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Home-made yogurt

22 May 2014 I've found that whisking my freshly-chilled batch leaves it rather grainy. For no particular reason, I recently decided to pour off the tablespoon or so of clear liquid from the top of the Thermos before pouring the yogurt into its container. Result: a much smoother and creamier product. Why? I have no idea!


I suppose I could be classified as a ’yogurt addict’ because I eat around 200 grams for breakfast most days, mixed with my home-blended muesli. I’ve been doing this for quite a few years, and for most of that time my yogurt-of-choice was Danone Activia. Because our village shops - Co-op and One-Stop - don’t sell Activia, the regular alternatives have always been Onken and Yeo Valley, and the latter has become my real favourite as it’s lusciously thick and creamy, more tangy than the others and, surprisingly, contains only 4.2% fat.

My loyalty to Yeo Valley became even stronger after seeing their premises in the West Country and a recent BBC-TV series on the food industry, which identified Danone as one of the scarier food manufacturers.

I particularly like cold stewed prunes with my muesli, and I buy French ’Jumbo’ prunes from Holland and Barrett - by far the best I’ve found in this country!

Anyway, I have recently seen both Nigel Slater and Hugh Fearnley-Whittingstall making their own yogurt on TV and, being (as evidenced by this site) an obsessive food experimenter, I decided to have a go (the cost-saving would be fairly trivial, but I suppose every little helps!). Yesterday (10 January 2011) was the day.

After a bit of googling (and as a faithful Observer reader) I decided to trust Nigel. In the absence of milk powder I went right back to basics for the first trial.

Now, after about 16 weeks, I feel pretty confident, so - for those without the patience of interest to follow my experiments - here is my recipe...

The recipe

Pour a litre of milk - preferably whole milk, but I’ve used both whole and a half-and-half mix of whole and semi-skimmed, depending on what’s been in the fridge, and both have been fine - into a saucepan and start heating. When this is getting warm, add 45 grams of powdered skimmed milk (readily available from supermarkets) and stir to mix thoroughly (I use a wire whisk).

Meanwhile, fill your vacuum flask (I use a stainless-steel Thermos) right to the top with boiling water and screw the top on. This both sterilises the flask and preheats it so the mixture won’t lose temperature when poured in.

You need a fairly accurate thermometer for reliable results - I have a mid-range digital thermometer (about £20 from Nisbets) but a cheaper one will do.

When the milk mix reaches 85°C, turn off the heat, empty the flask, pour in the mix and screw in the stopper. This temperature isn’t critical, as explained below, but the next one needs to be pretty accurate. This will ultra-pasteurise the mix and denature the milk proteins in a beneficial way, giving you a thicker yogurt..

After 30 minutes (also not highly critical and also explained below) pour the mix back into the saucepan. Re-fill the flask with boiling water.

Leave the mix to to cool to 46°C - about half an hour should do it, reheating if it cools too far. I don’t know how critical this temperature is, but I suspect it’s the optimum for the bacterial action we’re after. I think the bacteria go on working as the mixture cools, but a temperature much higher would kill your bugs, so if you’re not sure of your thermometer err on the low side.

Add one teaspoon (more if you like) of your favourite live plain yogurt and stir in. Once you have some home-made yogurt, use this instead for further batches - mine’s been going fine for months now.

Again, empty the flask and pour in the mix. Stopper it and leave. I’ve left it for anything between eight hours and 24, and it seems to get more tangy if you leave it longer. However, the 24-hour batch (an accident - I forgot to decant it!) wasn’t as good either in flavour or in texture.

When you’ve waited long enough, open the flask and check that the mix has set. If it has, you’ll probably need to get a long-handled spoon or something similar to break the set before you can pour your yogurt into a container and put it in the fridge. When cold, beat it until smooth with a balloon whisk.

The worst bit is cleaning all the coagulated protein off the inside of the flask: I use a long-handled washing-up brush.

Now the history...

Yogurt Mk I

It is essential to sterilise - or at the very least pasteurise (see below) - the milk to ensure that the yogurt-making bacteria have no unwanted and potentially dangerous competition. I didn’t want to bring the milk right to boiling point, because this forms a skin and changes the flavour, so I settled for raising one litre of whole milk to 90°C in a saucepan on the gas hob. I then turned off the gas and left the pan with my latest digital thermometer probe immersed in the milk and the lid on until the temperature had dropped to 46°C.

Meanwhile, I thoroughly washed a one-litre stainless steel Thermos flask and filled it with boiling water, both to sterilise it and to ensure that the temperature of the milk wouldn’t drop when poured in.

When the thermometer read 46°C, I stirred in four level tablespoons of Yeo Valley yogurt (natural, of course!) and brought the temperature back up to 46°C on a very low gas, stirring constantly with the thermometer probe. I then poured the mixture into the Thermos, screwed down the top and put the cup on too for extra insulation. (With this flask, you can unscrew the whole plastic top rather than just the stopper, which gives a wider mouth to pour into.)

That was at about 1pm. Early in the evening I tried a quick teaspoonful: smooth, mild and quite thick. I decided to give it some more time as I like a strong lactic flavour - and then forgot to do anything at bedtime! At 8am, however, it was still very palatable, so I decanted it into a basin, slapped on some clingfilm and put it in the fridge.

Of course, when it was time for a late breakfast I couldn’t resist trying it with my muesli, although it was still slightly lukewarm. It was a bit lumpy, and I thought at first that the whey had separated out, but this proved not to be the case.

Pretty good, I thought, though still rather mild for my taste. I’m looking forward to trying it fully chilled.

More research

At this point I thought I’d better check out a bit of the science, so I consulted Harold McGee’s superb On Food and Cooking: the Science and Lore of the Kitchen. I’ve referred to this magnificent book many times in The Online Cookbook: it is, quite simply, the definitive source of reference for the home cook interested in the scientific side of cooking - and McGee is, by the way, a close friend and collaborator of Heston Blumenthal.

I had had a narrow miss with my choice of 90°C for the preheating, which wasn’t bad considering that I was simply wanting to sterilise the milk without cooking it.

Pasteurisation is done at much lower temperatures: holding the temperature at 62°C for 30 minutes and at 71°C for a mere 15 seconds will have the same effect, which McGee says will ’destroy all disease-causing organisms and most others as well’. As we know from experience, though, pasteurised milk that hasn’t been unsealed will go sour even in the fridge after a few days, whereas sterilised (UHT) milk (the stuff the French use, which tastes like watered-down Carnation) can be stored at room temperature for months.

It turns out that holding the milk for yogurt at ’about 85°C’ and ’holding it there for about half an hour’ both pasteurises the milk very thoroughly and ’denatures the whey proteins to some extent, unfolding the initially compact molecules into longer structures that increase the viscosity - thicken the texture - of the liquid. And in the case of yogurt, it results in a firmer gel that is less prone to separating into curds and whey.’ If you want to know why this happens, you’ll have to buy the book - and if you’ve read this far you probably should!

The other point for attention is the common use of powdered milk in yogurt. Apparently this provides more milk protein without adding water or much fat, helping to make a thicker, smoother yogurt. Nigel Slater uses two tablespoons per half-litre.

So the next batch will he heat-treated according to McGee’s information above and will include powdered skimmed milk...


At 1pm, 24 hours after putting the yogurt in the flask, I tasted the fully-chilled product. The texture is a bit lumpy, and if anything it’s a bit tangier than the Yeo Valley yogurt I used to innoculate it, so maybe 20 hours is a bit too long. Thick and creamy, though, with absolutely no sign of the whey separating out. Not bad for a first attempt!

13 January 2011

I’ve just eaten the last full helping of my first batch of yogurt - there’s enough left in the bowl to provide four tablespoons to innoculate the next litre, with maybe a bit to be used in a dessert. At one stage, I thought I could see some whey separating out, but I think it more probably condensation that had formed on the clingfilm covering the bowl: the texture of the yogurt itself hasn’t changed.

Yesterday I got some ’instant dried skimmed milk’ from our village Co-op. I think the ’instant’ refers to the fact that it can be mixed with cold water. It was on the same shelf as Marvel, and is probably the same thing without the brand and marketing.

So today I will be making my second batch with the addition of the dried milk. The function of the dried milk is to add extra protein and lactose (milk sugar). The first is what coagulates to thicken the yogurt, under the influence of the lactic acid produced as the bacteria digest the sugar. The additional protein should produce a thicker yogurt, and possibly in the process a more acid one.

Obviously, as the dried milk in there to increase the protein content of the mix, it should be mixed in before heat treatment.

Yogurt Mk II

The litre of whole milk I bought this morning cost 87p, compared with £2.30 for two 500ml cartons of Onken yogurt - over 2½ times as much for the same money.

I weighed four tablespoons of dried skimmed milk at 38 grams (I’ll weigh rather than measure in future and, if the result is satisfactory, use 40 grams in future), intending to add it to the cold milk in the saucepan. Unfortunately I was so preoccupied with the problem of heating the mix to 85°C and keeping it there that I only spotted the dried milk still in the scale-pan about ten minutes into the heat-treatment! Not to worry: in it went for the remaining 50 minutes.

To make reconstituted skimmed milk, this quantity of dried milk would be mixed with 380ml of water , so it should add around 40% to the protein and lactose in the yogurt mix.

For most of this time I managed to keep the milk within a degree of the required temperature by adjusting the gas-ring by minute amounts - and McGee does say ’about 85°C’. However, this meant staying with the process for the whole hour, stirring almost constantly with the thermometer probe - not something I’ll want to do every week. So the next experiment will be to use the water-bath I bought for my sous vide’ experiments - perhaps putting half a litre of mix in each of two jugs immersed in the temperature-controlled water.

Meanwhile, the Thermos had been rinsed and filled to the brim with boiling water, which was left to cool until the hour was up. At 12:15pm, when the temperature had dropped to 46°C, in went four tablespoons of the previous batch of yogurt. I plan to sample the result before going to bed before deciding whether or not to leave it overnight.

40 grams of the Co-op dried milk contains 14.25 grams of protein, 20 grams of carbohydrate (all sugar) and only a quarter of a gram of fat, giving a total of 140 kilocalories. What happens to this in yogurt-making I’m not sure. The protein should stay about the same and the sugar should be consumed to produce lactic acid, I guess.

Disorganised again! I forgot to check this batch before bed, but did so at 2:45am when I had to come down for some cough medicine! It was already thicker than Mk I, so I extracted the yogurt from the flask into a 2-pint pudding basin as before. (I emphasise ’extracted’ because I had to insert a long-handled spoon and disturb the yogurt before it would come out, and even then quite a lot was left at the bottom of the flask, which a good shake dislodged. The basin was clingfilmed and put in the fridge to await breakfast time.

The texture was similar to that of the previous batch - somewhat lumpy - and the flavour perhaps a little stronger. Very acceptable, though. This morning (16 January) I decided to risk whisking it before breakfast. It took quite a bit of work to get it smooth, but the result was very creamy.

Don’t change two variables in one go!

I should have remembered this fundamental rule of experimentation and tried first the heat-treatment alone and then the addition of powdered milk alone before combining the two. However, since the result was excellent I probably won’t bother backtracking, and may never know which of the two changes made the most difference!

17 January

Yogurt still smooth and free from separated whey this morning. Also, my old mate Pete ’Voltarol’ tells me he used to make yogurt, also in a Thermos, and sometimes strained it to get thicker Greek-style yogurt. I checked this out, and it seems to be about draining off the whey rather that actually straining the yogurt. I don’t know if it works if the yogurt has no ’free’ whey.

Yogurt Mk III

24 January 2011 This batch was made two days ago. After the tedium of keeping the temperature of the milk at ’about 85°C’ for an hour, I had what seemed like a brainwave: heat the milk to a bit over that temperature and tip into a pre-heated Thermos. I couldn’t really see or taste any significant difference when I decanted the yogurt yesterday (if anything this was a bit more difficult than with the previous batch, so a bit thicker), and after whisking to a smooth texture I had some with my muesli this morning. The texture and flavour were just as good as before, so I will stick to this process unless I’m distracted by any more sudden inspirations!

Yogurt Mk IV

31 January 2011 (Monday) I made a new batch, as above, at Saturday lunchtime. The only difference was that, when I decanted it from the Thermos, I did so into a larger basin so that I could whisk it more vigorously before tipping it into a smaller one - this saved a lot of splatter. I had some on my muesli this morning and it was just as good as before.

Yogurts Mk V, VI, VII and VIII

25 February 2011 (Friday) Four more batches done, including one whipped with the handheld electric mixer, and I’m still very happy with the taste and viscosity of the result. It’s not as smooth as the commercial product - a little granular - but if anything the taste is cleaner, eliciting a satisfied ’Aaaaaah!’ as the last scrapings from the muesli dish go down. One batch was accidentally left in the Thermos for nearly 24 hours, but seemed none the worse for this.

The reason this week’s report is on a Friday is that I used some of the yogurt in a 50:50 mix with Patak’s Madras curry sauce. I wondered if this mixture might split, but it was fine, cooling this fierce curry enough to make it acceptable to Patricia, who has a low pain threshold when it comes to chilli.

So that’s eight litres of very acceptable yogurt for, counting the milk and the skimmed-milk powder, under £8. The same quantity of Yeo the Valley product would have cost £18.40. As far as I can see, I’ll be able to go on innoculating each batch from the previous one indefinitely. So it will be worth investing in...

A more suitable Thermos

I’ve been using a tall, slim stainless steel Thermos, which is fiendishly difficult to clean because the yogurt deposit left behind sticks very firmly to the metal. I had a look at the Thermos Online site and found a wide-mouthed one among the food flasks. Amazingly this turned up overnight. It’s not as deep as the skinny one, has a wider mouth and 200ml more capacity. I’ll be able to can scrub it out, rather than relying on squirting water in and shaking it around, then resorting to bleach, and there will be a bit of headroom so I don’t spill as much of my mixture.

I chose to buy this from Thermos direct desite it being a couple of quid on Amazon, because I needed new tops and stoppers for our two skinny flasks. When I read the instructions for the new flask, I found out why the seals on these had failed: I’d put them on a hot programme in the dishwasher and Thermos only recommend warm water and washing-up liquid followed by a hot rinse. Be warned.

Yogurt Mk IX

3 March 2011 Again, ignoring my own warning not to change two variables at once, I made this week’s batch in the new flask using only three rather than four tablespoons of the previous batch as a starter. After only eight hours the yogurt was fully set and at least as good as any previous ones. The new flask proved much easier to clean, first because I could get a brush to the bottom but also because it has an easy-clean surface treatment.

I intend to reduce the quantity of starter progressively because it seems sensible to use as much ’old’ yogurt as possible, so the next batch will be made with two tablespoons...

Yogurt Mk X

8 March 2011 I really should read my own notes! I came back to this page to double-check that the heat treatment temperature is85°C (had a senior moment and doubted my own memory) and realised that the time required according to The Blessed Harold is about half an hour - not the hour I’ve been using so far. So the tenth batch has just gone in the Thermos for 30 minutes (with only two tablespoons of starter this time).

I’m doing this batch just five days after the last one because I’ve been eating a banana with yogurt and Billingtons’ molasses sugar - even darker and more strongly flavoured than dark musovado - for pudding at dinner time. Delicious!

It was only when I was pouring the milk out of the Thermos after the heat-treatment phase that I remembered I hadn’t put in any milk powder - obviously this was a day for senior moments. I stirred the powder in and completed the process with no more errors. Although the milk powder hadn’t had the benefit of the heat treatment, I couldn’t detect any difference from the previous batches.

This batch was made with two tablespoons of starter, and didn’t take any longer to thicken. The next will be done with just one spoonful...

My yogurt isn’t quite as tangy as the Yeo Valley product that provided the original starter. Very nice, but just a bit bland. The lactic acid comes from the bacteria fermenting the lactose (milk sugar) in the milk and acts as a preservative because not many bacteria like an acid environment (hence pickling in vinegar). So I started wondering if I could improve the tang by adding some lactose. To my surprise, there seems to be plenty on offer in small quantities in the USA, but none in anything but huge sacks in the UK. It seems that the only way to increase the lactose content, and therefore the acidity of the finished product, would be to increase the amount of powdered milk. I might up this from 40 to 50 grams in the next batch.

But that will be changing two variables at once again! But hey - there’s only a litre of milk at risk!

Yogurt Mk XI

14 March 2011 This time I remembered to mix the powdered skimmed milk (50 grams this time) to the litre of fresh whole milk before heat-treating the mix. At 85°C the mix was poured into the Thermos, which had been filled with boiling water to sterilise and pre-heat it. After half an hour this time (!) the mix was poured back into the saucepan to facilitate cooling. Once it had cooled to 46°C, a fairly precisely measured tablespoon of the previous batch was added and the mix was poured from the pan into the Thermos and sealed. Six hours later it had set fairly firmly.

This batch was definitely thicker than before, but if anything a bit too thick - almost chalky in texture. If I’m honet, I don’t think the taste was any more acid than before.

Yogurt Mk XII

22 March 2011 This time I split the difference, adding 45 grams of powdered milk, and innoculated the milk with just one level teaspoon of the previous batch. To my delight, it set just as well as the previous lot, so I’m now down to one twelfth of the original four tablespoons!

Yogurt Mk XIII

28 March 2011 I was short of whole milk when the need for this new batch arose, so I used 50/50 whole and semi-skimmed, again with 45 grams of powdered milk and one level teaspoon of the previous batch. There was not much difference - maybe the all-whole-milk is a little smoother. At some point, I might try Nigel Slater’s suggestion of adding a little cream for a luxury product...

And on it goes...

26 April 2011 Today’s will be about my 18th or 19th batch - I’ve stopped keeping count because the yogurt seems totally reliable. The last batch got left in the Thermos for almost 24 hours - by accident rather than design, because I made it in the morning and forgot to decant it before going to bed. It was fine, and very noticeably more tangy, so there is obviously some lacose left in the mix after 12 hours. I think I might try this again.

23 May 2011 Just an update... I can’t be bothered to count back to see what generation my friendly little bacteria are now, but it must be at least the 20th and the yogurt is still excellent - if anything, getting better. As with the sourdough, there seems to be a good case for keeping one culture going rather than starting a fresh one.

Interruption for a holiday

We left for a two-week visit to Normandy on the 22 August, and before going I stashed the last dregs of the current batch of yogurt in the fridge to see if it would survive to start the next. When we got back on the 5 September I was disappointed to find two orange and one black spot on the surface. Given that the mold that forms the rind on some of the more fragrant French cheeses (including Livarot) is bright orange, I decided not to risk using the stash as a starter. So this morning’s breakast included freshly-purchased Yeo Valley yogurt, the left-overs of which will be used to kick off a new generation of yogurt. I found this a bit mild compared with my home-made product.

The good news is that starters from the first batch (10 January) had made excellent - maybe even improving - yogurt for over seven months.

9 September I made a new batch using one tablespoon of Yeo Valley yogurt two days ago and had it for breakfast today. A bit milder than my previous batch, but I’m sure we’ll be back to normal after a few generations.

5 October The most recent batch, using one teaspoon of the previous one, was only left in the flask for about eight hours. The result is milder and not as thick, but still very palatable.

17 October The next batch was left from mid-morning until bedtime and by then had set really well. Until now I’ve been giving each batch a good beat by hand with a wire balloon whisk, after which it thickens nicely overnight in the fridge. This time I decided to try our stick-blender. Bad move - it smoothed the yogurt nicely but it didn’t thicken again, so I’d obviously broken down the protein structure too much. Otherwise, as nice as usual!

15 November The latest batch was left in the flask for a little over 12 hours and was very firmly set. Once into its fridge container, it was left for two days before beating with the wire balloon whisk. The result was very thick and pretty smooth. So it obviously pays not to beat the yogurt until it’s really cold.

A new recruit Last week I visited my accountant to get last year’s business accounts and tax return sorted. As a bit of light relief we chatted about home-made bread (she’s a bread-machine user) and yogurt. Today I got the draft accounts attached to an email which contained the following: P.S. Just to let you know on Sunday I had a successful attempt at yogurt making, using the thermos flask and thermometer method! I used UHT semi-skimmed milk, and a bought yogurt as starter. Yogurt was fairly thick, smooth, mild, and delicious. I started another batch this morning experimenting using the Sunday yogurt as a starter, so fingers crossed.

29 November I forgot to decant the last batch until it had been in the flask for about 23 hours. Perhaps it ended up a little tangier than the previous 12-hour batch, but tastes just as good and is very thick.

2 July 2012 The lack of updates since the end of November indicates that there have been no problems keeping the culture started on the 9 September. Recently I?ve been using semi-skimmed milk instead of whole - simply because we buy gallons of this every week and only buy whole milk when needed for sauces and desserts - and the results have been just as satisfactory as the ?full fat? ones. Today, I?ve started a batch with mostly whole milk because that was left over from the cheese sauce for a moussaka, topped up with a small amount of semi-skimmed.

8 May 2013 The most recent batches have been made entirely with semi-skimmed milk. I fill the big Thermos to the bottom of the neck (1200ml), tip it into the pan, rinse the flask and fill it with boiling water. As the milk is heating I whisk in 50 grams of powdered skimmed milk. When the thermometer hits 85°C I turn off the gas, empty the flask and pour in the hot milk. After 30 minutes I decant it back into the pan to cool and refill the flask with boiling water. It takes about another 30 minutes to cool to 47°C, at which point I stir in about a dessertspoonful of the last batch of yogurt and tip the result back into the flask. The big innovation is that I now ferment the yogurt for a full 24 hours, which makes it a lot thicker and not excessively sour. A rest in the fridge and a vigorous whisking have it ready for breakfast.

Personal site for Paul Marsden: frustrated writer; experimental cook and all-round foodie; amateur wine-importer; former copywriter and press-officer; former teacher, teacher-trainer, educational software developer and documenter; still a professional web-developer but mostly retired.

This site was transferred in June 2005 to the Sites4Doctors Site Management System, and has been developed and maintained there ever since.