Department of Biology, Tufts University, 200 Boston Ave., Medford, MA, 02155, USA.
Tufts University Sensory and Science Center, Tufts University, 200 Boston Ave., Medford, MA 02155, USA; Department of Chemistry, Tufts University, 62 Talbot Ave., Medford, MA, 02155, USA.
Food Microbiol. 2018 Dec;76:543-552. doi: 10.1016/j.fm.2018.07.011. Epub 2018 Jul 20.
The rinds of surface-ripened cheeses have expected aesthetic properties, including distinct colors, that contribute to overall quality and consumer acceptance. Atypical rind pigments are frequently reported in small-scale cheese production, but the causes of these color defects are largely unknown. We provide a potential microbial explanation for a striking purple rind defect in a surface-ripened cheese. A cheese producer in the United States reported to us several batches of a raw-milk washed-rind cheese with a distinctly purple rind. We isolated a Proteus species from samples with purple rind defect, but not from samples with typical rind pigments, suggesting that this strain of Proteus could be causing the defect. When provided tryptophan, a precursor in the indigo and indirubin biosynthesis pathway, the isolated strain of Proteus secreted purple-red pigments. A Psychrobacter species isolated from both purple and normal rinds also secreted purple-red pigments. Using thin-layer chromatography and liquid chromatography-mass spectrometry, we confirmed that these bacteria produced indigo and indirubin from tryptophan just as closely related bacteria make these compounds in purple urine bag syndrome in medical settings. Experimental cheese communities with or without Proteus and Psychrobacter confirmed that these Proteobacteria cause purple pigmentation of cheese rinds. Reports of purple rinds in two other cheeses from Europe and the observation of pigment production by Proteus and Psychrobacter strains isolated from other cheese rinds suggest that purple rind defect has the potential to be widespread in surface-ripened cheeses.
表面成熟奶酪的外皮具有预期的美学特性，包括独特的颜色，这有助于整体质量和消费者接受度。在小规模奶酪生产中经常会出现非典型的外皮色素，但这些颜色缺陷的原因在很大程度上尚不清楚。我们为表面成熟奶酪中明显的紫色外皮缺陷提供了一个潜在的微生物解释。美国的一位奶酪生产商向我们报告了几批带有明显紫色外皮的生牛乳洗皮奶酪。我们从有紫色外皮缺陷的样本中分离出了一种变形菌属，但从有典型外皮色素的样本中没有分离出，这表明这种变形菌属可能是造成这种缺陷的原因。当提供色氨酸（靛蓝和靛玉红生物合成途径的前体）时，分离出的变形菌属菌株会分泌出紫红色色素。从紫色和正常外皮中分离出的一种 Psychrobacter 属也分泌出紫红色色素。通过薄层色谱和液相色谱-质谱联用，我们证实这些细菌可以从色氨酸中产生靛蓝和靛玉红，就像在医疗环境中与紫色尿袋综合征相关的细菌产生这些化合物一样。有或没有 Proteus 和 Psychrobacter 的实验奶酪群落证实，这些变形杆菌会导致奶酪外皮出现紫色色素沉着。欧洲另外两种奶酪出现紫色外皮的报告以及从其他奶酪外皮中分离出的 Proteus 和 Psychrobacter 菌株产生色素的观察结果表明，紫色外皮缺陷有可能在表面成熟奶酪中广泛存在。
