and GB/T 1536–2021 (2021) has specific requirements for the color and POV of cold-pressed rapeseed oil.The determination of oil color is a test item that must be mastered by grain and oil quality inspectors(Sunet al.2002);additionally,POV is the main index to evaluate the deterioration of vegetable oil and is commonly applied to measure oil quality (Shiozawaet al.2007).

The proportions of the different fatty acids and phenolic compounds are associated with oil quality;besides,the fatty acid composition and ratios largely determine the nutritional value of vegetable oils (Ajayiet al.2006).An earlier study reported that linoleic acid and α-linolenic acid have cholesterol-lowering,blood lipid-lowering,and cardiovascular disease prevention functions (Yang and Chang 2011).Moreover,phenolic compounds and vitamin E have strong antioxidant properties,which can prevent and reduce the damage of lipids in the cell membrane by producing peroxides and maintaining the integrity of tissues (Chernget al.2013;Sunet al.2018).

Previous research on harvesting methods and operation periods focused on the yield and content of rapeseed oil.However,the effects of harvesting methods on the characteristic quality indexes of rapeseed oil are still unclear.Herein,this study investigated the effect of different harvesting methods and harvesting periods on oil color,POV,tocopherol content,fatty acid composition,and PTP of two rapeseed varieties in Huanggang and Xiangyang.Current findings could add knowledge regarding the influence of different harvesting methods and harvesting periods on the oil quality of rapeseed.

2.Materials and methods

2.1.Experimental conditions

This present study used two rapeseed cultivars,Huayouza62 (HZ62),provided by Huazhong Agricultural University,and Zhongshuang11 (ZS11),provided by Oil Crops Research Institute,Chinese Academy of Agricultural Sciences,with an oil content of 41.46 and 49.04%,respectively.The study was carried out at Huanggang Academy of Agricultural Sciences(114°55′5′′E,30°34′24′′N) and Xiangyang Academy of Agricultural Sciences (111°50′43′′E,31°46′52′′N),Hubei Province,from September 2018 to May 2019.

2.2.Preparation of rapeseed oil

Combined harvesting was done in a split-plot design,with varieties as the main zone and different harvesting periods of CK1 to CK7 as sub-plot (Huanggang and Xiangyang were harvested 23 and 26 days after final flowering as CK1,respectively,with an interval of 3 days).Combined mechanical harvesting was simulated through direct threshing after manual cutting down at 10 cm of stem above the ground in three replications of each treatment.The split-plot design was used in the twostage harvesting,with variety as the main plot and cutting time of D1,D2,and D3 as sub-plot (20,26,and 32 days(Huanggang) and 23,29,and 35 days (Xiangyang) after final flowering and post-ripening time of 3,6,and 9 days as sub-sub plots.Two-stage mechanical harvesting was simulated by manually cutting down at 10 cm of stem above the ground and threshing after a certain time of post-ripening in three replications of each treatment(Table 1).

Table 1 The cutting time and post-ripening design in combined harvesting and two-stage harvesting

The meteorological data for the two trial sites,from final flowering to harvest of rape,are shown in Fig.1.The average daily temperatures at Huanggang and Xiangyangwere close from final flowering to maturity of rapeseed.However,the average daily relative humidity,atmospheric pressure,and rainfall at Huanggang were higher than at Xiangyang.In contrast,the average daily solar radiation was lower than at Xiangyang,resulting in earlier rapeseed maturity and three days earlier harvest at Huanggang than Xiangyang (Baiet al.2021).

Fig.1 Meteorological data for two trial sites in Huanggang and Xiangyang.

The rapeseed seeds were sun-dried,then divided into mesh bags and stacked separately to store in the seed hanging storage room with proper ventilation at Huazhong Agricultural University from May to September 2019.The average temperature during storage was 27.06°C,the relative humidity of the air was around 76.1%,and the moisture content of the rapeseed ranged from 8.0 to 9.0%.In September 2019,about 200 g of rapeseed was pressed with a German CA59G screw-type oil press (Germany IBG Inc.,Cologne,Germany),and oils were collected in 50 mL tubes and stored at–20°C for further analysis.

2.3.Determination of color

The color of rapeseed oils was determined using Lovibond colorimetric method (Karraret al.2020).Briefly,samples were placed into the spectrophotometer cell of 25.4 mm,and the measurement was carried out in the Lovibond colorimeter (Shanghai Merwell Biotechnology Co.,Shanghai,China).

2.4.Determination of peroxide value (POV)

The POV was determined according to the standard method of the International Milk Federation (Shinet al.2018).Samples were added to 9.8 mL chloroformmethanol mixture,vortex for 2–4 s,then 50 μL of NH4SCN solution was added to the mixture after vortex 50 μL of FeCl2solution was mixed.The mixture was kept at room temperature for 5 min,the absorbance value (As)was measured at 500 nm using a Multiskan Sky full wavelength enzyme calibrator (Thermo Fisher Scientific Co.,Shanghai,China),and water was taken as the blank control (Ab).

wherePOV,peroxide value (mmol kg–1oil);m,mass of the weighed oil (g).

2.5.Determination of tocopherol

The tocopherol of rapeseed oil was determined according to AOCS Official Method Ce 8-89.Oil sample(2 g) was weighed into a 25-mL volumetric flask and dissolved withn-hexane to constant volume.The mixture was filtered through a 0.22-μm filter membrane.The supernatant was analyzed using HPLC (Shimadzu Scientific Instruments Inc.,Tokyo,Japan) withn-hexaneisopropanol (99.5:0.5,v/v) as the mobile phase according to Huanget al.(2019).

2.6.Determination of fatty acid composition

The fatty acid composition was determined according to Ganet al.(2012).Samples were saponified with 0.5 mol L–1NaOH in 1.5 mL methanol at 100°C for 5 min and then cooled to room temperature.The saponified sample was mixed with 2 mL of methanol for methylation and heated at 100°C for 3 min.After cooling,2 mLiso-octane and saturated 1 mL NaCl were mixed with a vortexed,and supernatant was analyzed by gas chromatography (Agilent Technologies Inc.,Palo Alto,USA).

2.7.Determination of polarity of total polyphenols(PTP)

The content of PTP (mg 100 g–1) was measured according to the Folin-Ciocaileu Colorimetry method (Koskiet al.2002).Oil sample (2.5 g) was dissolved in 3 mL ofn-hexane,1.5 mL of 80.0% methanol solution was mixed and shaken at room temperature for 5 min,and the supernatant was transferred to a 50-mL of colorimetric tube.Polar total phenol extract (0.5 mL) was taken into a 10-mL centrifuge tube,followed by adding distilled water (5 mL) and Folin-phenol reagent (0.5 mL).Then,1 mL saturated Na2CO3solution was added.Finally,the solution was diluted with distilled water to volume,mixed with a vortex,and allowed to stand at room temperature(1 h);then,the absorbance of the sample was measured at 765 nm wavelength (Yanget al.2013).

2.8.Statistical analysis

The experiment was triplicated using a split-plot design,and means were expressed as ±standard deviation(SD) of triplicates for each sample.The significance of differences was analyzed using Duncan’s multiple range test (DMRT) at aP＜0.05 significance level.

3.Results

3.1.Effect of different harvesting methods on the color

Generally,there is a relationship between the color and the quality of the oil,whereas the Chinese rapeseed oil standard has specific requirements for the color.Herein,the Lovibond colorimeter value was used to indicate oil color,and the color of different treatments was evaluated through red value (R-value).In Xiangyang and Huanggang,the R-value of the HZ62 variety extracted from the combined harvesting was recorded at 30.2 and 25.0% lower than that obtained from two-stage harvesting,respectively (Table 2).

On the other hand,The R-value of the D series group with two-stage harvesting was higher than the CK group of combined harvesting.Although the R-value of D3L6 (35 days after final flowering and 6 days of post-ripening) was the lowest for two-stage harvesting,it was significantly higher than the CK5 of combined harvesting (Table 2).

Moreover,the ZS11 variety displayed that the R-value of two-stage harvesting was mostly greater than the CK of combined harvesting in Xiangyang and Huanggang.However,in Xiangyang,the lowest R-value of rapeseed oil was 22.5% lower in two-stage harvesting than incombined harvesting.Overall,the R-value of the pressed rapeseed oil prepared from the combined harvest was approximately 27.6% lower than that of the two-stage harvest (Table 2).

Table 2 Lovibond color (Red/Yellow) of Huayouza62 (HZ62) and Zhongshuang11 (ZS11) rapeseed oils obtained through the combined and two-stage harvesting

3.2.Effect of different harvesting methods on the peroxide value (POV)

Our results showed that the POV of HZ62 and ZS11 harvested by two methods decreased with the extension of days after final flowering.The lowest POV was noted at D1L3 and D1L6 for two-stage harvest,while in the combined harvest groups,the lowest value was recorded at CK5 and CK6 (38 and 41 days after final flowering) in Xiangyang and Huanggang of both varieties,respectively,as shown in Fig.2-A and B.

Furthermore,the POV showed a decreasing trend of fluctuation with the increasing post-ripening time for D groups,while it showed a downward trend before CK5,then began to rise.The two-stage harvesting was slightly lower than combined harvesting in the D1 and D2 series and significantly higher than combined harvesting in the D3 series for the HZ62 variety (Fig.2).

On the other side,the POV of the two-stage harvesting differs significantly from the combined harvesting in ZS11.D2L6 was the lowest,while significantly higher than CK5.Overall,the POV value of the pressed rapeseed oil prepared from the combined harvest was approximately 5.7% lower than that of the two-stage harvest (Fig.2).

3.3.Effect of different harvesting methods on tocopherol contents

The contents and components of tocopherol in rapeseed oil were significantly affected by cutting time in combined harvesting.Moreover,the total tocopherol contents were increased from 32 to 44 days,and it reached a maximum average of 100.40 μg g–1oil 44 days after the final flowering in HZ62 at Xiangyang and Huanggang.

In ZS11,α-tocopherol,γ-tocopherol,and total tocopherol contents showed a fluctuating increasing trend in Xiangyang and Huanggang.However,there was a nonsignificant difference in total tocopherol contents between HZ62 and ZS11 varieties in Xiangyang and Huanggang when the harvesting time was 23–29 days.Under the condition of two-stage harvesting,the tocopherol contents of rapeseed oil were significantly affected after the final flowering and post-ripening time.

In the HZ62 variety,the post-ripening process profoundly affected the D1 series,and the tocopherol contents reached a maximum at 9 days of post-ripening.Additionally,α-tocopherol,γ-tocopherol,and total tocopherol contents were significantly increased with the extension of the post-ripening time in the D2 and D3 series.However,in the ZS11 variety,the tocopherol contents and its components were increased with increasing the final flowering days,and the time of postripening had a significant effect on tocopherol contents 38–44 days after final flowering.

Furthermore,with the prolonged post-ripening days (6–9 days),α-tocopherol,γ-tocopherol,and total tocopherol contents were increased significantly,and the maximum average of total tocopherol contents was recorded as 94.53 μg g–1oil at 9 days of post-ripening in Xiangyang and Huanggang.

At the same time as the final flowering and postripening,the tocopherol contents of HZ62 were significantly higher than ZS11 (8.0,25.6,and 18.4%),respectively,in Xiangyang,while the non-significant difference between the two varieties in Huanggang.However,the HZ62 in Huanggang had significantly higher α-tocopherol and γ-tocopherol contents than the ZS11 at D1L3 (16.8 and 18.3%),D2L6 (17.6 and 13.5%),and D3L9 (36.3 and 14.7%).Conclusively,the tocopherol contents in HZ62 were higher in Huanggang and Xiangyang versus ZS11.The total tocopherol contents of the pressed rapeseed oil prepared from the combined harvesting were 15.8% lower than that of the two-stage harvesting (Fig.3).

Fig.3 Tocopherol content of rapeseed oils obtained through different harvesting methods in Huayouza62 (HZ62) and Zhongshuang11(ZS11) at Huanggang and Xiangyang.A and C,tocopherol content of rapeseed oils in combined harvesting at Huanggang and Xiangyang,respectively.B and D,tocopherol content of rapeseed oils in two-stage harvesting at Huanggang and Xiangyang,respectively.Different letters indicate significant differences according to Duncan’s multiple range tests at P＜0.05.Error bars indicated SD of three replicates.

3.4.Effect of different harvesting methods on the composition of major fatty acids

For the D1 series of HZ62,the saturated fatty acid(SFA) content of the rapeseed oil harvested in twostage harvesting were higher than the combined harvesting;in contrast,SFA was slightly less in D2 and D3 series (Table 3).The linoleic acid content of twostage harvesting was higher than that of the combined harvesting,but the difference in linolenic acid content was smaller.For the ZS11 variety,the SFA content of two-stage harvesting rapeseed oil was also higher than combined harvesting.In CKs,the SFA content of HZ62 and ZS11 declined with increasing the harvesting time in Xiangyang and Huanggang;while in the D group,the SFA content was reduced as the time after final flowering extended in HZ62 and ZS11,and the reducing value was greater in D1–D2 (Table 3).

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Generally,the D group of linoleic and linolenic acid contents (polyunsaturated fatty acids (PUFA)) showed a fluctuating decline and rise trend,respectively.The linolenic acid content was increased with increasing the time after final flowering and post-ripening,while linoleic acid content declined as the length after final flowering extended and increased as post-ripening days were prolonged in HZ62 and ZS11.Overall,the content of linoleic and linolenic acids of the pressed rapeseed oil prepared from the combined harvesting was 2.0 and 0.5% lower than that of rapeseed oil from the two-stage harvesting,respectively (Table 3).

3.5.Effect of different harvesting methods on the content of polar total phenolics (PTP)

Our study displayed that PTP content had significant differences between both varieties of rapeseed oil in combined harvesting groups,as shown in Fig.4.The PTP content in HZ62 ranged from 9.77 to 65.77 mg 100 g–1in Huanggang and from 18.26 to 54.44 mg 100 g–1in Xiangyang;the highest PTP content was noted at 29 days (CK3) and 38 days (CK5) after the final flowering in Huanggang and Xiangyang,respectively (Fig.4).

For two-stage harvesting,there were significant differences in the content of PTP in different varieties of rapeseed oil.The PTP content in the D1 and D3 series decreased as the post-ripening time extended;an opposite trend was found in the D2 series for HZ62 in Huanggang.Moreover,there was still a notable difference in the PTP content among various treatments of postripening time (Fig.4).At 3 and 6 days of post-ripening,the maximum PTP content was noted as (19.59±1.47) mg 100 g–1(3 days) and (22.68±2.02) mg 100 g–1(6 days)in the D1 and D2 series,respectively,at Huanggang.In HZ62 at Xiangyang,the maximum PTP content was listed as (30.53±1.50) mg 100 g–1(at 6 days of post-ripening)and (37.18±1.50) mg 100 g–1(after 6–9 days of postripening) for D1 and D2,respectively,while it was noted as (35.09±1.94) mg 100 g–1at D3L9.In general,at 3 days of post-ripening,the PTP content was highest in D2 and lowest in D1,while it increased with increasing the final flowering time after 9 days of post-ripening.

In ZS11 at Huanggang,the PTP content peaked at 9 days of post-ripening in the D1 and D2 series;while at D1L9 and D2L9,the values were recorded (58.12±2.33)and (28.71±1.02) mg 100 g–1,respectively,in ZS11 at Huanggang.Meanwhile,in Xiangyang,the PTP content decreased significantly with the post-ripening time extension,which was noted as (60.47±2.10) mg 100 g–1in D1L3,which was higher than in D3L3 (Fig.4).Moreover,after 6 days of post-ripening,the PTP content was reduced as the final flowering time extended,whereas it was recorded as the highest in the D2 series after 9 days of post-ripening.Overall,the PTP content in combined harvesting was 28.6% lower than that of the two-stage harvesting (Fig.4).

Fig.4 Polarity of total polyphenols (PTP) content of rapeseed oils obtained by different harvesting methods in Huayouza62 (HZ62)and Zhongshuang11 (ZS11) at Huanggang and Xiangyang.A,PTP content of rapeseed oils in combined harvesting.B,PTP content of rapeseed oils in two-stage harvesting.Different letters indicate significant differences according to Duncan’s multiple range tests at P＜0.05.Error bars indicated SD of three replicates.

4.Discussion

Generally,there is a relationship between the color and the quality of the oil.The Chinese rapeseed oil standard has specific requirements for the color,and consumers normally prefer lighter rapeseed oil.The color of the oil depends on the oil’s quality,processing method,degree of refining,and storage conditions (Sydowet al.2021).For most vegetable oils,the color quality is affected by the R-value on the Lovibond colorimeter.When the yellow value (Y-value) is fixed,the higher the R-value leads to darker oil color andvice versa(Muzzioet al.2014).

The R-values of the two varieties of rapeseed oil decreased with the extension of the final flowering days and achieved the lowest value at CK5 of combined harvesting,followed by an upward trend.It is speculated that the long time after final flowering,the higher the maturity of rapeseed,and the content of various substances gradually stabilized;thereby,R-value became stable.For the D1 series,the R-value of ZS11 showed a notable decreasing trend in the post-ripening processvs.HZ62.With the extension of the final flowering days (after 33 days),the maturity of rapeseed was increased,and the R-values of both rapeseed oil were decreased and gradually stabilized.Whereas,the R-values of rapeseed oil of both varieties were increased at 6–9 days of postripening.In terms of color,it is better to harvest 35 days after the final flowering and 3–6 days of post-ripening.

On the other hand,POV is an essential index for oil oxidation degree and is closely linked to food safety.Moreover,hydroperoxide,the main factor affecting the POV,is formed by the oxidation of oil and can be further decomposed into secondary oxidation products,such as alcohols,aldehydes,and ketones.The previous study proved that the lower the initial POV leads to a slower oxidation rate (Niki 2009).Moisture has a notable effect on the POV of oil,as water in the oil leads to an increase in dissolved oxygen and promotes the formation of peroxide (Akinosoet al.2010).On the other hand,water molecules may also interact with some prooxidants or antioxidants,affecting the oxidation of the oil.Furthermore,moisture also facilitated the growth of microorganisms,which in turn accelerated the oxidative deterioration of oil.

In this investigation,the POV of combined and twostage harvesting rapeseed gradually declined with the extension of time after final flowering.Presumably,moisture decreased to a stable level during the maturation of rapeseed,and the oxidation reaction weakened.The water content was higher in the D1 series,and postripening could effectively contribute to decreasing the water content;hence,there was a significant difference in POV around its maturity.In addition,rape straw contains a small amount of water;there may be a water transport effect when the post-ripening time is short (Wanget al.2017).In short,the POV tends to be stable after 9 days of post-ripening.

Furthermore,the oil oxidation is influenced by light and temperature;as rapeseed is exposed to light and high temperature for a long time,its internal oxidation rate will be accelerated,resulting in high POV of rapeseed oil(Martinezet al.2013;Vaidya and Eun 2013;Olabarriaet al.2018).The POV of pressed rapeseed oil in the D3 series was higher than in the D1 or D2 series,and this trend remained for different post-ripening times.It is speculated that the oxidation of oils has become stable in the D1 and D2 series.Moreover,the rapeseed suffered from further oxidation in the treatment of D3,which was a longer light time (Doet al.2015).The appropriate harvesting method is recommended as 35 days after final flowering and 6–9 days of post-ripening to the evaluation of POV.

Tocopherol is a natural antioxidant in vegetable oils and an essential nutrient for humans (Fereidoon and Adriano 2016).The primary tocopherol contents in coldpressed rapeseed oil are γ-tocopherol (61.5–62.6%) and α-tocopherol (37.0–38.5%).The human body cannot synthesize tocopherol and only be obtained through external intake.The strongest antioxidant capacity is α-tocopherol (Seppanenet al.2010).

Furthermore,water molecules may interact with some pro-oxidants to promote oxidation reactions,affecting the tocopherol contents.In this study,the extension of the post-ripening time of the two rapeseed varieties had a significant effect on the content of tocopherol in the D1 series.This was probably due to the higher moisture of the D1 series,and post-ripening could effectively reduce the moisture and increase tocopherol contents (Sigeret al.2015).Also,the contents of tocopherol significantly increased with the extension of the final flowering days,which could be related to the continuous decrease of water content in rapeseed.Overall,the content of tocopherol was the highest 35 days after final flowering and 6–9 days of post-ripening.

The fatty acid composition was an essential factor affecting the nutritional value and health functions of rapeseed oil (Ruenwaiet al.2010;Layeet al.2018).The fatty acids in rapeseed oil mainly included palmitic,oleic,linoleic,linolenic,stearic,and small amounts of arachidonic acids.Thede novofatty acid synthesis system synthesized SFA of 18C and shorter carbon chain SFA such as palmitic and stearic acids;then,PUFA was generated through desaturation in the fatty acid desaturation system (Layeet al.2018).

Moreover,oleic acid is desaturated by desaturase to produce linoleic acid or further obtain PUFA,such as linolenic acid (Yanget al.2021).The decrease of SFA with the extension of days after final flowering was caused by the conversion of SFA to other long-chain fatty acids during maturation (Chenet al.2002).Meanwhile,linoleic acid content declined with the prolongation of days after final flowering,which may be because the critical period of linoleic acid synthesis is at the late maturity stage of rapeseed (Yinet al.2016).Linoleic acid is transformed into a certain amount of other carbon chain fatty acids during the maturity stage after final flowering;therefore,the content of linoleic acid has little change or decreased in this study (Poirieret al.2001).The content of linolenic acid rose with fluctuation as the time after final flowering extended,which may be because some linolenic acid could also be reversed to linoleic acid or oleic acid during late seed maturation (Leonardet al.2004;Zhang Y Wet al.2020).

Phenolics are compounds that contain two or more phenolic hydroxyl groups (-OH) in their molecular structures.Polyphenols,found in most plant bodies,could contribute to inhibiting the oxidation of low-density lipid proteins,eliminating free radicals,preventing cardiovascular diseases,and promoting anti-tumor and anti-platelet coagulation (Vazet al.2018).The PTP content in rapeseed oil was higher at D2L6 in HZ62 and ZS11 varieties,which speculated that the earlier cutting time could reduce the consumption of nutrients by respiration (Zhang Let al.2020).Generally,the content of PTP was greater in HZ62 than in ZS11 and higher in Xiangyang than in Huanggang.It could be related to the higher latitude of Xiangyang (32.04° north latitude)vs.Huanggang (30.44° north latitude),where the larger temperature difference between day and night facilitated the formation of more nutrients (Reedet al.2007;Baiet al.2021;Schlubachet al.2021).

Additionally,oil production is influenced by climatic,genetic,and agronomic factors and their interactions.Previous work showed that climatic factors such as temperature and rainfall influenced plant physiologic behavior and the chemical characteristics of seed oil(Temimeet al.2006).High temperatures might reduce the starch,protein,and oil content of oilseed crops (Maestriet al.2002).Ahmadi and Barhani (2009) reported that supplying sufficient water to rapeseed crops,particularly at flowering and pod formation,significantly improved the yield and quality of the crop.

Meteorological data for this trial were obtained from China Meteorological Administration.From final flowering to harvest of rape,the average daily temperature in Xiangyang (19.10°C) was 6.5% lower than that in Huanggang (20.33°C).It is also known from our study that Xiangyang generally performed better than Huanggang for rapeseed oil,which is consistent with the results of a previous report (Temimeet al.2006).The average daily solar radiation,relative humidity,and atmospheric pressure in Xiangyang were non-significantly different from those in Huanggang,but the total rainfall in Xiangyang (74.1 mm) was 49.8% lower than that in Huanggang (147.7 mm).In summary,the overall quality of rapeseed oil was better at an average daily temperature of 18–21°C,rainfall of 1.58–3.36 mm,and solar radiation of 12–14 MJ m–2,with combined harvesting 38–41 days after final flowering;two-stage harvesting about 35 days after final flowering and 6 days of post-ripening.This showed that the lower rainfall and temperatures during the maturing process of rapeseed are conducive to better oil quality.

5.Conclusion

The properties of rapeseed oil are related to their genotypes,R-value,POV,tocopherol contents,fatty acid composition,PTP content,and different harvesting methods.Increasing final flowering time enhances rapeseed maturity and decreases the R-value.Moreover,the color of cold-rapeseed oil was influenced by the time after final flowering during combined harvesting.The oil color improved in CK5–CK6 (41–44 days after final flowering),while the R-value was the lowest in the D3 series (35–38 days after final flowering) among twostage harvesting groups.The POV decreased initially and then increased with increasing growth time after final flowering,which was the lowest in CK5–CK6 and D2L9(corresponding to CK6 in the same period) in HZ62 and ZS11 varieties.Furthermore,total tocopherol contents in the oil increased with fluctuations in combined harvesting groups with a maximum in CK5–CK6,while it increased significantly with the prolonged harvesting period and post-ripening time with a maximum at D3L6.Additionally,Xiangyang and HZ62 were superior to Huanggang and ZS11 in PTP content,with the highest content at D2L6(35–38 days after final flowering and 6–9 days of postripening),and it was higher in the two-stage harvesting than combined harvesting;besides,SFA content was lower 41–44 days after final flowering.Overall,the rapeseed oil obtained 41–44 days after final flowering of combined harvesting or 35 days after final flowering and six days of post-ripening of the two-stage harvesting could achieve better quality.Further studies about the effect of different harvesting methods on the sterol compounds or antioxidant activity of rapeseed oil,such as the DPPH and FRAP,are necessary to comprehensively assess the oil quality.

Acknowledgements

This work was supported by the National Key R&D Program of China (2021YFD1600502).

Declaration of competing interest

The authors declare that they have no conflict of interest.