On-farm testing: the next frontier for raw milk cheese?

When a chemical salesman suggested Graham Kirkham, the farmhouse cheesemaker behind Kirkham’s Lancashire, switch from the usual pipeline cleaning powder to a supposedly better liquid version earlier this year, he thought, “why not?”  

“We watched the [coliform] counts start going up the day after,” Graham told a room of cheesemakers. Coliforms are bacteria used as hygiene indicators in raw milk. “Within three days, we were like, oh my goodness, these have gone scary. There’s something wrong,” he said. He quickly switched back to the powder product. The coliform counts dropped. 

Graham caught the problem right away thanks to his on-farm testing set-up. Every day, he tests a sample of milk for coliforms. 24 hours later, he has the results. “If we hadn't got that, how long would we have continued using that [product] there?” Graham said.  

A greater ability to spot red flags before they become serious problems would be powerful for raw milk cheesemakers. That’s why Graham, alongside Jonny Crickmore, the farmhouse cheesemaker behind Baron Bigod, presented the method to their colleagues at the Specialist Cheesemakers Association (SCA) Farm Visit in June.  

A Petrifilm plate used for milk testing is held up to the light

Here at Neal’s Yard Dairy, we are supporting Graham, Jonny, and other farmhouse cheesemakers as they pioneer this everyday approach to raw milk testing. Why? More control means even safer raw milk cheese – and more people enjoying it.  

Raw milk, pasteurisation, and pathogens 

To understand why testing raw milk matters, you have to understand the risks – and benefits – of raw milk cheesemaking. Pasteurisation kills bacteria that can make you sick. It also kills other, benign bacteria – including some that contribute to the flavour and texture of cheese. 

High quality raw milk, on the other hand, contains a diverse array of those cheese-improving microorganisms. “All things being equal, that is going to make a cheese with way more interest, length, and complexity than if that milk were pasteurised,” said Bronwen Percival, our technical director.  

Raw milk farmhouse cheesemakers aim to harness the good organisms without letting in the bad ones. That requires an excellent standard of cleanliness for their cows, employees, and equipment. “The standard of what we’re making, our products – they need to be better than just industry standard,” said Jonny Crickmore, farmer and cheesemaker at Fen Farm Dairy. 

A cheesemaker at Fen Farm Dairy tests milk (Photo courtesy of Jonny Crickmore)

To make sure they’re doing enough to keep pathogens out of their milk, farmhouse cheesemakers often rely on laboratory testing. One common test is for coliforms. "There’s no legal limit to the coliform levels in milk used for cheese, but coliforms can be an indicator of fecal contamination, which can be an indicator of problems with milk production practices,” Bronwen explained. “As cheesemakers, we should be looking for low coliforms in our milk.” 

Laboratory testing costs money and takes time. “You might be waiting four days to get the test results back from the lab,” Bronwen said. If a farmhouse cheesemaker tests their milk every two weeks, that could mean up to 18 days between a contamination incident and a positive result. 

On-farm testing allows farmhouse cheesemakers to take more frequent samples and get quicker results. 

A quick, simple way to test for bacteria 

“When you start thinking you're going to start testing, you think it’s going to cost a fortune and it’s going to be really, really difficult,” Graham told a room of SCA members in June. “It’s the opposite. It’ll take five minutes.” 

Example Petrifilm plates from the Specialist Cheesemakers Association Farm Visit

Let’s get specific. Here is a list of what you need to start testing milk for coliforms. 

1. Incubator: The incubator provides a temperature-constant environment for the bacteria to grow – or not. As of this writing, you can find one on Amazon for £119. 

1. Petrifilms: To test for bacteria, you need a medium on which they can grow. A company called Neogen produces Petrifilms that support testing coliforms and total bacteria. A pack of 50 coliform Petrifilms costs approximately £80 from a specialist supplier. 

1. Pipettes: Packs of pipettes to collect milk samples are available on Amazon for less than £10. 

Once you have this equipment, you can follow these basic steps. 

1. Collect sample: With clean, gloved hands, collect a 1 millilitre sample of milk. 

1. Inoculate Petrifilm: Lift the protective film off the plate, then dispense the sample onto the centre. Replace the film. 

1. Label your sample: Write the date and any other relevant details on the Petrifilm. 

1. Incubate: Set the incubator to 30 degrees Celsius. Leave for 24 hours. 

1. Interpret: Check the Petrifilm for growth. Each spot represents one colony forming unit.  

There is guidance on all this and more available online.  

A safe future for raw milk cheese 

Daily coliform testing is more than just a stopgap between laboratory tests. It’s a source of information farmers and cheesemakers can use to improve.  

When Jonny heard that rubber joins in the milk pipeline — currently standard best practice within the dairy industry — could lead to contaminated milk, he removed them and replaced them with welded stainless steel. Because of his daily testing regime, he saw the effect right away. “Our numbers dropped an awful lot,” he said.  

While Jonny is currently pasteurising the milk he uses to make cheese, day-to-day coliform monitoring allows him to sell raw drinking milk with confidence. Raw milk may contain up to 100 coliforms per millilitre, according to the Food Standards Agency. Jonny passed around a spotless Petrifilm that tested milk from Fen Farm. “No coliforms. Zero coliforms,” he said. 

While Petrifilm testing is a step forward, there is further still to go. A test that reveals a high level of coliforms tells you that your system needs investigation. It doesn’t tell you that pathogens are present – or if so, which ones. Neal’s Yard Dairy is working with a group of researchers at Imperial College London to scope out the feasibility of doing just that.  

“[The researchers] have developed a test kit that is designed to allow people in remote locations with minimal infrastructure to do PCR testing,” Bronwen said. "So, DNA testing for presence or absence of whatever organism you desire, on site with no centrifuge and no special equipment." Cheesemakers in France have used similar technology to test for pathogens before using raw milk to produce cheeses like Camembert. 

More widely available testing could mean more cheesemakers producing raw milk cheese with confidence – and more of us eating it with confidence, too.  

At Neal’s Yard Dairy, we are working to make daily milk testing more accessible. If you are a cheesemaker interested in bringing in-house testing to your dairy, please feel free to contact us.