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So you forgot about your yogurt and it has been in the incubator for longer than you usually ferment it for. You wonder if it is still good or will I get sick or worse from eating it, how long is too long to leave yogurt in an incubator or should I just throw it out and start again.
The longer yogurt ferments the less lactose remains and the lower the pH the yogurt will have. This has the effect of making yogurt taste sour and may cause it to separate between curds and whey. Unless there is mold growth or other signs of spoilage it is safe to consume over fermented yogurt.
As the yogurt incubates the pH of the yogurt continues to drop making the yogurt more and more sour. For two more reasons why your yogurt is sour read Three Resons Why Your Homemade Yogurt is Sour. There you will learn what you can do to adjust your yogurt making to get exactly what you want.
How long is too long to incubate Yogurt
This is an interesting question which requires a broader answer than X number of hours is okay but once you get to Y number of hours it is ruined. Yogurt is produced by a living active culture of various bacterial species mostly made up of lactic acid bacteria. They consume the lactose in the milk as food and produce lactic acid. If you want to know more about lactose free yogurt check this article here.
After 48 hours the yogurt will be too tart to eat plain. The longer the yogurt incubates the less lactose is left in the milk and more lactic acid which makes the yogurt increasingly sour. After 48 hours it will be difficult to keep the yogurt from separating due to the low pH.
Depending on the purpose of the yogurt the ideal incubation time will vary. For an indepth explanation and step-by-step instructions of how to make yogurt using three differnet methods read “Three Simple Ways to Make Homemade Yogurt“.
Incubation time for sweet creamy yogurt
6-8 hours
This yogurt is great for fruit toppings, smoothies or just to eat with a spoon. The lactic acid bacteria has had a chance to lower the pH to below 4.6 where the proteins in the milk begin to form a loose matrix causing it to gel. Most of the liquid in the milk remains associated with the protein so has a soft creamy consistency.
Incubation time for yogurt with lower lactose content
12-24 hours
Yogurt made for a lower lactose content needs to be incubated for a longer period of time. This allows the lactic acid bacteria to consume more of the lactose in the milk making it easier to digest for those who are lactose intolerant. The resulting yogurt will be less sweet and quite sour as there will be less sugar and more lactic acid in the yogurt.
This is the incubation time recommended for those who are lactose intolerant. Although at this point there is still lactose in the yogurt it is drastically reduced. Checkout this article if you want to learn how to make low or no lactose yogurt.
Incubation time for unrefrigerated conditions
24-36 hours
The longer the milk is incubated the less lactose and more lactic acid. Lactic acid bacteria are acidophilic which means they grow best in acidic environments. This makes them very tolerant to the low pH conditions of long incubated milk.
As the milk continues to acidify the proteins begin to disassociate with whey in the milk, a process called Syneresis. This causes it to separate into curds and whey. By draining this whey off the yogurt becomes thicker, more like cheese which will last longer in unrefrigerated conditions.
There are several ways to make super thick yogurt besides the draining off of the whey (the basis for Greek yogurt). Check this article out for many other options
36-48 hour yogurt and beyond
36-48 hour yogurt is highly acidic with little lactose left in it for the lactic acid bacteria to consume. This makes for very tart yogurt which will be hard to keep from separating. It is great for making tangy cheese spread, cheesecake and gravies.
It is hard to keep the yogurt from separating at this point as the proteins in the milk will have a strong attraction to each other forming dense clumps which expel the liquid whey. You can either use the yogurt as is by stirring it or blending it into smoothies, gravies or dips or you can drain off the whey by hanging it.
Effect of Incubation Time on Yogurt Thickness
From the time you add the yogurt culture to the cooled pasteurized milk it will begin to acidify. It is this acidification which is responsible for the change in thickness of the yogurt.
The structure of milk protein is made up of three forms of milk caseins, one of which is found on the surface of milk micelles. Micelles are structures which give a solution solubility. Once the pH drops to 4.6 the surface caseins lose their ability to repel one another causing the milk to gel.
The longer the milk is incubated the lower the pH and the stronger the bonds get between the micelles. The structures will continue to grow forming a matrix throughout the milk producing yogurt. Once the milk has thickened it will continue to form stranger and stronger bonds until the liquid in the milk can no longer remain in suspension. When this happens it dissociates itself with the solids and the yogurt separates.
The length of time the yogurt incubates affect its thickness to a point. Once it reaches the stage where the liquid can no longer remain in suspension it will break down.
Effect of Incubation Time on Yogurts Tartness
The tartness of yogurt is affected by two things. The amount of lactose and the amount of lactic acid in the yogurt. The less lactose and the more lactic acid the tarter the yogurt will be.
Incubation time affects the tartness of yogurt by allowing the lactic acid bacteria more or less time to consume the lactose and produce lactic acid. In the first stages of incubation the lactic acid bacteria grow quickly which consumes much of the lactose producing lactic acid. This will lower the pH of the yogurt from 7 to 4.6 in 4-8 hours depending on the temperature and amount of culture used. Once the amount of lactose in the milk drops below the amount needed to support the lactic acid culture their population will begin to fall. This slows the rate the yogurt acidifies taking it longer to lower the pH further.
Effect of Incubation Time on Yogurt Culture
Yogurt cultures have a wide variety of lactic acid bacterial species in them. Commercial yogurt is made up of two main types: Lactobacillus bulgaricus and Streptococcus thermophilus but heirloom style yogurts have a vast number of species which makeup the consortium.
Each species has different energy needs and will grow at different rates depending on the temperature, pH and availability of food. Each species of LAB has a different doubling time which is the amount of time it takes for the bacteria to double its population. This affects the composition of the culture to favor the fast growing bacteria over the slower ones while there is food availability.
Once the pH begins to drop the most acid tolerant species begin to have the upper hand and can continue to multiply where others are hindered in their growth. Long incubated yogurt gives the best chance for all species to be able to influence the composition of the yogurt.