基于LF-NMR和HS-SMPE-GC-MS分析烤牛脂水分分布及关键挥发性风味物质

采用低场-核磁共振技术、顶空固相微萃取结合气相色谱-质谱联用技术对秦川牛牛脂烤制过程中水分分布及关键挥发性风味物质进行研究。结果表明:在烤制过程中牛脂L^(*)值明显下降(P<0.05),a^(*)值、b^(*)值先上升后下降。结合水的相对含量在整个烤制过程中变化不显著(P>0.05),而不易流动水先上升后下降,自由水整体下降(P>0.05)。弛豫时间T_(23)和T_(24)逐渐减小(P<0.05),水分子的自由度逐渐降低,说明此过程中水分子与脂质、蛋白质分子之间的结合更加紧密。电子鼻能对烤牛脂样品进行有效区分,并且电子鼻数据主成分分析结果显示醇类、芳香类、萜类以及有机硫类化合物受烤制时间影响显著。在牛脂样品中共检测出95种挥发性物质,1-甲氧基-2-丙醇、1-辛烯-3-醇、己醛、乙酸、壬醛、壬酸和乙酰胺含量较高且普遍存在于每个样品中,酮类、杂环类、胺类化合物主要存在于烤制后期。气味活度值(odor activity value,OAV)法鉴定出19种关键挥发性物质,其中大部分物质OAV在烤制12 min时最大。

Corresponding Factors Influencing Crude Oils Assay Using Low-field Nuclear Magnetic Resonance

In this study, the main factors influencing the measurements by means of the off-line low-field 1H NMR in the lab were discussed base on a robust calibration model established by the PLS algorithm using 255 crude oil samples. The preheating temperature had a great influence on the viscosity of oil samples and the resolution of spectral analysis. The repeatability of spectral measurements was impacted by the metal and wax content of the oil samples. For the case of high wax content oils, the wax species began to crystallize in the course of determination that could affect the repeatability of spectral measurements. These factors have evidenced why the preheating devices and filter unit are necessary when low field NMR system is used in the online analysis process. The investigation is very important for the on-line application of the low field NMR.

Influence of CaO-based expansive agent,superabsorbent polymers and curing temperature on pore structure evolution of early-age cement paste

Cracks easily generate in concrete at early age owing to the shrinkage deformation.CaO-based expansion agent(CEA)and superabsorbent polymers(SAP)have been extensively used for the mitigation of concrete shrinkage.The macroscopic properties of concrete are highly determined by the microstructure.In this study,the influence of CEA and SAP addition on the pore structure evolution of cement paste under different curing temperatures was evaluated via low-field nuclear magnetic resonance spectroscopy.Test results indicated that,in cement paste,a higher CEA content led to a higher porosity and a larger most probable pore diameter(MPPD).Meanwhile,SAP addition increased the porosity and MPPD of CEA cement paste at early age but decreased them after 7 d,and a higher SAP content always brought a higher porosity and MPPD.Furthermore,the addition of SAP led to a lower porosity and MPPD of CEA cement paste than that of plain cement paste after 14 d.Moreover,the porosity and MPPD of CEA cement paste decreased first and subsequently increased as the curing temperature raised.

焙烤对宁夏滩羊羊脂水分分布和关键性香气成分的影响

以宁夏盐池滩羊肉羊脂作为研究对象,运用低场-核磁共振技术研究烤制过程中羊脂水分分布及迁移规律。采用顶空固相微萃取提取不同烤制过程中羊脂挥发性成分,并结合气相色谱-质谱、电子鼻进行分析。结果表明:羊脂L*随着烤制时间的延长而下降,a*、b*随烤制时间的延长呈上升趋势。羊脂烤制过程中弱结合水的相对含量呈增加趋势,而不易流动水相对含量呈下降趋势。T_(23)随烤制时间的延长呈明显减小趋势,说明随烤制进行水分子与大分子物质结合更紧密。在10个烤羊脂样品中共检出78种挥发性成分,其中醛类12种、醇类11种、酮类9种、烯烃类4种、酸类10种、酯类12种、杂环类12种、其他物质8种。己醛、壬醛、1-辛烯-3-醇、3-羟基-2-丁酮、7-甲基-3-亚甲基-1,6-辛二烯和甲苯普遍含量较高。酯类物质主要存在于羊脂烤制初期,而杂环类化合物,特别是吡嗪类化合物主要存在于烤制后期。电子鼻数据的主成分分析结果表明,烤制过程对羊脂中的氮氧化物、芳香族化合物、有机硫化合物、醇类有显著影响,对烷烃、氢化物和氨类化合物影响不大。

Evolution of distribution and content of water in cement paste by low field nuclear magnetic resonance

The low field nuclear magnetic resonance (NMR), as a nondestructive and noninvasive technique, was employed to investigate the water distribution and content in cement paste with different water-to-cement ratio (w/c ratio) during early and later hydration stages. From the water distribution spectrum deduced from relaxation time distribution in paste, it is suggested that the water fills in the capillary pores at initial period, and then diffuses to the mesopores and gel pores in hydration products with the hydration proceeding. The decrease of peak area in water distribution spectrum reflects the transformation from physically bound water to chemically bound water. In addition, based on the connection between relaxation time and pore size, the relative content changes of water in various states and constrained in different types of pores were also measured. The results demonstrate that it is influenced by the formation of pore system and the original water-to-cement ratio in the paste. Consequently, the relative content of capillary water is dropped to less than 2% in the paste with low w/c ratio of 0.3 when being hydrated for 1 d, while the contents are still 16% and 36% in the pastes with w/c ratios of 0.4 and 0.5, respectively.

Pore structure formation and hydration characteristics of cement paste with temperature rising inhibitor

The early-age thermal cracking easily generates and severely impairs the durability of concrete.The temperature rising inhibitor(TRI)was utilized to regulate the temperature evolution by controlling the cement hydration process.This paper aimed to investigate the pore structure formation and hydration characteristics of cement paste containing TRI by low-field nuclear magnetic resonance.The experiment showed that the T_(2) peak of cement paste shifted from 7.32 ms to 0.23 ms regardless of TRI addition.But the pattern of pore structure formation was changed with TRI addition,that is,the pore structure formation was delayed,and the pore successively shifted to left in two parts.In addition,TRI addition significantly prolonged the duration of gel pore formation and greatly decreased the increase rate of gel water,which implied that TRI introduction hindered the growth of C-S-H,and subsequently decreased the hydration rates and delayed the main hydration peak.Meanwhile,TRI dissolved and diffused rapidly at 40℃,delaying the hydration of cement paste seriously.Moreover,TRI brought about the C-S-H nucleation homogeneous and the ion concentration uniform,which might reduce the localized curvature occurring on the sheet of C-S-H,and then decreased the T_(2) intensity of capillary water and gel water.

Low-field NMR micro coils based on printed circuit board technology

Radiofrequency coil is one of the most important components for a nuclear magnetic resonance(NMR)instrument.In this article,some planar micro coils with an inner diameter of 2 mm and number of turns that varied from 1 to 11 were investigated based on the printed circuit board(PCB)technology.The electrical characterization of micro coils show that self-resonant frequencies are larger than 200 MHz.Then,an NMR measurement platform with a static magnetic field of 0.66 T was constructed and the signal to noise ratio(SNR)values of the NMR were analyzed.It was found that the SNR is optimal when the turn number of the micro coils is six and the excitation time of a 90°pulse is 0.8?s.Finally,we used the micro coil with six turns to study the transverse relaxation rate of copper sulfate pentahydrate aqueous solution with different concentrations.It was found that the transverse relaxation rate is proportional to the solution concentration.Results from the micro coil were verified by measurements using a Bruker Minispec MQ60.