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What causes cheese to turn pink?

Various attributes of a cheese, including both flavor and appearance, contribute to the final quality of the product. During the production of some cheeses, microbial processes can cause strange quality defects, often with colorful outcomes. Researchers in University College in Cork, Ireland identified the microbial culprit behind a notorious pink cheese defect. In this Science Digested, Adam Shutes from the Boston Cheese Cellar explains what they found.

Occasionally you will find cheese which will have a pinkish hue, or ‘pinking’, often around of just under the rind. Most often affected are aged Cheddars, though also occasionally Swiss alpine cheese, and some Italian cheese (such as Parmigiano Reggiano & Fontina Val d’Aosta) are affected too.

To the cheesemaking industry, this pinking is a defect, and if caught before shipment, it will mean discarding or repurposing of the entire cheese wheel resulting in a financial loss for the cheesemakers. It should be noted, however, that this discard is for aesthetic reasons only, and pinking is of no danger to the consumer.

The cause of this pinking has never been conclusively explained, although many hypotheses have been put forward:

  • A direct chemical change of cheese constituents, such as a change in annatto (the natural colour used in cheeses) to other colored chemicals such as carotenoids due to sunlight; or by the Maillard reaction of cheese components.
  • A microbial action on the components of cheese by certain lactic acid starter bacteria, or propionic acid bacteria, such as Lactobacillus helveticus used frequently in Swiss-style cheese.

Despite all this conjecture, no definitive answer to the pinking question has been provided, until now, when a recent study by Quigley et al. from University College in Cork, Ireland. They have conclusively identified the chemical causing the pinking, as well as the microbe responsible for its transformation.

On the initial assumption that a microbe was responsible for the pinking, they first took a broad genetic look across samples of cheese that showed pinking and some control cheese that did not. They used a technique which specifically examined the 16s rRNA gene, a gene which evolves very slowly due to its involvement in a crucial step of cellular activity (protein synthesis) and can therefore be used to identify a microbe’s position in a taxonomic tree, the phylum, genus & family etc.

The 16s rRNA sequencing showed that both pinking and control cheese contained many of the same microbes, yet cheese with pinking contained an extra microbe from the phylum, Deinococcus-Thermus, family and genus, Thermaceae and Thermus respectively. This phylum of microbes are known as thermophiles, since they are able to, and indeed prefer to, grow in environments where the temperature is greater than 65 °C.

The investigators then used a second genetic technique called ‘shotgun sequencing’ which examines a snapshot of all the microbes and their genetic make-up within pinking cheese and non-pinking control samples. At this level of genetic examination, they were able to go a step further then the genus-level Thermus identification, and could pinpoint specific strains present in pinking cheese, and not within control samples. They found that Thermus thermophilus was the most dominant strain, a thermophilic microbe which was first isolated from a hot spring in Japan.

In samples from pinking cheese, they also detected genes specifically involved in carotenoid synthesis – the pinking cheese also contained the Thermus thermophilus strain. Following onwards with this lead, they used a spectrophotometer (instruments that measure the absorbance of different colors of light) and observed the presence of the carotenoid lycopane in areas overlaying the pinking areas of the cheese that also contained thermophilus.

Together the presence of the carotenoid lycopane in the pink areas of cheese, as well as the presence of Thermus thermophilus in only cheese showing pinking, and ultimately the presence of carotenoid synthesizing genes within these samples, strongly suggest a link between the microbe, the carotenoid, and pinking cheese.

In order to prove that the presence of thermophiles drives the creation of lycopane and the pinking, they performed a crucial experiment.

The investigators made four cheeses using exactly the same methods, except for the different microbes they added. Three out of the four cheeses contained Thermus thermophilus with lactic acid starter cultures and varying levels of propionic acid bacteria. The fourth control cheese contained only the lactic acid starter and propionic acid bacteria. The cheese made with Thermus thermophilus addition showed increased carotenoid production and pinking, and the cheese made without Thermus thermophilus lacked any form of pinking at all. The cheese which had the highest levels of propionic acid bacteria also showed the most pinking, suggesting interplay between the two microbes.

To make this story complete, the authors asked the question: Where is the Thermus thermophilus coming from in the cheesemaking process?

Samples taken from two different cheesemaking facilities were analyzed for the presence of Thermus thermophilus and they showed that the microbe was only found in the hot water supply pipes of each facility, rather than milk vats or other equipment. This, of course, makes complete sense from what we know about the hot spring origin of Thermus thermophilus.

One important caveat for this study: it is likely that there are several different causes of pink pigments in cheese. For example, a previous post on MicrobialFoods.org explored how interactions between bacteria and fungi growing on cheese rinds might cause pink or red colors to occur in cheese pastes. How often Thermus thermophilus is the culprit behind pink cheese, versus other microbes, remains to be determined.

The initial aim of the study was to identify the culprit, either chemical or biological, of the pinking of cheese, and now armed with this information, strategies can be designed to prevent the pinking occurring at all, and save cheesemakers precious finances to create more great cheese.

Post written by Adam Shutes, owner of the Boston Cheese Cellar.

For more details on the study, please see Quigley, Lisa, et al. “Thermus and the Pink Discoloration Defect in Cheese.”mSystems 1.3 (2016): e00023-16.

There are 2 comments on this article

  • Gyulai Lehel Jozsef says:

    Hello,

    I would like to ask you if you met somehow a case like ours, when the cheese; 3-4 weeks purple spots appear strong, especially cheese crust but inside. Laboratory tests show an infection with Proteus but started to become skeptical about it.

    Reply to this comment
    • Hello Gyulai –

      This is such an interesting observation because we are actually studying in my lab how Proteus can make cheese rinds purple! I will send you a detailed email about this. Thanks for your note!

      Ben

      Reply to this comment

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