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Yogurt has a long and interesting history which has only recently included commercial manufacturing. For most of its existence it has been made by those who are consuming it. There are a wide variety of heritage yogurts which have been cultured for hundreds of years coming from many parts of the world.
Yogurt can be made safely at home by maintaining a clean environment, using fresh milk and using an active starter. When making yogurt with store bought yogurt always use fresh yogurt. If a heritage yogurt starter is used ensure the conditions for the starter are met to ensure success.
Commercial yogurt is made with a few isolated bacterial strains of lactic acid bacteria which are easy to control and provide repeatable results making for a consistent product. The drawback of this is that the culture is unstable and can be overwhelmed by unwanted bacteria and yeast.
Heritage starters are made up of many bacteria species and are very stable. The culture is affected by the environment with the fluctuation of the population of the various species found in the culture. This variability allows the culture to protect itself from unwanted microorganisms making heritage cultures very robust.
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Is my homemade yogurt safe to eat?
While store bought yogurt can get moldy in a matter of a week or two in the fridge homemade yogurt tends to have a longer shelf life. The difference is how they are made and the ingredients of each type.
Commercial yogurt is made up of only a few bacterial species with additives frequently included meant to improve consistency, maintain structure during transport and to add flavor or texture to the yogurt. These additives are usually not digestible by the lactic acid bacteria but can only be used for food by molds and other fungi. This makes it susceptible to spoilage. The higher pH of commercial yogurt also makes it more susceptible to unwanted microorganism growth.
Homemade yogurt made from powdered starters or fresh commercial yogurt can be consumed safely for 2 to 3 weeks, watch for mold growth on the surface. Heritage Yogurt has a longer shelf life as the wide variety of microorganisms which make up the culture resist contamination.
Yogurt made at home is perfectly safe to eat if it was made from fresh milk, inoculated with a healthy starter and incubated within the proper temperature range. There are some things which can happen during the fermentation which cause it to be different from store bought yogurt. None of the following differences are indications of spoilage.
Clear liquid on the surface
Yogurt with clear liquid on the surface is perfectly fine to consume. For an in depth explanation of what it is and why it forms read this post but in short the liquid is called whey and is the water soluble part of the milk which was not included in the gelling milk as it formed yogurt. It is full of protein, vitamins and minerals.
Whey is what most dairy based protein powders are made from. It is collected mostly from cheese making, filtered and freeze dried. For a more detailed explanation of how this happens read “Is the Clear Liquid on Top of Yogurt Edible“
Grainy texture
Although grainy yogurt may seem strange, it will not hurt you if you consume it. Grainy yogurt can form if the yogurt is incubated too long, at too high a temperature or with milk which has been scorched.
Sometimes when old milk is used which is close to going off the pH of the milk is dangerously close to curdling. When yogurt is made it is pasteurized prior to inoculation which kills off any unwanted bacteria but because the milk is close to curdling the addition of the acidifying bacteria from the culture causes the proteins in the milk to associate strongly with each other forming the grainy texture.
The grainy texture is formed by the proteins in the milk becoming tightly bonded together expelling whey and forming small dense structures. Grainy yogurt can be used for blended drinks, sauces or dips without worry and if the texture does not bother you you can still consume it with a spoon.
Curdled milk surrounded by clear liquid
The usual cause of this is over fermentation. Lactic acid bacteria can produce enough lactic acid to lower the pH of the milk below 4.0. This produces a strong bond between the various proteins found in the milk and expels the water soluble whey from the solids. This produces a curdled milk product which is very sour.
Although it is still edible most people find the tangy flavor too much to consume normally. Instead it can be used to make tangy cream cheese, spicy salsa dip or vinegar free salad dressing.
Stringy or ropey yogurt
Stringy or ropey yogurt is produced when certain lactic acid bacteria in the culture get over populated. These bacterial species are found in most heritage starters and are usually kept in check by the other bacteria in the culture but sometimes the environmental conditions are just right for their growth.
If this happens you can still consume the yogurt by using it in baking goods, smoothies or blended cream soups. You can continue using the culture which has made the stringy/ropey batch being careful to control the environment. If it continues to produce stringy/ropey yogurt, get some new starter for your next batch.
Really sour yogurt
Sour yogurt is not something which most consumers are familiar with as store bought yogurt rarely gets overly sour before it begins to grow mold around its edges. This is because yogurt manufacturers will ferment the milk for the shortest time possible and then add additives. These additives are often starches which lactic acid bacteria cannot use for fuel. Starches can only be broken down into shorter chain sugars by mold, not something you want in yogurt.
Homemade yogurt when made with heritage starters will continue to acidify the milk until there is little sugar left. This makes for very sour yogurt which can last a very long time. This type of yogurt can be used to make tangy cream cheese, savory dips and sauces and used in baking.
How do you know if homemade yogurt is bad?
Like any other food yogurt can go off and should not be consumed. Although it is rare to get food poisoning from yogurt it can become home to unwanted spoilage bacteria and yeast which make it unsuitable to eat.
Yogurt which smells different, has fuzzy growths on the container or surface of the yogurt should be thrown out. These are signs of spoilage bacteria getting a foothold in the yogurt. Once such bacteria have a foothold they continue to spread throughout the yogurt.
Fuzzy growth on the top of the jar
Any colored fuzzy growth on the surface of a container of yogurt is an indication of mold growth. Mold has a life cycle which includes four stages, spore, germ, hypha, mature mycelium. The first two of these stages are done before there is any visible indication. Fuzzy growth is an indication that the mold has reached a mature stage with extensive growth of mycelium throughout the yogurt.
Some people scrape the mold off the surface and consume the yogurt on the bottom but this is not recommended as mold sends an extensive network of threads down into its food source to extract food energy. These threads go very deep and cannot be seen. Some molds produce toxins which are unhealthy to consume it is best to just throw the batch out and start again.
Green spots on the surface
Green spots found on yogurt shows the mold has progressed to the hypha stage of growth. The mold has begun to send threads down into the yogurt to extract food energy. This will taint the flavor of the yogurt as well as can cause sickness. Throw it out and start again.
Can you get botulism from homemade yogurt?
Botulism is a form of food poisoning caused by Clostridium botulinum. This bacterium is found in all environments throughout the world where it is found in the soils, on fruits and vegetables and on most surfaces. When Clostridium botulinum is provided with an environment which favors its growth it can produce large amounts of toxin which can be lethal.
When making yogurt the conditions for the overgrowth of Clostridium botulinum are absent which makes it impossible for it to produce enough toxin to be harmful to humans. It cannot survive in acidic conditions below a pH of 4.5 and have many competitors in the culture during the short acidification period of the yogurt.
Botulism became prevalent when methods of canning began to take off. Canning relies on the destruction of all bacteria and spores which can grow in the canned food. Since Clostridium botulinum has spores which are very hardy it can survive when most other bacteria cannot. If this happens it can grow without any competitors and will produce ample amounts of toxin which if consumed can kill.
The culture which produces yogurt can easily outcompete Clostridium botulinum. This eliminates the chance that it can produce enough toxin to cause any ill effects.
Is homemade yogurt better for you than store bought?
Homemade yogurt does have many benefits which store bought yogurt does not have but to make such a broad statement would be wrong. Yogurt of any type has been shown to have beneficial effects for most people. Whether one is better than the other depends on several things.
Homemade yogurt has more or less lactose, fat and protein content depending on the milk used, incubation time and type of starter. This is beneficial for those who cannot have regular yogurt. Store bought yogurt is consistent, has the same vitamins, minerals and pre/probiotics and is conveniently available.
This means that if you are lactose intolerant then homemade yogurt will be better for you as you can control the amount of lactose left in the milk by lengthening the incubation time. If you are on a high fat low carb diet then homemade yogurt can be made with high fat milk producing a thick custard like yogurt which can be used for sauces and dips (eliminating the need for thickeners).
On the other hand if you are short on time or inclination homemade yogurt is not a good option as it does take commitment. In that case it is best to consume store bought yogurt as any yogurt is better than none.
Store bought yogurt is made in a very controlled environment using only a few lactic acid bacterial species. This can be beneficial to those who are sensitive to fungi because homemade yogurt cultures may have yeasts in them (a simple form of fungus) which can aggravate fungi sensitive people.