糌粑加工过程中稳定同位素指纹分馏效应分析

李继荣，张唐伟，次仁德吉，杨小俊，次顿

糌粑加工过程中稳定同位素指纹分馏效应分析

李继荣，张唐伟，次仁德吉，杨小俊，次顿

（西藏自治区农牧科学院农业质量标准与检测研究所/农业农村部农产品质量监督检验测试中心（拉萨），拉萨 850032）

【目的】系统分析青稞原料、炒制青稞和磨粉糌粑中稳定碳、氮、氢和氧同位素的差异，揭示糌粑加工过程中青稞原料、炒制青稞和磨粉糌粑稳定碳、氮、氢和氧同位素的组成特征及相关性，为青稞及其制品产地溯源提供理论与技术支撑。【方法】2018年从西藏自治区日喀则市糌粑加工作坊分别采集炒制青稞和磨粉糌粑各11份，同时在对应地点采集青稞原料11份；实验室模拟糌粑加工过程的青稞原料和炒制青稞各8份。利用元素分析-同位素比率质谱仪（EA—IRMS）测定青稞原料、炒制青稞和糌粑中的稳定碳、氮、氢、氧同位素。结合单因素方差分析及LSD或Games-Howell多重比较分析探究稳定碳、氮、氢、氧同位素在青稞原料、炒制青稞和糌粑间的差异；逐步判别分析区分雅鲁藏布江和年楚河青稞及其制品；独立样本T检验分析水磨和电磨加工糌粑样品稳定碳、氮、氢、氧同位素差异；配对数据T检验分析模拟试验中青稞原料、炒制青稞样品稳定碳、氮、氢、氧同位素差异，皮尔逊相关分析解析青稞原料和炒制青稞样品稳定碳、氮、氢、氧同位素的相关性。【结果】青稞原料、炒制青稞和糌粑间稳定碳、氮、氢、氧同位素比值无显著差异；稳定氮同位素对不同流域来源青稞判别率为72.7%，稳定氮、氧同位素对不同流域来源炒制青稞判别率为90.9%，糌粑判别率100%；水磨和电磨加工糌粑稳定碳、氮、氢、氧同位素比值无显著差异；模拟试验中青稞原料与炒制青稞稳定碳、氮、氢、氧同位素比值无显著差异，青稞原料与炒制青稞间稳定碳、氮同位素存在显著正相关（＜0.05）。【结论】糌粑稳定碳、氮、氧同位素与炒制青稞稳定碳、氮、氧同位素间分馏效应不显著；青稞及其制品中稳定同位素存在一定地域性；糌粑加工过程中使用电磨或水磨，对糌粑稳定碳、氮、氢、氧同位素值无影响；青稞原料稳定碳、氮同位素反映糌粑稳定碳、氮同位素特征；利用稳定同位素技术可以实现对糌粑的原产地溯源。

糌粑；青稞；稳定同位素指纹；溯源性；分馏；西藏

0 引言

【研究意义】食品产地溯源技术是有效实施食品原产地追溯、保护名优特产品的重要技术手段[1]。作为藏族人民最爱吃的食物之一，糌粑中含有丰富的营养物质，具有热量高、抗寒耐饥、降胆固醇、易于保存和制作的特点[2-4]。糌粑是由青稞经除杂、清洗、晾干、翻炒、磨粉等工艺制成的粉状食物[5]。传统研磨方法除部分人力研磨外，大多是水磨碾磨，随着电力资源的丰富，磨面机在糌粑加工过程中得到广泛应用[6]。党君[7]的研究显示拉萨市青稞脂肪酸含量高于青海、甘肃、云南地区的青稞，营养成分（水分、灰分、蛋白质、脂肪、淀粉）表现为日喀则市优于云南迪庆地区。稳定同位素是用于植源性农产品产地溯源的有效指标[8-13]。研究糌粑加工过程中稳定同位素的组成特征，有助于扩大稳定同位素指纹图谱技术的应用范围，可为青稞产地溯源及青稞产业链追溯提供理论和技术支撑。【前人研究进展】稳定同位素指纹图谱技术具有灵敏度高，实验操作简便，可较好地区分被追踪物质是新加入的还是试验系统固定的等优点，已被广泛应用于农产品产地溯源中[14]。该技术主要应用于谷物[12,15-20]、果品[21-24]、茶叶[25-27]、经济作物[13,28]、蔬菜[11,29-31]等农产品产地溯源。稳定同位素指纹图谱技术应用于谷物产地溯源的研究对象多为小麦[12,19,32-34]和水稻[17,20,35-36]。常用的测定指标有δ13C、δ15N、δD、δ18O、δ34S和86Sr/88Sr等[14]。LIU等[19]研究结果显示，利用δ13C、δ15N、δD对新乡、杨凌和石家庄冬小麦产地溯源的判别率为77.8%，结合δ13C、δ15N、δD和86Sr/88Sr的判别率达到98.1%。Wadood等[32]的研究结果表明小麦籽粒及其产品（面条、煮熟面条）间的δ13C、δ15N、δ18O无显著差异，且δ13C、δ15N、δ18O可以用于对小麦籽粒及其产品（面条、煮熟面条）产地溯源。Fraser等[37]的碳化试验表明，低温（230℃以下）加热谷物对其δ13C影响不大，δ15N值平均富集1‰。【本研究切入点】糌粑加工过程中稳定同位素是否存在分馏，进而应用于青稞及其制品产地溯源还不清楚，电磨或者水磨产糌粑稳定同位素比值是否存在显著差异也未见相关报道。【拟解决的关键问题】研究不同成分、不同加工方式糌粑稳定同位素差异，探究应用稳定同位素指纹图谱技术进行青稞及其制品产地溯源的可行性，为青稞及制品的产地溯源提供理论参考。

1 材料与方法

1.1 试验材料

2018年11月从西藏自治区日喀则市雅鲁藏布江段和年楚河段糌粑加工作坊分别采集炒制青稞6份和5份，对应磨粉糌粑6份和5份，同时在对应地点采集青稞原料6份和5份，采样点见表1；实验室模拟糌粑加工过程的青稞原料8份和炒制青稞8份。

表1 采样点信息表

1.2 试验方法

1.2.1 样品前处理 蒸馏水清洗青稞籽粒，除去表面附着物，青稞籽粒和炒青稞放入60℃烘箱内48 h烘干至恒重，烘干后的样品用药用粉碎机粉碎，过200目筛；糌粑样品放入60℃烘箱内48 h烘干至恒重，过200目筛；处理好的样品放入自封袋中备用。

1.2.2 糌粑模拟试验 称取300 g青稞籽粒平均分成2份，每份150 g，一份样品按照1.2.1所述进行处理，直接用于δ13C、δ15N、δD和δ18O检测；一份样品进行润麦，添加超纯水（Milli-Q，Millipore，USA），调整青稞含水量达到15%，润麦时间3 h。不沾锅中加入200 g蒸馏水清洗干净自然晾干的沙子，放在电炉上加热温度至230—240℃[38]，倒入青稞籽粒迅速翻炒，待青稞爆腰率达85%以上时过60目筛分离青稞和沙子[6]。青稞原料和炒青稞按照1.2.1所述进行处理。试验用沙子放入60℃烘箱内48 h烘干至恒重，过200目筛进行δ13C、δ15N、δD和δ18O检测。

1.2.3 样品测定 使用万分之一天平称取6.5 mg样品放入锡箔杯中包样进行稳定碳、氮同位素检测，称取1 mg样品放入银舟中包样进行稳定氢、氧同位素检测。元素分析仪（vario PYRO cube，Elementar，Germany）联稳定同位素质谱仪（IsoPrime100，IsoPrime，UK）进行稳定碳、氮同位素检测。元素分析仪（Flash EA2000型）联稳定同位素质谱仪（MAT253型）进行稳定氢、氧同位素检测。使用标准品为IAEA-600、IAEA-601、IAEA-CH-7，仪器对δ13C、δ15N、δ18O和δD的连续测定精度＜0.2‰。

稳定同位素比值表示样品与标准品之间偏差的千分数：

δ（‰）=[(sample/standard)-1]×1000

式中：指13C或15N或18O或D；R=13C/12C或15N/14N或18O/16O或2H/1H；sample为被测样品的同位素比值；standard为标准品的同位素比值。

1.3 数据处理及质量控制

使用软件Excel 2007对数据进行整理，SPSS 20对数据进行统计分析，使用单因素方差分析对糌粑生产过程中的青稞原料、炒制青稞和磨粉糌粑样品的δ13C、δ15N、δD和δ18O值进行分析。青稞样品统计检验前，用Kolmogorov-Smirnov和Levene统计量分别检验所有数据的正态性和方差同质性，满足方差齐性时采用LSD多重比较，不满足方差齐性时采用Games-Howell多重比较法进行分析。使用逐步判别分析对雅鲁藏布江和年楚河流域样品进行判别。使用独立样本T检验对电磨和水磨研磨糌粑的稳定同位素（δ13C、δ15N、δD和δ18O）进行分析。使用配对数据T检验对糌粑模拟试验中青稞原料和炒制青稞样品中的稳定同位素（δ13C、δ15N、δD和δ18O）进行分析，使用Pearson相关分析青稞原料和糌粑中稳定同位素（δ13C、δ15N、δD和δ18O）相关性。

2 结果

2.1 加工对糌粑稳定同位素特征影响及差异

2.1.1 糌粑加工过程中稳定同位素特征及差异 糌粑加工过程中青稞原料δ13C值介于-25.27‰—-23.89‰，炒青稞的δ13C介于-25.58‰—-24.27‰，糌粑的δ13C介于-25.60‰—-24.01‰；糌粑加工过程中青稞原料δ15N值介于0.55‰—4.69‰，炒青稞的δ15N值介于-0.38‰—6.74‰，糌粑的δ15N值介于1.02‰—6.90‰；糌粑加工过程中青稞原料δ18O值介于12.38‰—21.76‰，炒青稞的δ18O值介于10.63‰—21.79‰，糌粑的δ18O值介于12.81‰—23.28‰；糌粑加工过程中青稞原料δD值介于-206.76—-150.91‰，炒青稞的δD值介于-207.88‰—-160.45‰，糌粑的δD值介于-194.09‰—-159.40‰。如表2所示，δD值相对δ13C、δ15N和δ18O值标准差较大。

表2 糌粑加工过程中稳定碳、氮、氧和氢同位素（平均数±标准差）

同一列不同小写字母表示差异显著（＜0.05）。下同

Different lowercase letters in the same column indicate significant differences (＜0.05). The same as below

单因素方差分析结果显示（图1、图2），糌粑加工过程中青稞原料、炒青稞和糌粑间的稳定碳、氮、氢和氧同位素间均无显著差异，δ13C单因素方差分析结果为(2,30)=0.15（＞0.05），δ15N单因素方差分析结果为(2,30)=0.024（＞0.05），δ18O单因素方差分析结果为(2,30)=0.864（＞0.05），δD单因素方差分析结果为(2,30)=0.618（＞0.05）。

2.1.2 不同加工方式糌粑稳定同位素差异 水磨和电磨加工糌粑δ13C、δ15N、δ18O、δD值如表2所示，水磨糌粑的δ13C值介于-25.27‰—-24.01‰，电磨糌粑的δ13C介于-24.86‰—-24.31‰；水磨糌粑的δ15N值介于1.98‰—6.90‰，电磨糌粑的δ15N值介于1.02‰—4.51‰；水磨糌粑的δ18O值介于15.41‰—23.28‰，电磨糌粑的δ18O值介于12.81‰—21.66‰；水磨糌粑的δD值介于-184.90—-159.40‰，电磨糌粑的δD值介于-194.09‰—-166.85‰。两组加工方式间均无显著差异。

表3 不同加工方式产糌粑样品稳定碳、氮、氧和氢同位素（平均数±标准差）

图1 糌粑加工过程中稳定碳、氮同位素

图2 糌粑加工过程中稳定氢、氧同位素

独立样本T检验结果显示（图1、图2），水磨糌粑稳定碳、氮、氧和氢同位素比值与电磨糌粑稳定碳、氮、氧和氢同位素比值无显著差异，δ13C的9=-0.59（＞0.05），δ15N的9=1.0219（＞0.05），δ18O的9=0.56（＞0.05），δD的9=0.161（＞0.05）。

2.2 模拟加工对糌粑稳定同位素特征影响及相关性

2.2.1 模拟糌粑加工过程青稞原料与炒青稞样品稳定同位素差异 模拟糌粑加工过程中青稞原料与炒青稞样品δ13C、δ15N、δ18O、δD值如表4所示，配对数据的T检验结果显示（图1、图2），青稞原料δ13C、δ15N、δD、δ18O与炒青稞δ13C、δ15N、δD、δ18O值之间无显著差异，值分别为7=0.27（＞0.05）、7=1.402（＞0.05）、7=0.175（＞0.05）和7=-0.94（＞0.05）。试验用沙子δ13C、δ18O、δD比值分别为21.438‰、-0.59‰、-169.21‰，δ15N比值未检出。

表4 模拟糌粑加工过程中青稞原料与炒青稞样品稳定碳、氮、氧和氢同位素

2.2.2 模拟糌粑加工过程中青稞原料与炒青稞样品稳定同位素相关性分析 模拟糌粑加工试验得到的青稞原料和炒青稞之间的稳定碳、氮、氧和氢同位素比值图如图3—6所示。Pearson相关分析结果显示青稞原料δ13C、δ15N与炒青稞δ13C、δ15N存在显著正相关，相关系数分别为=0.719（＜0.05）、=0.79（＜0.05）；青稞原料δD、δ18O与炒青稞δD、δ18O无显著相关性，相关系数分别为=0.124（＞0.05）、=0.163（＞0.05）。

2.3 不同流域青稞及其制品产地溯源判别分析

雅鲁藏布江和年楚河流域青稞及其制品逐步判别分析结果显示，稳定氮同位素比值可以作为不同流域青稞原料判别指标，回代检验判别率和交叉检验判别率均为72.7%。稳定氮同位素比值和稳定氧同位素比值可以作为不同流域炒青稞和不同流域糌粑判别分析指标。不同流域炒青稞回代检验判别率和交叉检验判别率均为90.9%，不同流域糌粑的回代检验判别率和交叉检验判别率均达到100%。如图7所示，雅鲁藏布江流域青稞及其制品稳定同位素比值主要落在稳定氮、氧同位素图的左下方，而年楚河流域青稞及其制品稳定同位素比值主要落在稳定氮、氧同位素图的右上方。雅鲁藏布江流域青稞及其制品稳定氮同位素较年楚河流域青稞及其制品稳定氮同位素贫化；雅鲁藏布江流域炒青稞和糌粑稳定氧同位素较年楚河流域炒青稞和糌粑稳定氧同位素贫化。

图3 糌粑加工过程中稳定碳同位素比值

图4 糌粑加工过程中稳定氮同位素比值

图5 糌粑加工过程中稳定氧同位素比值

图6 糌粑加工过程中稳定氘同位素比值

图7 不同流域青稞及其制品稳定氮、氧同位素比值

3 讨论

近年来，基于稳定同位素特征的谷物产地溯源技术已成为谷物地理标志保护的重要手段。模拟青稞原料加工炒青稞的过程中，230—240℃加热约2 min，青稞爆腰率便可达到85%，本研究中青稞原料与炒青稞间δ13C的结果与Fraser等[37]的碳化试验结果相同，氮δ15N结果相比Fraser等[37]得出的加热使δ15N值平均富集1‰的结果有所不同，其原因可能与青稞加热时间较短，美德拉反应[39]未能导致青稞δ15N的变化有关。

炒青稞加工为糌粑的主要方式是研磨，炒青稞和糌粑间稳定同位素无显著差异，进一步说明研磨处理对样品δ13C、δ15N、δD、δ18O值无影响。电磨或水磨对糌粑δ13C、δ15N、δD、δ18O无影响，此结果与杨乐等[40]得出的直接剪碎或液氮研磨对羽毛的δ13C、δ15N值无影响的结果相似。调查结果显示水磨相对电磨研磨的糌粑保质期较长，但是对于一些小作坊来说，水磨相对电磨研磨存在一定的季节限制，当冬季水流减小或无水时，水磨研磨糌粑则不可行。

判别分析结果显示，青稞及其制品稳定同位素存在一定的地域特征性。雅鲁藏布江发源于西藏西南部喜马拉雅山北麓的杰马央宗冰川，是世界上海拔最高的一条大河，年楚河是雅鲁藏布江的一级支流[41]。位于雅鲁藏布江干流谢通门县δD为-138.2‰，δ18O为-14.6‰；年楚河δD为-112.4‰，δ18O为-10.3‰[42]。李继荣等[42]对2014年西藏主要水体稳定氢、氧同位素研究结果显示水体δD、δ18O的取值范围分别为-152.06‰—-19.05‰、-16.96‰—4.66‰，青稞δD较水体δD偏贫化，青稞δ18O较水体δ18O偏富集，这一结果与LIU等[18]得出的脱脂小麦δD与0—20 cm土壤水δD呈正相关的结果不同，其原因可能是由于本试验中的青稞及其制品样品为2018年采集，而水体δD、δ18O数据为2014年样品，由于不同年际间气候条件（温度、湿度、降水量等）的不同，导致δD、δ18O比值不同[43]。但LIU等[18]的研究对象为低海拔地区冬小麦，属于小麦属植物，而本研究对象为高海拔青稞样品，属于大麦属植物，不同属、不同海拔植物δD、δ18O与土壤水δD、δ18O相关性是否一致需要做进一步的研究。另外，由于本研究用于判别分析的样本较少，尚需增加样本量对判别分析结果做进一步验证。

4 结论

青稞及其制品稳定同位素存在一定的地域特征性；糌粑加工过程中使用电磨或水磨，对糌粑稳定同位素δ13C、δ15N、δ18O、δD值无显著影响；模拟糌粑加工的炒青稞与青稞原料稳定同位素指纹无显著差异。稳定同位素指纹分析技术可以应用到青稞及其产品产地溯源中。

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Fractionation Effect of Stable Isotopic Ratios in Tsamba Processing

LI JiRong, ZHANG TangWei, CIREN DeJi, YANG XiaoJun, CI Dun

(Institute of Agricultural Product Quality Standard and Testing Research, Tibet Academy of Agricultural and Animal Husbandry Sciences/Supervision and Testing Center for Farm Products Quality, Ministry of Agriculture and Rural Affairs, Lhasa 850032)

【Objective】Our study mainly analyzed the difference of stable carbon, nitrogen, hydrogen, and oxygen isotopes, and revealed the characteristics and correlations of stable carbon, nitrogen, hydrogen, and oxygen isotopes in raw highland barley material, highland barley stir-frying, and milling tsamba in tsamba processing, which could provide a theoretical and technical basis for geographical origin traceability of highland barley and its products. 【Method】 We collected 11 samples of both stir-frying highland barley and milling tsamba from Xigaze (Tibet) tsamba processing workshop in 2018, and 11 samples of raw highland barley material were collected simultaneously from corresponding sites; 8 samples of both raw highland barley material and stir-frying highland barley were collected by the simulation of tsamba processing in the laboratory. Stable carbon, nitrogen, hydrogen, and oxygen isotopes were measured by element analysis-isotope ratio mass spectrometer (EA-IRMS). The one-way analysis of variance was combined with LSD or Games-Howell multiple comparison analysis to analyze the difference of stable carbon, nitrogen, hydrogen, and oxygen isotopes from perspectives of raw highland barley material, stir-frying highland barley, and tsamba. Stepwise discriminant analysis was employed to distinguish highland barley and its products from Yarlung Tsangpo River and Nianchu River. We used independent - sample T test to discover the difference of stable carbon, nitrogen, hydrogen, and oxygen isotopic between water milling tsamba and electric grinding tsamba. Paired T test was adopted to analyze the difference of stable carbon, nitrogen, hydrogen, and oxygen isotopes in raw highland barley material and stir-frying highland barley samples in the simulation experiment. And Pearson correlation analysis was used to analyze the correlation of stable carbon, nitrogen, hydrogen, and oxygen isotopes in raw highland barley material and stir-frying highland barley. 【Result】 No significant difference was found in stable carbon, nitrogen, hydrogen, and oxygen isotope ratios among raw highland barley material, stir-frying highland barley, and tsamba. The highland barley discrimination rate of stable nitrogen isotope from different watersheds was 72.7%, and the stir-frying highland barley discriminant rate of stable nitrogen and oxygen isotopes from different watersheds was 90.9%, whereas the tsamba discriminant rate was 100%. No significant difference was found in stable carbon, nitrogen, hydrogen, and oxygen isotope ratios between water milling tsamba and electric grinding tsamba. In the simulation experiment, there was no difference in stable carbon, nitrogen, hydrogen, and oxygen isotope ratios between raw highland barley material and stir-frying highland barley, while significant positive correlation was found in stable carbon and nitrogen isotope ratios between raw highland barley materials and stir-frying highland barley (＜0.05). 【Conclusion】 The fractionation effect of the stable carbon, nitrogen, and oxygen isotopes between stir-frying highland barley and tsamba was not significant. The stable isotopes in highland barley and its products were regional. In the tsamba processing, the use of either electric grinding or water milling had no effect on the stable carbon, nitrogen, hydrogen, and oxygen isotope ratios of tsamba. Simulation of tsamba processing experiment results showed that stable carbon and nitrogen isotopes in raw highland barley material could reflect the stable isotopes characteristics of those in tsamba. Therefore, stable isotope technology could be used for realizing the geographical origin traceability of tsamba．

tsamba; highland barley; stable isotope fingerprint; traceability; fractionation; Tibet

2019-06-03；

2019-08-23

西藏自治区科技重大专项（Z2016B01N04，ZD20170014，XZ201801NA04，XZ201901NA04）、国家大麦青稞产业体系（CARS-05-02-06）

李继荣，Tel：18089980869；E-mail：ljr18697179656@163.com。通信作者次顿，Tel：13989086593；Fax：0891-6868491；E-mail：13989086593@163.com。通信作者张唐伟，Tel：13518997809；Fax：0891-6868491；E-mail：zhangtangwei04@163.com

（责任编辑 赵伶俐）