How much of the weight of yoghurt is due to bacteria?

How much of the weight of yoghurt is due to bacteria?

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I recently started making all kinds of bacteria-processed foods by myself (such as yoghurt, but also Sauerteig and so on), and I found myself wondering how much of the weight of these products is actually due to the bacteria themselves in the end. But I couldn't find anything useful on the internet.

So, specifically for yoghurt, how much of its weight is due to the bacteria, i.e. dead and/or living bacterial cells, but not including the products of their metabolism outside the cells?

I would also be happy about numbers on other bacteria- or fungi-processed foods, but I wanted to keep the question simple.

Interesting question and it will only be possible to do some estimation. The problem is to know the exact bacterial composition as well as the mass of the bacteria. To start, it is necessary to know the number of bacteria in a defined amount of yoghurt. There are scientific publications on this topic, but the numbers in fresh produced yoghurt are in the area of 108 bacteria per gram yoghurt and declines over time. Based on the numbers in reference 1 and 2 I use 2x108 bacteria per gram fresh yoghurt.

The other number we need is the mass of one bacterium. This is complicated, as bacteria are very light due to their size and cannot be easily weighted. There are some numbers available (we imply here that all bacteria weight the same, which is of course not the case), and according to them, one bacterium weighs about 1x10-12 grams.

Mutiplying both numbers gives the amount of bacteria in one gram of yoghurt, around 0.0002 gram or 0.2mg. One typical sized yoghurt of 150 gram therefore contains about 30mg of bacteria. The weight of the bacteria can thus be neglected against the total weight.


  1. Survival of Yogurt Bacteria in the Human Gut
  2. Handbook of Food and Beverage Fermentation Technology
  3. Mass of a Bacterium

Everything you always wanted to know about fermented foods

Are fermented foods the best thing since…bread?

Delicious homemade Kimchi (fermented cabbage). It’s alive! Click for a closer look.

Fermented foods, such as sauerkraut and kombucha, have become popular for health reasons. I have made my own sauerkraut in the past and have recently made the tasty, fermented Korean side dish, kimchi. I did it not only for the taste but also for the hope that the bacteria responsible for the fermentation of the cabbage — lactic acid bacteria (LAB) — would contribute to the diversity of my gut microbiota.

As a research scientist in the field of bacterial pathogenesis, this made sense to me. Now that I have started blogging about health and fitness and have been writing more in depth articles about health related topics, I started wondering what research has been done on the health benefits of fermented foods. Can the bacteria in fermented foods even survive the harsh conditions of the human gastrointestinal (GI) tract, particularly the stomach?

I was amazed to learn that the fermentation of food has been used by humans for thousands of years as a way to preserve foods, and that the health benefits go beyond their microorganisms (don’t worry, citations are provided below). The fermentation process enhances the nutritional quality of food by contributing beneficial compounds such as vitamins, and by increasing the bioavailability of minerals. Probiotics, including those found in kimchi, have a range of positive effects on health, including the improvement of various intestinal inflammatory conditions, positive impacts on the immune system and even weight loss, and can alter the composition of the gut microbiome.

However, these effects mostly depend on whether the bacteria actually make it in sufficient numbers to the colon. And let me tell you, the journey to the colon is one harsh and dangerous ride!

Why Does Yogurt Taste Sour?

Our food scientist Jared Levan has soloved the mysteries of brain freeze, the emotions caused by chopping raw onions and the source behind J ell-O’s Jiggles. Today he takes on yogurt’s sour taste.

While most informed consumers, like you, know that yogurt is loaded with protein, calcium and a slew of other nutrients, few know the science behind its characteristic taste and texture. Have you ever wondered why every spoonful of your favorite fruit-on-the-bottom snack has such a tang to it?

Why does yogurt taste sour?
Plain and simple, yogurt is the result of bacterial fermentation of milk. All you need is pasteurized milk and bacteria. During this process, bacterial strains such as Lactobacillus and Bifidus — just two of the many types of lactic acid microorganisms — convert lactose sugars in milk into lactic acid. Standard starter cultures often number in the millions, and these little guys are active as hell now you know why yogurt always contains “active culture” in the list of ingredients.

With such a high volume of active bacteria chugging away, the end result becomes supremely tangy. For some, that sour taste is anything but tasty, so many varieties of yogurt come adorned with additives ranging from honey to fruit preserves to balance out the flavor.

What is yogurt so thick?
Again, this is a direct result of the acidic environment created by fermentation. When casein proteins found in the milk are exposed to the highly acidic environment created by bacterial metabolism, they begin to curdle and form the thick yogurt texture you know and love. If you like your yogurt extra thick, stick with Greek-style strained yogurts — the straining process thickens by removing a massive amount of excess water without the need for increasing the fat content.

Terms and Concepts

  • Live cultures
  • Bacteria
  • Microorganisms
  • Microscopic
  • Cells
  • Species
  • Agar plates
  • Bacteria colonies
  • Anaerobic chamber
  • Anaerobic
  • Sterilize


  • How can you tell if bacteria are living somewhere?
  • What is a bacteria colony?
  • Why should bacteria from yogurt be grown in anaerobic conditions?
  • What are some species of bacteria that you have seen listed on yogurt containers?

Antibiotic Treatment

Antibiotics are medications that can destroy bacteria in your body. Besides killing the harmful bacteria that cause infections, antibiotics also kill good bacteria present in your intestines. These helpful bacteria, also referred as your normal flora, promote intestinal health, produce vitamins and hormones and prevent the growth of bad bacteria found in your intestines. Taking antibiotics can disturb the balance between these two types of bacteria and lead to overgrowth of the bad bacteria.

Other Considerations

Professional medical guidelines neither recommend yogurt nor its avoidance for heartburn, as there isn't enough evidence to prove harm or benefit. Keeping a heartburn diary can help you determine how yogurt affects you. Change one food in your diet at a time and note its effects on your symptoms. When trying yogurt, note the kind consumed. Because food isn't the only factor that can contribute to acid reflux, also consider heartburn triggers such as certain medicines, smoking, overeating and lying down too soon after eating. If your heartburn continues to be a problem, consult your doctor since frequent heartburn can damage your esophagus and cause other health problems.

The basic recipe for yogurt involves adding two strains of live bacterial cultures -- Lactobacillus bulgaricus and Streptococcus thermophilus -- to warm pasteurized milk, according to the U.S. Food and Drug Administration. The bacterial cultures ferment the milk and cause lactic acid production, the source of yogurt's uniquely tangy taste. To this basic product, manufacturers may add sugar, fruit, honey, artificial sweeteners or other ingredients, as well as additional strains of bacteria.

TCBY Nutrition Guide

Probiotics, often called good bacteria because of their beneficial qualities, exist in the intestinal tract. Many supplements and foods such as:

  • yogurt
  • buttermilk
  • miso
  • tempeh
  • kim chi
  • some pickles
  • sauerkraut
  • some soy drinks also contain probiotics

Several studies have suggested that the probiotics in yogurt, including Activia, may help to reduce the undesirable bacteria in the digestive tract that cause infection or inflammation. Probiotics may also replace good bacteria that have been killed by taking antibiotics.

Diversity is one of the keys to a healthy gut

Not only was I lacking in variety, but the gangs of bacteria that were hanging out in my guts were less than savoury. My report revealed 65 times more Clostridium perfringens than average, and 211 times more E. coli – both associated with gastrointestinal disease.

“These results indicate you have a very unhealthy microbiome,” stated my report.

Now, I could make the excuse that I’d just been on a work trip and had probably eaten something dodgy. But according to Spector it would be unusual to tip the balance quite that far from a one-off infection.

But what about my good bacteria? Fewer than 100 species of bacteria cause infectious disease. The thousands of types of microbes found in our gut are, as the author Douglas Adams would say, ‘mostly harmless’. So how did I fare on the good guys?

Top of the ‘most desirable’ list are microbes like Akkermansia and the tongue-tying Christensenellaceae. Both are associated with protection against weight gain. Methanobrevibacter helps squeeze more calories out of food, meaning you can eat less. Oxalobacter helps to prevent the formation of kidney stones.

How many of these beneficial bacteria did I have? Zero.

Not only was I dumped at the bottom of the class – my guts had been served a detention. And they weren’t allowed out until they took a long hard look at their behaviour and decided to change.

Clostridium perfringens is associated with gastrointestinal disease (Credit: SPL)

So what can I do to improve my microbiome? Variety is the key, apparently, and diversifying your diet helps diversify your bacteria.

Why eating yogurt may help lessen the risk of breast cancer

One of the causes of breast cancer may be inflammation triggered by harmful bacteria say researchers.

Scientists say their idea- as yet unproven -- is supported by the available evidence, which is that bacterial induced inflammation is linked to cancer.

The paper in the journal Medical Hypotheses is by Lancaster University medical student Auday Marwaha, Professor Jim Morris from the University Hospitals of Morecambe Bay NHS Trust and Dr Rachael Rigby from Lancaster University's Faculty of Health and Medicine.

The researchers say that: "There is a simple, inexpensive potential preventive remedy which is for women to consume natural yoghurt on a daily basis."

Yoghurt contains beneficial lactose fermenting bacteria commonly found in milk, similar to the bacteria -- or microflora- found in the breasts of mothers who have breastfed.

Dr Rigby said: "We now know that breast milk is not sterile and that lactation alters the microflora of the breast.

"Lactose fermenting bacteria are commonly found in milk and are likely to occupy the breast ducts of women during lactation and for an unknown period after lactation."

Their suggestion is that this lactose fermenting bacteria in the breast is protective because each year of breast feeding reduces the risk of breast cancer by 4.3%.

Several other studies have shown that the consumption of yoghurt is associated with a reduction in the risk of breast cancer, which the researchers suggest may be due to the displacement of harmful bacteria by beneficial bacteria.

There are approximately 10 billion bacterial cells in the human body and while most are harmless, some bacteria create toxins which trigger inflammation in the body.

Chronic inflammation destroys the harmful germs but it also damages the body. One of the most common inflammatory conditions is gum disease or periodontitis which has already been linked to oral, oesophageal, colonic, pancreatic, prostatic and breast cancer.

The researchers conclude that: "The stem cells which divide to replenish the lining of the breast ducts are influenced by the microflora, and certain components of the microflora have been shown in other organs, such as the colon and stomach, to increase the risk of cancer development.

"Therefore a similar scenario is likely to be occurring in the breast, whereby resident microflora impact on stem cell division and influence cancer risk."

Fermented foods: the science of sauerkraut

Fermented foods are very much “en vogue” at the moment. Some are dubbing the sector a “mega trend in the making”. But biochemically speaking, fermentation is a metabolic process where microorganisms like yeast turn sugars in foods into alcohols and acids. That’s how we make wine and beer of course. The same processes can produce delicious and nutritious foods too - lactic acid bacteria breaking down the sugars in milk will make cheese and yoghurt, for example. And sauerkraut - a tangy german side dish made from fermented cabbage, is also made from lactic acid bacteria. Eva Higginbotham’s been trying her hand at making some.

Eva - I love to cook, but I’ve never fermented anything myself before, and so sauerkraut, a relatively quick fermentation process that requires only 2 ingredients - cabbage, and salt, seemed like a good place to start.

So, I lightly rinsed the outside of my cabbage, chopped it up thinly, and then added some salt in a big bowl. Then, I massaged the salt into the chopped cabbage, and after a few minutes of TLC the cabbage became a bit limp and released a lot of water. I grabbed handfuls of my wet cabbage and loaded it into a clean jam jar before pouring over the remaining salty liquid in the bowl. Finally, I took the core of the cabbage and used it as a weight to hold the cabbage under the salty, cabbage-ey brine, before I covered the jar with a clean paper towel and rubber band to leave at room temperature to ferment.

I’d been following a few different recipes I found online, but, being a Naked Scientist, I wanted to understand what the science was behind the different steps. So I called up University of Cambridge chemist Ljiljana Fruk…The first thing is that I had to wash everything that was going to be touching the cabbage really carefully. Why is that?

Ljiljana - Because on every surface that we have and use there are microorganisms living, and some of them might be really beneficial to the microorganisms involved in fermentation and others could be having destructive action, which means that you need to wash as many microorganisms off of your equipment, so that the microorganisms that you use in fermentation come mainly from the cabbage leaf.

Eva - The first thing I had to do was chop the cabbage very thinly, and then I added salt to a big bowl. What does the salt do? And does it matter how much you add?

Ljiljana - Salt is very beneficial for us, but if we take too much of it we will ultimately die. And so it happens to microorganisms as well. So a certain amount of salt will be damaging to the bad microorganisms, which you don't want to have in fermentation, but it will be tolerated by bacteria, which you need for fermentation of the cabbage. But the balance is very fine. So lactic acid bacteria, which you need for the cabbage fermentation, will tolerate up to 8% of salt for weight, which is good. But if you add a little bit more, you will kill it as well so you will not get anything. So by carefully controlling the amount of the salt, you are promoting the health and the growth of fermentation bacteria, but you are removing the microorganisms that might cause it to rot, or they might cause some other effects, which you don't want to have. So you are just allowing the salt to act as a kind of distinguisher between the bad and the good.

Eva - When I put the cabbage into the jar I then had to pour the brine - so all the salty liquid that had come out of the cabbage - on top of the chopped up cabbage to make sure it was all covered. Why is it important to make sure the chopped up cabbage is covered in brine?

Ljiljana - It's important because you are preventing the access of oxygen closer to the areas of fermentation. So fermentation will happen within the leaf and on the leaf of the cabbage. So you want to keep this area, where fermentation is happening, oxygen free and rich in salt. And so you need to make sure that you're covering the area of your cabbage so the oxygen from the air cannot get in.

Eva - The other thing is, from what I saw, I had to use a breathable cloth to cover the cabbage. I'm wondering why I should need to use a breathable cloth if we're trying to keep oxygen out?

Ljiljana - In the process of fermentation you might have different processes, chemical reactions happening. And in some of those reactions you produce gasses. And one of the gasses is carbon dioxide, and you wouldn't like to accumulate the carbon dioxide in a large amount within the whole fermentation vessel because carbon dioxide could dissolve a little bit in the liquid and it could increase the acidity of the whole system. And then again, if you increase the acidity, this might be detrimental to some of the microorganisms.

Eva - What can I expect to see through the glass? Are there going to be any visual changes?

Ljiljana - Yeah. So you might see that the liquid which you might have noticed becomes a little bit turbid, which is not as transparent. And you might see the change in the texture of the cabbage. If you see something black or brown, probably it's about the time to remove everything and throw it away. But as long as you just see slight color changes in terms of the liquid, or maybe changes in the texture of the leaf, that should be still okay.

Eva - Most importantly, what should I plan on eating it with?

Ljiljana - Well, depends if you are looking for the vegetarian version or non-vegetarian version! But what I like the most is if you ferment your cabbage correctly, it can have such a wonderful flavour. And you can just add a little bit of garlic to it, maybe a tiny bit of pepper, and have a wonderful salad. I love it. Particularly this time of the year to give me a boost of C vitamin as well.

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