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Yogurt has a slightly tangy flavor accompanied by sweetness from the leftover sugar in the milk so when you add it to a bowl of fruit or on top of cereal it provides a nice tangy addition to the flavor. But sometimes it can be overly tangy, making your mouth pucker and totally over power the other flavors in a recipe.
Homemade yogurt can become sour by a long fermentation time, warm temperatures and over inoculation. These conditions encourage lactic acid bacterial growth which use more milk sugar producing lactic acid. The reduction of sugar with a corresponding increase in lactic acid makes sour yogurt.
Any one of these conditions can have a great effect on how sour your yogurt will get. It takes a while to perfect your yogurt making technique so that you get a yogurt which you like. For some it is a slightly sweet yogurt with lots of remaining lactose and for others it will be a sour yogurt with little lactose and high amounts of lactic acid.
How fermentation time can make sour yogurt
Yogurt is a living fermentation which continues even after it has been refrigerated. The lactic acid bacteria which make up the culture uses the lactose in the milk as food energy and produces lactic acid.
As the lactic acid level increases the proteins in the milk gel with other components on the milk and the yogurt thickens. This happens once the milk has reached a pH of 4.6. Once the milk has thickened it can be considered yogurt and refrigerated to slow the fermentation down. This results in a sweet tangy yogurt which is high in milk sugar (lactose) as the lactic acid bacteria have not had a chance to consume all the sugar in the milk.
As fermentation time increases, lactic acid bacteria consume lactose in milk and produce lactic acid. Lactic acid bacteria can rapidly consume large amounts of lactose in a short period of time and left long enough most lactose will be used to make lactic acid making for sour yogurt.
There are other affects time has on your yogurt beyond the pH of the milk. To learn more about the full effects of incubation time checkout this article.
How temperature affects how sour yogurt will become
Another factor which affects the growth of lactic acid bacteria is temperature. Warmer temperatures encourage the rapid growth of lactic acid bacteria. This rapid growth uses a lot of energy which is supplied by the lactose in the milk.
With an optimal growth temperature range for yogurt is 108℉ to 115℉ (42℃ – 45℃). When you ferment your yogurt at the high side of the temperature range it encourages the bacteria to grow exponentially and can convert high amounts of lactose to lactic acid in a short period of time.
Outside of this range the lactic acid bacteria will have problems multiplying. A cooler temperature will slow its growth which can lead to unwanted microorganism growth causing off flavors and spoilage. Once the temperature surpasses the 115℉ (45℃) the lactic acid bacteria will begin to die off leaving room for higher temperature tolerant microorganisms to grow. These include yeasts and some molds.
How over inoculation affects how sour yogurt will become
It is the lactic acid bacteria which are the star players in yogurt production. In the other two conditions it was the growth of the bacterial colony which was discussed. With more time the colony can grow larger and in warmer conditions the colony multiplies faster.
Using a large amount of starter culture, whether it is fresh store bought yogurt, powdered yogurt starter or a heirloom yogurt culture, shortens the fermentation time. The high amount of active lactic acid bacteria consumes the lactose quickly which sours the yogurt.
Here you are just starting with a large colony to begin with! It does not need to grow rapidly or have extra time to increase in numbers. When lactic acid bacteria are put in an environment which is conducive to growth it will multiply quickly and consume large amounts of sugar in the process. With a large initial colony this multiplication progresses quickly, essentially jump-starting the fermentation process.
Other reasons why homemade yogurt can get overly sour
Using milk with a high sugar content
Not all milk has the same amount of lactose in it. Skim milk has a higher lactose content than homogenized milk. Lactose is the sugar which the yogurt culture uses to acidify the milk. The more sugar in the milk the higher amount of lactic acid the culture can make.
To make a highly sour yogurt use low fat milk and incubate a bit longer. This will give the lactic acid bacteria time to acidify more of the lactose and produce more lactic acid as a result.
Milk was too old
Milk which has been around for a while will have increasing amounts of bacterial species growing in it. Pasteurization at the source of milk production helps to control the amount of spoilage bacteria in the milk (including several species of lactic acid bacteria) but it does not eliminate them entirely.
As a result in time they begin to multiply and consume the lactose in the milk. This makes the milk pre-acidified before you even start the yogurt making process and as a result can produce overly sour yogurt.
Several conditions combined
It may not be that you are fermenting your yogurt overly long or at too high a temperature or with too much starter. It may be a combination of all three. Small changes in these three conditions favoring the microbial growth can drastically change the acidity level or the yogurt.
How to prevent homemade yogurt from becoming too sour
Keep a log
If you are consistently getting yogurt which is too sour for you then start keeping a log which records the three main players in the game: temperature, time and inoculation volume. When you have a log you will be able to adjust one of these three conditions and gage the result. If it is still not to your liking you can adjust other factors.
Once you have a yogurt you like it will be easy to reproduce regularly and if you get a batch which is not what you want you can tell which of the three conditions changed.
Milk was heated too long or at too high a temperature
Heating the milk before making it into yogurt is an important step for making thick flavorful yogurt. It affects the proteins in a positive way enabling them to react with one another better but it also reduces the pH of the milk (makes it more acidic).
The acidity level of the yogurt is one of the conditions which your taste buds register as sourness. By heating the milk too long you pre-acidify the milk a little which can be distinguishable from milk which has been heated a shorter time or at a lower temperature.
Automate the process
When most people start making yogurt they start with a jar and some type of makeshift incubation chamber. These are varied from ovens with the pilot light on to coolers with hot water bottles but all these makeshift chambers are hard to control and temperature can vary from batch to batch.
To prevent this, invest in a yogurt maker or temperature controlled incubation chamber. These are great for controlling the temperature which tends to be the hardest variable to control.
Yogurt makers come in all shapes and sizes from simple insulated chambers to plugin appliances which control the temperature over a long period of time. They can be found at kitchen supply stores or online.
Other methods of automation include having dedicated tools for making yogurt such as measuring cups for the amount of milk and starter, the container(s) for fermenting the yogurt and mixing utensils for ensuring the starter is evenly mixed.
Benefits of making sour yogurt
For some people sour yogurt is the end goal. Making sour yogurt ensures a low amount of lactose in the yogurt which is good for those who are lactose intolerant. It also keeps longer, makes better tangy greek yogurt and is great for savory recipes.
Sensitivity to lactose
Some people have a hard time digesting the milk sugar found in dairy products, even commercially fermented milk products like yogurt or sour cream but if the milk is fermented for a long period of time the amount of lactose left in the milk is small enough to allow them to digest it with no problem. To learn more about low lactose yogurt checkout this article here.
Yogurt of this type is regularly fermented for 24 hours or longer. This gives the lactic acid bacteria enough time to reduce the lactose content in the milk enough to make it digestible but it also sours the yogurt much more than normal yogurt.
To help the sour taste use higher fat milk. The high fat content in the milk tends to smooth out the sour flavor giving the yogurt a more creamy texture and buttery flavor which masks some of the sour flavor in the long fermented yogurt.
Yogurt for non-refrigerated conditions
Before refrigeration was available ambient environmental temperatures caused milk to spoil quickly. Making yogurt was a method of milk preservation. Lactic acid bacteria acidified the milk which protected it from spoilage, producing an environment not conducive for spoilage microbial growth.
Today taste trumps longevity and we prefer a sweeter product. In fact we often add sugar to the final product with fruit on the bottom yogurt or vanilla flavoring with added sugar. Yogurt which has a low pH and low sugar levels will last longer than yogurt which is sweeter with a higher pH. It is the acidity level and lower amount of easily consumable sugar which increases the shelf life of the yogurt.
Quick Bread baking
Quick bread baking uses a chemical reaction to create the rising effect. The reaction of the rising agent like baking powder or baking soda with the acid in the liquid part of the batter produces CO2 which gets caught in the thick batter causing it to become fluffier.
When you use sour yogurt as a replacement for the milk in the recipe there is more acid available to react with the baking soda or powder. This is the same effect you get when using buttermilk for buttermilk pancakes.