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Once you have been making yogurt for a while you will probably have a batch or two which don’t seem quite right. Your yogurt may have gelled but it seems slimy or stringy. It holds together in long strings and sticks to the spoon.
Slimy yogurt contains excessive biofilms produced by the lactic acid bacteria in the starter culture. Biofilms are normally produced to protect bacteria from environmental damage but overproduction can happen if the temperature is lower than 47℃, the milk too sweet or the pH remains above 6.0.
The challenge with making yogurt is that it is a living culture which changes as its environment changes. When yogurt does not turn out as expected the first assumption is that the culture has been contaminated and/or the yogurt has gone bad. This is untrue as the various bacterial species found in yogurt culture have the capability to produce high amounts of biofilm (it is how they protect themselves from environmental conditions as well as infection).
If you take the advice of some uninformed “experts” not only will you throw out yogurt which can be used but you will not be able to learn the real reason why it has gone slimy on you. The environmental conditions can have as much influence on yogurt’s texture and characteristics as the makeup of the culture.
What to do to prevent slimy, stringy or ropey yogurt
The three things which make the greatest difference in the consistency of yogurt are the temperature of incubation, the makeup of the milk substrate and the rate the pH drops. Any one of these can lead to one or more of the lactic acid bacteria to overgrow and produce a higher than normal amount of biofilm.
How temperature influences the texture of yogurt
The temperature of the milk at inoculation time is an important part of making yogurt. There are two different types of yogurt cultures on the market:
Mesophilic
Mesophilic yogurt culture is made up of lactic acid bacteria which are capable of growing rapidly in milk at room temperature. There are many different types of yogurt cultures from all over the world which have been developed through history. Some types of mesophilic cultured milk products are meant to be highly stringy or ropey such as viili which is said to have a viscous texture (it doesn’t pour well) and holds together.
Thermophilic
Thermophilic yogurt requires temperatures above normal ambient temperatures to grow rapidly and without additional heat the culture will find it hard to produce good textured yogurt. It will take a longer time and the results will not be guaranteed.
If the temperature of the milk is not up to the correct temperature at the start of the inoculation then the species of lactic acid bacteria which are more active in lower temperatures will get a jump on the others. Oftentimes it is these species which can produce high amounts of biofilm.
To prevent this from happening, bring the temperature of the milk to the proper incubation temperature by either warming it in a warm water bath or pasteurizing it (which has other benefits). Once the milk is within the correct incubation range add the starter and stir.
How the makeup of the milk affects the texture of yogurt
Milk comes in a wide variety of types from low fat to whipping cream, powdered, canned and ultra-pasteurized. It is usually categorized by its fat content but when it comes to making yogurt the fat content is only half the story.
Lactic acid bacteria do not use fat as a food source, instead they use simple sugar like lactose, glucose, galactose and sometimes fructose. Lactose is the sugar which is found in milk and is made up of two molecules of sugar stuck together, glucose and galactose.
As the amount of fat in the milk decreases the sugar content increases. This increases the potential amount of lactic acid bacteria in the yogurt, which in turn increases the amount of potential biofilm which can be produced.
The addition of dehydrated milk (which is often done commercially) to the yogurt culture is done to improve the texture of the yogurt and it can have a positive benefit to the texture of the end product but it can also cause excessive biofilm production due to the high amount of sugar in the powder.
To prevent your yogurt from going slimy due to excessive sugar content use a higher fat content milk. This will slow the growth of the lactic acid bacteria a little and produce a thicker creamier yogurt.
How the pH of the milk affects the texture of yogurt
The lactic acid bacteria responsible for making the most biofilm and therefore producing a slimy texture are most active in a higher pH. It has been found that to produce the most biofilm a pH of about 6.0 is the sweet spot. The longer your milk is above this pH the stringier or ropey it will be.
The main reasons why it may take longer for the pH to drop are too low a temperature which was discussed above and not using enough starter culture. The starter culture inoculates the milk with specific bacteria strains which immediately begin using the lactose in the milk to grow.
Lactic acid bacteria produce the biofilm as a method of protection from environmental damage. Damage can come from pathogens growing in the same medium, since most unwanted microorganisms found in milk grow best in basic environments and work to maintain a basic pH. The longer the milk’s pH stays high the more biofilm the lactic acid bacteria will produce to protect itself.
To ensure this does not happen make sure you use enough starter to inoculate the milk. Some homemade yogurt makers like using very small amounts of starter to improve texture and to a certain degree this can work in some cases but with too little starter the lactic acid bacteria can struggle to acidify the milk fast enough
Use at least a tablespoon of culture for every cup of milk to ensure it will be able to quickly lower the pH and prevent the slimy, stringy or ropey texture.
Is it safe to eat slimy, stringy or ropey yogurt?
Yogurt with a slimy texture may not have the best feel in the mouth but it is not going to harm you. The slimy texture is not harmful although it can be unpleasant to use for a topping or to simply eat with a spoon.
Yogurt which is slimy, stringy or ropey is high in healthy biofilms. It helps probiotics to survive the stomach, can coat surfaces displacing pathogenic bacterial biofilm and helps colonization of probiotic bacteria by occupying vacant space allowing bacteria growth.
Biofilms produced by lactic acid bacteria are currently being studied for use in protecting industrial food processing equipment from contamination from pathogenic bacteria with positive results. The biofilms can adhere to surfaces which are occupied by pathogenic bacteria and displace them providing a healthier environment for the food to be processed.
They are also being used to help keep cows udders healthy from antibiotic resistant pathogens which affects the health of the milk. These studies are important for the scientific community to distinguish between good and bad biofilms. As long as your milk is fresh, your culture is robust and you work in a clean environment a batch of slimy yogurt will be perfectly fine for consumption.
What can I do with slimy, stringy or ropey yogurt?
Yogurt with a slimy texture may be fine and healthy for consumption but it may not be a pleasant experience. Especially if you are used to store bought yogurt which has been produced to have a consistent texture with the use of thickeners such as corn starch or guar gum. Homemade yogurt is not subject to the strict environmental controls which the dairy industry has to follow (for good reasons).
When you get a batch of yogurt which has a poor texture rather than throwing it away you can use it for:
Morning smoothie
The morning smoothie is a regular part of my morning. It includes a wide variety of ingredients but is great for using yogurt with poor texture. The blender does a great job of blending the ingredients together and it is impossible to tell if the yogurt had a slimy texture to it in any way.
Vegetable dip
Vegetable dip benefits from yogurt with a sticky texture like that of slimy, stingy or ropey yogurt. The biofilm is meant to stick to surfaces to enable the lactic acid bacteria to colonize areas. This sticky texture helps the dip to stick to the vegetable when dipped into the dip.
Quick bread
Quick bread includes such baked goods like biscuits, pancakes and muffins. All of which benefit from a viscous texture which helps to trap CO2, which makes for fluffier baking. Also the acidic nature of the yogurt gives the rising agent an added boost.
Yeasted bread
This is a great tip for those who like sourdough bread but can’t be bothered with culturing and caring for a starter. By adding a little yogurt to the dough the bread takes on a more sourdough flavor. The lactic acid bacteria in the yogurt grow quickly when mixed with the flour and a little water which produces ample CO2.
Use normal bakers yeast which is basically Saccharomyces cerevisiae (the yeast first isolated in wine production). The added lactic acid bacteria compete a little with the yeast and produces unique flavors reminiscent of sourdough bread.
Cream soups
Cream soups are usually made with fresh cream which is full of lactose, something which many people have a problem digesting. Avoid this by using yogurt which has been fermented for at least 24 hours. Yogurt with an unpleasant texture can be used for making cream soups by replacing the cream with yogurt and lowering the acidic ingredients.
Marinades
Marinades are usually a mixture of acidic ingredients with thickeners which help the marinade to stick to the meat. The thickener could be sugar, flour or fat. The acid helps the flavors in the marinade to penetrate the meat while the thickeners provide a way to keep the spices in contact with the meat.
The addition of slimy yogurt adds both the thickener and the acid. Yogurt with a sticky texture adheres to the meat well eliminating the need for carbohydrate filled thickeners. The low pH of properly fermented yogurt (about 4.0 depending on the incubation time) (internal link to 24 hour yogurt) adds the needed acid. All you need to add is the flavors and you are good to go.
Soft cheese
Soft cheese is easily made with yogurt by hanging it in a clean cloth above a bowl for 12-24 hours. The longer the yogurt hangs the firmer the cheese will become. Using yogurt with poor texture makes a good cream cheese for making icing, cheesecake or cheese dips.
Creamy icing
Creamy icing is a popular topping for carrot cake, muffins or any other sweet baked good. It is usually made with cream cheese which provides the thick texture to the icing. Yogurt can be used to make cream cheese yogurt in a two step process.
- Hang the yogurt to make yogurt cream cheese
- Make the icing with the thickened yogurt cream cheese
Using yogurt with an unpleasant texture helps the icing stick to the baking and adds a creamier texture to the icing which remains soft longer than normal cream cheese icing.
Gravy
Gravy is thickened with some type of starch which produces long strings of gluten. The gluten mixes with the juices in the bottom of the pan. Gluten forms stringy chains of carbohydrates which stick to the food it is poured on.
Slimy yogurt is full of biofilm which is designed to stick to the sides of surfaces to help the lactic acid bacteria to colonize a new area. This ability also helps gravy made with yogurt stick to your food. It thickens the gravy without the addition of carbohydrates.