Department of Nutrition and Food Sciences, University of Vermont, Burlington 05405.
Department of Nutrition and Food Sciences, University of Vermont, Burlington 05405.
J Dairy Sci. 2018 Sep;101(9):7714-7723. doi: 10.3168/jds.2018-14712. Epub 2018 Jun 30.
Surface crystallization and radial demineralization of Ca, P, and Mg occur in smear-ripened cheese. Furthermore, crystals of ikaite, struvite, calcite, and brushite have been identified in cheese smears by powder X-ray diffractometry (PXRD), and ikaite and struvite exist in smears as single crystals. Polarized light microscopy (PLM) is a simple, inexpensive, and well-established method in geology to detect and identify single crystals. However, use of PLM to identify cheese crystals has not been reported previously. The specific objectives of this research were (1) to identify crystals in cheese smears using selected PLM criteria; (2) to compare identification by PLM against PXRD; and (3) to develop and evaluate a novel treatment for smear material to improve crystal analyses by both PLM and PXRD. Duplicate wheels of 4 cheeses produced by different manufacturers were obtained from retail sources. Scrapings of surface smears were prepared and analyzed by PLM and PXRD by previously described methods. Crystals were categorized by PLM based on angle of extinction (AE), birefringence behavior under crossed polarizers and quartz filters, and size and shape (circularity) by image analysis. Crystals observed by PLM fell almost exclusively into 2 readily differentiated groups based on birefringence behavior and estimated angle of extinction. Group 1 (n = 18) were highly birefringent with AE = 88-92°, whereas group 2 (n = 28) had no birefringence with AE = 13-26°. Group 2 crystals were significantly larger and more circular than group 1 crystals. Group 1 and 2 were identified as struvite and ikaite, respectively, based on known birefringence and AE characteristics. Struvite was identified in all 4 cheeses by PLM but in only 3 cheeses by PXRD. Ikaite was identified in 3 cheeses by PLM but in only 2 cheeses by PXRD. These discrepancies occurred because the smear scrapings from 1 cheese contained excessive amorphous matter that caused extreme background noise, potentially obscuring diffractogram peaks that may have been present. To minimize noise, smear scrapings were dispersed in aqueous NaOH (pH 10) before analyses, which resulted in consistent results by PXRD and PLM. The method also rendered high-quality images by PLM. Data suggest that PLM may offer a simple and inexpensive means to identify struvite, ikaite, and possibly other single crystals in cheese smears.
表面结晶和径向脱矿质的钙、磷、镁发生在涂抹成熟奶酪。此外，晶体的文石、鸟粪石、方解石和磷酸氢钙已经确定在奶酪涂片粉末 x 射线衍射（PXRD），和文石和鸟粪石存在于涂片单晶。偏光显微镜（PLM）是一种简单的、廉价的和成熟的地质方法来检测和识别单晶。然而，PLM 用于鉴定奶酪晶体尚未报道。本研究的具体目标是（1）使用选定的 PLM 标准来识别奶酪涂片上的晶体；（2）比较 PLM 和 PXRD 的鉴定结果；（3）开发并评估一种新的涂片材料处理方法，以提高 PLM 和 PXRD 的晶体分析效果。从零售来源获得了来自 4 家不同制造商生产的 4 个车轮的表面涂片刮屑，并通过之前描述的方法用 PLM 和 PXRD 进行了分析。晶体通过 PLM 基于消光角（AE）、交叉偏振器和石英滤光片下的双折射行为以及通过图像分析的大小和形状（圆度）进行分类。通过 PLM 观察到的晶体几乎完全分为 2 个基于双折射行为和估计消光角易于区分的组。第 1 组（n = 18）具有很高的双折射性，AE = 88-92°，而第 2 组（n = 28）没有双折射性，AE = 13-26°。第 2 组晶体明显大于第 1 组晶体。第 1 组和第 2 组分别被鉴定为鸟粪石和文石，基于已知的双折射和 AE 特性。PLM 鉴定了所有 4 种奶酪中的鸟粪石，但只有 3 种奶酪中的 PXRD 鉴定了鸟粪石。PLM 鉴定了 3 种奶酪中的文石，但只有 2 种奶酪中的 PXRD 鉴定了文石。这些差异是由于 1 种奶酪的涂片刮屑中含有过多的无定形物质，导致背景噪声极大，可能掩盖了可能存在的衍射峰。为了最小化噪声，在分析之前将涂片刮屑分散在碱性氢氧化钠（pH 值 10）中，这导致 PXRD 和 PLM 得到一致的结果。该方法还通过 PLM 生成了高质量的图像。数据表明，PLM 可能提供了一种简单而廉价的方法来识别奶酪涂片上的鸟粪石、文石和可能的其他单晶。
