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Monday, 20 April 2020

Cheese

This guest article was written by Maxwell Holle from the University of Illinois.

A simple glass of milk holds the potential to become hundreds of different types of cheeses with a variety of different flavours. But with so many diverse flavours and styles, how can we identify an off-flavour?

Image by corinnabarbara from Pixabay.

 

What is cheese?


Cheese is milk that we force into a gel by either acid, heat, or enzymes. While making cheese, we try to remove as much water as possible, which also means losing some milk proteins. The simplest cheeses stop here and are known as fresh cheeses; they have a relatively salty and neutral taste. However, most cheeses take on their characteristic flavours and smells during the ageing and ripening stage, where diverse groups of microorganisms get involved. These microorganisms generate the characteristic flavours of some of our favourite cheeses.


Flavour generation


Flavour compounds are created by microorganisms metabolising the sugar present in the cheese (yes, there are sugars in cheese!), and breaking down the cheese’s fat and protein. The type of microorganism (such as bacteria or mould) and their species have a dramatic effect on the flavour of the end product. In cheddar cheese, each specific strain of Lactobacillus paracasei can breakdown the fats in the cheese in noticeably different ways, despite being the same species[1].

Photo by Alana Harris on Unsplash.
Depending on which Lactobacillus a cheese maker chooses, the final flavour might have an enhanced buttery aroma, or even spicy aroma[2]. Some microorganisms can even be added during the cheesemaking process to help mimic certain flavours. For example, traditional raw milk Tetilla cheese has a very strong and specific flavour. By adding a certain bacteria and a yeast during the cheesemaking process, pasteurised milk Tetilla cheese can be made that mimics the raw milk Tetilla cheese flavour[3]. Some bacteria may produce pleasant flavour compounds, but there some such as Enterobacteria and Pseudomonas increase unpleasant aromas, often because they release sulfur compounds[4] – and the smell of sulfur is the smell of rotten eggs.

Cheese ripening is a naturally difficult process, and it’s complicated by multiple variables. The cheese starter culture (the bacteria added by the manufacturers to make cheese) composition leads to cheeses with different smells and tastes even when they are from the same batch[5][6]. In fact, how “many” flavour compounds (or categories of flavour compounds) are created is something we really don’t know, making it hard for cheese makers to manipulate cheese ripening.

Cheese flavour


Scientists use a variety of different techniques to identify the compounds present in cheese. Many rely on extracting the compounds from the cheese, separating them from each other, and then analysing them. One technique actually uses an electronic nose, to help recognise the compounds a consumer may smell[7]. The electronic nose recognises both simple and complex aromas and can be coupled with gas chromatography or mass spectrometry to determine what they are. Scientists use the electronic nose to discriminate between cheese types with different ages or even different sources[8]. Some cheeses, such as Danish blue cheeses, even have electronic nose “fingerprints”[9](so I guess it would be hard to make a counterfeit Danish blue!). However, these techniques can be limited. Just Knowing which chemicals are present does not necessarily mean knowing which chemicals are important for flavour. Because of how many flavour compounds there can be, and because tiny changes in their concentrations are so important, when a change naturally occurs it’s difficult to pinpoint exactly which chemicals made that change.

In the end, nothing beats traditional methods: scientists frequently rely on a panel of humans to determine if a cheese doesn’t taste good. Meanwhile, they’re still working on ways to find which balance of flavour compounds makes good cheese, while also working out how to control microorganisms so we can reproduce flavours across all cheeses. And good luck to them!


References
why don't all references have links?

[1] Stefanovic, E., et al., Evaluation of the potential of Lactobacillus paracasei adjuncts for flavor compounds development and diversification in short-aged cheddar cheese. Frontiers in microbiology, 2018. 9: p. 1506. doi: 10.3389/fmicb.2018.01506.
[2] Menéndez, S., et al., Effects of Lactobacillus strains on the ripening and organoleptic characteristics of Arzúa-Ulloa cheese. International Journal of Food Microbiology, 2000. 59(1-2): p. 37-46. doi: 10.1016/S0168-1605(00)00286-5.
[3] Centeno, J., et al., Recovering traditional raw-milk Tetilla cheese flavour and sensory attributes by using Kocuria varians and Yarrowia lipolytica adjunct cultures. International journal of food microbiology, 2017. 251: p. 33-40. doi: 10.1016/j.ijfoodmicro.2017.03.014.
[4] Khattab, A.R., et al., Cheese ripening: A review on modern technologies towards flavor enhancement, process acceleration and improved quality assessment. Trends in food science & technology, 2019. doi: 10.1016/j.tifs.2019.03.009.
[5] O’Riordan, P.J. and C.M. Delahunty, Characterisation of commercial Cheddar cheese flavour. 1: traditional and electronic nose approach to quality assessment and market classification. International dairy journal, 2003. 13(5): p. 355-370. doi: 10.1016/S0958-6946(03)00005-0. doi: 10.1016/S0958-6946(03)00006-2.
[6] O’Riordan, P.J. and C.M. Delahunty, Characterisation of commercial Cheddar cheese flavour. 2: study of Cheddar cheese discrimination by composition, volatile compounds and descriptive flavour assessment. International dairy journal, 2003. 13(5): p. 371-389. doi: 10.1016/S0958-6946(03)00006-2.
[7] Ampuero, S. and J. Bosset, The electronic nose applied to dairy products: a review. Sensors and Actuators B: Chemical, 2003. 94(1): p. 1-12. doi: 10.1016/S0925-4005(03)00321-6.
[8] Gursoy, O., P. Somervuo, and T. Alatossava, Preliminary study of ion mobility based electronic nose MGD-1 for discrimination of hard cheeses. Journal of food engineering, 2009. 92(2): p. 202-207. doi: 10.1016/j.jfoodeng.2008.11.002.
[9] And, J.T. and P.V. Nielsen, Electronic nose technology in quality assessment: Monitoring the ripening process of Danish blue cheese. Journal of food science, 2005. 70(1): p. E44-E49. doi: 10.1111/j.1365-2621.2005.tb09036.x.

Maxwell Holle Author Bio

Maxwell is a PhD candidate under the advisement of Dr. Michael Miller at the University of Illinois, Urbana-Champaign. Maxwell studies and develops new food safety solutions for fresh cheeses as well as teaches fermentation science at UIUC.

You may also be interested in reading our article on how we talk about flavour perception.

For more reading about sensory science, check out our articles on the senses and on smell.

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