That chunk of cheese on your plate owes its flavour to an elite gathering of microbes. 麻豆传媒 checks out the guest list
LOOK, I don鈥檛 want to put you off your festive food. But before tucking in, there鈥檚 something you should know: it鈥檚 not dead. At least, not all of it. Perched amid the tinsel, creatures are busily thriving, eating and reproducing. They are in the cheese.
In many houses the family feasts so common at this time of year are accompanied by a spread of the finest fermented dairy products that money can buy. No one who has caught a whiff of an over-ripe Stinking Bishop 鈥� once voted the UK鈥檚 smelliest cheese 鈥� will be surprised to learn that cheeses are home to microbes. But new technologies have allowed us to take a closer look at the startlingly complex lives of the bacteria, yeast and moulds within. 鈥淐heese is a veritable ecosystem,鈥� says Marie-Christine Montel, a microbiologist at the French National Institute for Agricultural Research (INRA) in Aurillac.
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So it is no surprise that microbiologists are trying to improve cheese-making. They want to make fermentation more consistent and reliable, to eliminate cases of food poisoning and to help develop ever finer, tastier cheeses.
These efforts are not always appreciated by traditional cheese-makers, however. 鈥淒airy scientists have come from the factory culture and they haven鈥檛 understood the benefits of the artisan system,鈥� says Randolph Hodgson, who founded the Specialist Cheesemakers Association in the UK.
鈥淒airy scientists come from the factory culture and they don鈥檛 understand the artisan system鈥�
Cheese-making has been practised for thousands of years, perhaps starting when herders stored milk in bags made from animals鈥� stomachs and found it created a tasty protein-based snack with a longer shelf-life than fresh milk. An enzyme called rennet, from the stomachs of mammals, causes the milk proteins to clump together into curds, leaving behind a watery liquid called whey.
The curdling is helped along by bacteria that turn milk sugars into lactic acid. Traditionally, these bacteria were naturally present in the milk or hanging around the cheese-making equipment. These days most cheese is made from milk that has been heat-treated, or pasteurised, to kill any harmful bacteria, and then mixed with starter cultures of carefully selected 鈥済ood鈥� bacteria. Raw milk is still used for some artisan cheeses, leading to more complex flavours, according to aficionados.
The curds undergo various cycles of fermentation, pressing and maturation, or 鈥渞ipening鈥�. Now the ecology starts to get interesting. Further cultures of bacteria or mould spores may be added, or other microbes floating around the creamery can get in on the act, helping to make the cheese so characteristic 鈥� Roquefort, for example, can only be ripened in certain caves in France. Mould-ripened cheeses such as Brie have the spores sprayed on, while washed-rind cheeses are bathed in liquids such as brine or brandy to encourage particular microbes.
Cheese-makers today range from artisan craftspeople working out of their kitchens to big businesses. Yet even in large creameries, cheese-making is often as much an art as a science, with results that are inherently variable and occasionally unpredictable.
Why so? The milk can vary depending on the cows鈥� health and their pasture. The wrong microbes might get in, or the right ones fail to, sometimes leading to catastrophe. Occasionally a batch of Stilton, for example, fails to develop its blue veins. According to Eric Spinnler, at INRA鈥檚 AgroParisTech research institute, cheese-making needs to move 鈥渁cross the border between know-how and science. At the moment, it鈥檚 a lot of know-how still.鈥�
In the last decade, however, new techniques usually associated with medical research, such as DNA sequencing and proteomics, have been used to study the cheese ecosystem. Spinnler and his colleagues have just sequenced the genome of Arthrobacter arilaitensis, a bacterium found in nearly every washed-rind cheese. They found the bacterium makes compounds that snatch iron from its environment; as most bacteria need iron, this kills off rival species (). Other bacteria have developed the ability to metabolise fatty acids, which are toxic to many other species.
Meanwhile, Spinnler鈥檚 colleague Fran莽oise Irlinger and her team have stripped down the ecosystem of a cheese to its bare essentials. In a project reminiscent of the way US geneticist Craig Venter is defining the minimal genome 鈥� or the least number of genes that can support life 鈥� the French team want to know the least number of species that can support a cheese.
They chose Livarot, a washed-rind cheese renowned for its strong smell, redolent of a farmyard. The team found it contains 82 strains of bacteria, yeast and mould. Through progressive rounds of testing, they whittled the ecosystem down to 10 strains that still resulted in the same familiar Livarot ().
After defining the 鈥渕inimal ecosystem鈥� they could pick it apart by removing strains one at a time. Taking out one of the yeasts, for example, resulted in the loss of some of the bacteria. This is partly because the yeast reduces the acidity of the cheese鈥檚 surface, allowing acid-sensitive bacteria to flourish.
Christine Dodd, a food microbiologist at the University of Nottingham in the UK has been dissecting what is sometimes called the 鈥渒ing of cheeses鈥� 鈥� the Stilton. This English cheese has a distinct architecture: a rind and a creamy white interior criss-crossed with blue veins produced by the Penicillium roqueforti mould.
The team began by using DNA sequencing to take a census of all the bacteria in a Stilton, before analysing slices of the cheese to see what lived where (). They found that different bacteria colonised the veins than did the white core, while one species, Lactobacillus plantarum, lived only under the rind. 鈥淲e know there鈥檚 a flavour difference in each of the sections,鈥� says Dodd. 鈥淭he different flora must be contributing to that flavour.鈥�
Flavour aside, the other main driver is food safety. Cheese can contain bacteria that cause listeriosis (Listeria monocytogenes) and other forms of food poisoning (Staphylococcus aureus and Escherichia coli); one study found that 2 per cent of cheeses sold in the UK have bacterial counts that breach European regulations, often because of poor shop storage ().
The riskiest products are thought to be those made from raw milk. Even if milk is pasteurised, listeria bacteria occasionally set up home if the cheese is one of the less acidic types, mainly the soft, mould-ripened ones such as Brie and Stilton. That is why raw-milk and soft cheeses are off-limits to those with weak immune systems and pregnant women.
Despite his reservations about dairy scientists, Hodgson hopes their work will have at least one benefit: improving confidence in the safety of raw-milk cheeses. 鈥淭his could help us understand why there are so few cases of food poisoning from raw-milk cheese when theory says it should be far more problematic.鈥�
Montel鈥檚 team, for example, have shown that the traditional French wood barrel, or gerle, used to make AOP Salers cheese, is coated with a biofilm of good bacteria that keeps out harmful ones. Other teams are studying 鈥渂acteriocins鈥� 鈥� molecules made by bacteria to kill off their competitors.
Could further study of artisan cheeses risk the loss of their mystique? Montel thinks not for the time being. 鈥淭he complexity and magic of our raw-milk cheeses are at the heart of our activities,鈥� she says, 鈥渂ut we are still a long way from revealing all the secrets.鈥�
