【作者】 刘丽艳；

【导师】 李琳；

【作者基本信息】 华南理工大学 ， 制糖工程， 2019， 博士

【摘要】 随着食品多样化需求不断增长,高品质的油脂必不可少,塑性脂肪能赋予食品良好的口感、风味及色泽,现已成为油脂工厂的主要产品。棕榈油因其产量大、价格低、加工特性良好等优点而广泛应用于塑性脂肪生产中。在加工或者储藏过程中,由于加工条件或环境温度不适宜,则油脂结晶中容易产生砂粒而使其口感变差,失去其应有的功能特性及操作性能。这一品质缺陷在棕榈油和牛油基塑性脂肪中尤为明显,每年造成巨大的经济损失。到目前为止,在食品工业界中冷链运输无法精确控制温度恒定的情况下,改善塑性脂肪的起砂问题主要依靠使用乳化剂。然而,目前乳化剂在塑性脂肪中的使用大多基于经验,乳化剂的作用规律及机理还不明确。任何材料的特定功能均是由其结构决定的,因此,从结构多角度层面探讨乳化剂对塑性脂肪结构的作用规律对理性控制塑性脂肪品质具有重要意义。本论文以棕榈硬脂为研究对象,系统地考察乳化剂的结构（单棕榈酸山梨糖醇酯、单硬脂酸甘油脂、单棕榈酸甘油脂）及用量（1%、2%、4%）对油脂固液转化、晶型衍变、结晶结构以及机械性能的影响机理,探讨乳化剂分子、塑性脂肪晶体网络结构与宏观物理性质之间的关联,揭示乳化剂分子导致油脂晶体网络多尺度结构变化所遵循的机制,以期对塑性脂肪品质进行理性控制。主要研究内容与结果如下:一、乳化剂对棕榈硬脂急冷过程的热性质及同质多晶衍变的影响采用DSC-XRD考察不同种类及浓度的乳化剂在急冷环节对棕榈硬脂结晶热力学及晶型衍变的影响,研究发现:分别以5℃/min、10℃/min、20℃/min三种速率冷却,随着冷却速率的降低,油脂样品结晶峰向高温区移动。所有油脂样品均出现两个结晶峰,分别代表不同的油脂组分,而乳化剂的添加对油脂体系结晶的影响主要体现在饱和组分的结晶上,当添加量达到2%和4%时,三种乳化剂添加下的棕榈硬脂样品的T_onset和T_p均显著高于空白棕榈硬脂样品。XRD同步监测降温过程（降温速率为10℃/min）的两个结晶阶段,发现所有油脂样品第一阶段均形成α晶型,在第二阶段α晶型转化为β′晶型,急冷过程中没有β晶型产生,且乳化剂的加入对晶型衍变没有显著性影响。二、乳化剂对棕榈硬脂等温结晶动力学及微观结构的影响分别在15℃、20℃、25℃、30℃,考察不同种类及浓度的乳化剂对棕榈硬脂等温结晶动力学、机械性质、微观结构的影响及相互之间的关联规律,发现:随着等温结晶温度的升高,油脂结晶平衡时的SFC值和硬度均显著降低。在15℃²5℃时,油脂体系瞬时成核,结晶曲线均表现为随时间变化的双曲线模式,与基料油中甘油三酯含有相同脂肪酸且结构相近的乳化剂延缓α晶型向β′晶型转化。在30℃时,棕榈硬脂结晶速率低,乳化剂通过改变油脂体系结晶机制从而影响结晶网络结构,使得油脂硬度差异较大,但这受到乳化剂种类及用量的影响。与空白棕榈硬脂比较而言,单硬脂酸甘油脂和单棕榈酸甘油酯的添加4%时,油脂体系瞬时成核,结晶细小,结构致密,硬度显著增大,而只需添加1%SMP就可达到同等促进结晶的效果,这与单棕榈酸山梨糖醇酯所含山梨醇酐的空间位阻有关。三、乳化剂对棕榈硬脂基模型起酥油结晶行为的影响模拟工业生产过程制备出模型起酥油,在25℃下,考察乳化剂的添加对模型起酥油硬度、SFC、结晶结构及晶型的影响,发现起酥油中的β晶型是在熟化的过程中经晶型衍变而来。模型起酥油的硬度与SFC值并非正向相关,通过影响纳米小板的大小进而影响晶体束的大小及聚集来赋予产品更高的抗屈服能力的。三种乳化剂均促进纳米级分子层结构的叠加,抑制纳米小板的进一步聚集,降低晶体束尺寸,但对纳米小板长度的影响不一致,这在纳米级结构层面证实了乳化剂结合到油脂结晶网络中是在结晶位点上发挥作用的。从脂肪酸组成差异度及分子结构差异度的对比上看,分子结构的差异度对塑性脂肪的晶体形态影响更大,而分子结构相同且异酸的乳化剂能有效抑制β′晶型向β晶型衍变,可抑制起砂,这对于塑性脂肪的宏观调控具有重要意义。四、储藏过程中乳化剂对模型起酥油品质的影响在温度波动储藏过程中（4℃、12 h-28℃、12 h作为周期往复）,模型起酥油的SFC值均随着温度波动次数的增加降低。在熔化-结晶的往复后期（7 d,14 d）,空白模型起酥油发生晶体聚集,液体油析出,硬度减小,β晶型比例最大,模型起酥油品质劣变最严重。三种乳化剂均有抑制β晶型衍变的效果,单硬脂酸甘油脂在重结晶的过程中吸附高温时熔化的甘油三脂分子形成较大尺寸的晶体,有效促进晶体成长和晶体束交联的作用,温度波动后期硬度较大,β晶型所占比例相对最小,且未见明显固液分层现象,温度波动次数较多的情况下（14次）单硬脂酸甘油脂抑制起酥油起砂效果最好。更多还原

【Abstract】 With the increasing demand for food diversity,high quality fat is absolutely necessary.Food can be endowed with fine taste,flavor and color by plastic fats,which have become major products by oil factories.Palm oil is widely applied in the manufacture of plastic fats due to its advantages such as large production,low price and fine processing characteristics.During processing or storage,sandiness is easily appeared in the crystallization of fat due to inappropriate processing conditions and ambient temperature,which generates worse tastes and results in losing its proper functional characteristics and operational performance.This quality defect is remarkably obvious in palm oil and beef tallow-based plastic fats,which leads to huge economic loss every year.Till now,as constant temperature during cold chain transportation hasn’t been achieved in food industry,emulsifier has been widely used to solve the problem of sandiness in plastic fats.However,the application of emulsifier in plastic fats is largely based on experience,thus,there has not been any definite conclusion for the functional rule or mechanism of emulsifier yet.It is obvious that any material’s special function is completely decided by its structure.Therefore,it is of great importance to discuss the effect of emulsifier on the plastic fats’structure from the multiple perspectives of structure for the rational control of plastic fats quality.In this paper,palm stearin is taken as the research object,which is added with 1%,2%,4%concentration of sorbitol monopalmitate,glycerol monostearate and glycerol monopalmitate,respectively.The effect of emulsifier’s structure and dosage on fat’s solid-liquid conversion,crystalline evolution,crystalline structure and mechanical properties is systemically discussed.The relationship between emulsifier molecule,plastic fats’crystalline structure and macrophysics properties is investigated.The mechanism of emulsifier molecule-induced multi-scale structural changes of lipid crystal network is revealed,so as to be rational to control the quality of plastic fats.（1）Effects of emulsifiers on thermal properties and the homogeneous poly-crystalline evolution of palm stearin during quenching process.Effects of emulsifiers with different types and concentrations on the thermodynamics of crystallization and on the homogeneous poly-crystalline evolution of palm stearin during quenching process was investigated by means of the DSC combining with XRD method.When the fat samples were cooled down at the rate of 5℃/min,10℃/min,or 20℃/min,it is found that the crystallization peaks shifted to the high temperature zone at low cooling rate.Addition of emulsifiers facilitated the crystallization of the saturated components in fat.Three emulsifiers with the addition dose of 2%or 4%all increased the T_onset and T_p level of palm stearin comparing with the untreated samples.Two crystallization peaks were observed for all tested samples and the first one corresponded to theαform consisting of the saturated components in fat while the other comprised of the unsaturated components was theβ’form.Theβ’form was evolved from theαform or directly produced from the fat,and theβform was not detected during quenching process.（2）Effect of emulsifiers on the isothermal crystallization dynamics and the micro-structure of palm stearinThe crystallization dynamics,mechanical properties,and microstructure of palm stearin with different emulsifiers were investigated during isothermal crystallization process at 15℃,20℃,25℃,orμm℃,respectively.The level of SFC and hardness of palm stearin remarkably decreased along with increasing the temperature of isothermal crystallization.The fat system formed instantaneous nuclei and the isothermal crystallization curves against time showed typical hyperbolic pattern at 15℃-25℃.Meanwhile,the emulsifier which contained the same fatty acids and similar structure as palm stearin postponed the transition fromαtoβ’form.The crystallization rate of palm stearin was low at 30℃.The structure of crystallization network and the fat hardness were affected by the emulsifier which changed the crystallization mechanism of the fat system.Besides,the influence of emulsifiers was structure and dose-dependent.Addition of 4%GMS or GMP prompted the instantaneous nucleation and facilitated the formation of compact fat crystallization with tiny nuclei and high hardness.Moreover,similar results were observed when 1%of SMP was applied,which was attributed to the steric hindrance of sorbitol anhydride in SMP.（3）Effect of emulsifiers on the crystallization behavior of palm stearin-based model shorteningThe palm stearin-based model shortening was prepared in terms of the simulated industrial production process.Then the hardness,SFC,crystalline structure,and crystalline form of the model shortening were investigated at 25℃.It is found that theβ-crystal was evolved from crystal during the ripening process.Emulsifiers improved the yield resistance of fat by affecting the size of nano-plates which had influence on the size and congestion of crystal beams.All three emulsifiers facilitated the superposition of nano-scale molecular layer structure,inhibited the congestion of nano-plates,and reduced the size of crystal beams.But they showed different impact on the length of nano-plates.This confirmed that the emulsifier binds to the oil crystallization network and plays a role in the crystallization site at the nanostructure level.On the basis of divergence of the fatty acid composition and molecular structure,it is suggested that emulsifiers with different molecular structure had higher extent of impact on the crystal morphology of plastic fat whereas emulsifiers with the same molecular structure but various fatty acids effectively inhibited the transition of crystal form fromβ’toβand sand-out.Therefore,it is of importance on the modulation of plastic fat.（4）Effect of emulsifiers on the quality of model shortening in storageThe SFC value of the model shortening decreased along with time during storage（4℃/12 h followed by 28℃/12 h in a cycle period of 24 h）.At the later stage in storage（7 d and 14 d）,the shortening without emulsifiers severely deteriorated owing to the aggregation of crystals,separation of liquid oil,reduction of hardness,and high proportion ofβcrystals.SMP,GMS,and GMP effectively ameliorated the production ofβcrystals and GMS showed the best capability in multiple cycles（14 times）.When GMS was used,it absorbed the triglyceride molecules melted at high temperature to form crystals with large size during recrystallization process.This phenomenon prompted the growth and cross-link of crystals.The showed higher hardness at the later stage of storage and had the less proportion ofβcrystals than other shortenings.Besides,the liquid-solid separation was not observed in the GMS shortening.更多还原