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Anyone who has been making their own yogurt for a while will eventually have the experience of the milk not thickening no matter how long it is incubated. When this happens to you it is not readily apparent why the milk does not thicken into the usual sweet tangy yogurt you have made so many times before.
Milk becomes yogurt when its pH drops below 4.6 where it begins to gel. There are a number of reasons milk does not reach this pH such as: weak starter, contaminated milk or equipment, improper incubation temperature and poor milk choices.
When you first start making yogurt you are usually very particular about following the steps outlined in the recipe. You wash all utensils, heat the milk to the correct temperature and incubate the milk under close scrutiny. Once a routine has been developed often the careful attention to the details begin to slide and you end up with a failed batch.
To prevent this and learn what you did wrong take some notes about what you did when making the failed batch and any changes you noticed of prior batches. This will give you some good clues regarding what you need to change in your system to prevent failure again.
Here are some of the most common reasons why your milk does not gel into yogurt.
Why a weak yogurt starter can lead to yogurt not thickening
The process of making yogurt requires the action of live lactic acid bacteria using the lactose in the milk for energy and producing lactic acid as a byproduct of their respiration. It is the lactic acid which lowers the pH of the milk which leads to the thickening of the milk into yogurt.
When the starter is not strong enough to dominate the milk during the incubation process unwanted bacteria and yeast strains may dominate the culture. Some of these unwanted bacteria produce an environment which is basic rather than acidic, preventing the milk from thickening.
Three reasons why a yogurt starter can become weak
- Too few lactic acid bacteria strains
Commercially made yogurt and direct set yogurt starters only contain a few (up to four) lactic acid bacteria strains in any measurable quantity. Some only contain two which makes it especially susceptible to contamination.
When making yogurt using commercial yogurt as a starter or direct set powdered yogurt packets it is recommended that you only make one or two batches using the backslopping method of yogurt inoculation. With so few lactic acid bacteria strains in the starter the chance of growth of unwanted bacteria is higher. This is because every microorganism has a niche in nature which it grows best.
Without a wide variety of lactic acid bacteria niches are left in the milk which the present lactic acid bacteria are not adapted for, be it cooler, exposure to oxygen or less agitation, whatever the reason this gives unwanted bacteria, mold and yeast an opportunity to get a foothold in the milk leading to a weak starter.
- Too much time between new batches
Yogurt is a dynamic food, the lactic acid bacteria in homemade yogurt will continue to consume the lactose in the milk lowering the pH of the milk. Unfortunately not all lactic acid bacteria are as acid tolerant as others. This leads to die off of some strains of lactic acid bacteria which reduces the viable lactic acid bacteria species which weakens the starter.
When using commercial yogurt or direct set yogurt packets don’t use the yogurt to start another batch once your yogurt is over two weeks old as the few lactic acid bacteria species can deteriorate quickly. If you are using heirloom starter the culture is much more robust but still needs to be fed fresh milk regularly to maintain a strong culture.
- Not enough sugar to feed the lactic acid bacteria
Lactic acid bacteria need sugar to grow. Lactose is the sugar in milk which the yogurt starter will break down into two smaller sugars called glucose and galactose. It then uses these two simple sugars in their respiration. If there is not enough lactose in the milk the yogurt starter can become weakened and susceptible to contamination.
Not all milk has the same amount of sugar in it. High fat milk is lower in sugar than skim milk and sheep and goat milk have less sugar than cow’s milk. If you like thick yogurt made with heavy cream use a fresh starter from commercial yogurt or a new direct set packet. When using a heirloom yogurt starter, alternate between high and low sugar content milk to ensure the lactic acid bacteria are fed properly. A recipe for making yogurt from high fat milk can be found here.
Why contaminated milk can lead to yogurt not thickening
Milk which is old or has been left at room temperature for a long period of time can become contaminated with unwanted bacteria and mold even if it has not been opened. Pasteurization is meant to control the number of spoilage bacteria in commercial milk, not sterilize the milk. As the milk ages or is placed in a warm environment the microorganisms left in the milk will begin to multiply.
As spoilage microorganisms grow they consume sugar in the milk leaving less for the lactic acid bacteria to use for respiration and can produce a basic environment. This means lactic acid bacteria in the starter have less fuel to acidify the milk to a pH of 4.6 where it will begin to gel.
Contaminated milk will introduce unwanted bacteria to a weak culture with few lactic acid bacteria strains in it. This is why using commercial yogurt or direct set packets as yogurt starters only work a few times reliably. Without the wide variety of bacterial strains in the culture it leaves opportunity for spoilage bacteria to get a foothold and eventually overtake the starter culture.
Heirloom yogurt is much more robust, preventing spoilage bacteria from getting a foothold in the culture but the use of contaminated milk can affect the flavor of the yogurt.
Why contaminated equipment can lead to yogurt not thickening
Using unclean equipment is another error which can creep in once you get familiar with making yogurt. With several successful batches under your belt it is easy to start to take shortcuts such as not washing the jars as thoroughly as you should or using utensils straight out of the drawer.
Using contaminated equipment can lead to the introduction of unwanted bacteria through food residue, unclean hands or contact with contaminated surfaces. These bacteria consume milk sugar and make the milk more basic, preventing lactic acid bacteria from acidifying the milk properly.
When making yogurt it is important to keep your equipment clean and in good order. Dispose of any equipment which are damaged or have scratches or grooves which can harbor unwanted microorganisms. Plastic containers are easily scratched during cleaning so before using such containers inspect them. Glass is easier to clean but avoid using metal scrubbers as they can scratch the surface of the glass.
Prior to making yogurt, wash all your equipment in hot soapy water, rinse with clean running water and let air dry. It is not necessary to try to sanitize your equipment with high heat or chemical means as your starter should be strong enough to prevent small numbers of unwanted bacteria from getting a foothold in your yogurt. This is especially true if you are using a heirloom yogurt starter as they are very robust and self protecting.
Why incorrect incubation temperature can lead to yogurt not thickening
The ambient temperature affects the growth of all microorganisms. They each have a specific temperature which they grow best in, this includes lactic acid bacteria. If your incubation temperature is either too hot or too cold it will affect the growth rate of the starter and can sometimes prevent its growth enough to allow other bacteria, yeast or mold to get a foothold in the starter.
Incorrect incubation temperature can kill the starter above 120℉ (48℃) or inhibit their growth below 100℉ (38℃). Either way the lactic acid bacteria growth will be inhibited allowing unwanted bacteria to grow. When incubating yogurt keep the temperature between 108-115℉ (42-46℃) for best results.
Some yeasts grow well in cool temperatures such as the yeast Saccharomyces pastorianus with a temperature range between (6-12℃). This yeast is used to produce lager commercially but is often found in the environment. Since yeast can grow on any simple sugar, milk which is not heated properly can become the medium for yeasts with similar temperature ranges.
Mold is also pervasive in the environment and can grow in all types of temperature ranges from hot to cool. They usually grow best in warm temperatures but some species of mold grow in cool temperatures. Mold can also utilize complex carbohydrates for energy unlike bacteria and yeast, making it difficult to eliminate from the environment.
Other bacterial species also have a different temperature range, some of which are considered spoilage and even pathogenic. These bacteria can produce a basic environment which can inhibit the growth of lactic acid bacteria further reducing the thickening ability of the lactic acid bacteria.
Why poor milk choices can lead to yogurt not thickening
For milk to thicken into yogurt it needs to have protein which will associate with one another to form a loose matrix. This loose matrix encoumpases all four componetes of the milk, lactose, water, fat and protein. If the milk choice you make does not have a good balance of these components the thickening power of the lactic acid bacteria is limited.
Poor milk choice reduces yogurt thickness by altering the balance of milk components. Enough sugar is needed to acidify the milk, adequate fat makes the yogurt creamy but too much prevents thickening, water thins out yogurt and the presence of protein provides the thickening power.
Why sugar is needed to thicken yogurt
Sugar comes in three forms: monosaccharides, disaccharides and complex carbohydrates. Monosaccharides come in three forms: glucose, fructose and galactose. These three sugars are the basic building blocks of other forms of sugar and starch.
The sugar in milk is called lactose and is made up of one molecule of glucose and one molecule of galactose. Lactic acid bacteria have the ability to break down the lactose in the milk into its component parts and use the simple sugars as fuel. As a result they produce lactic acid which acts on the protein in the milk causing it to form weak bonds with one another, thickening the yogurt.
Without enough sugar in the milk the lactic acid bacteria does not have enough fuel to produce the acid to thicken the yogurt. For yogurt to thicken it needs to have a pH of at least 4.6 and the amount of acid needed to lower the pH of the milk can vary depending on its original pH.
Milk choices like lactose free, nut and seed “milk” and any grain based milk products will either have too much sugar (due to added sugar) or not enough.
The effect fat content has on yogurt thickness
The amount of fat in the milk influences the texture of the yogurt. Low fat milk such as skim milk or 2% produces yogurt with a harder less creamy texture whereas high fat milk produces a creamy yogurt with less structure.
The fat in yogurt gives it its creamy texture by providing a barrier between the protein molecules preventing them from associating too closely together. This distance keeps the proteins from forming curds and forcing the water out of the solution.
How the amount of water in the milk changes its thickness
Water makes up about 87% of milk volume so it is an important component of the milk, making up the majority of the volume. It provides the yogurt with its soft and malleable texture which makes it so popular.
Too much water in the milk will make yogurt thin and runny, which is one of the reasons why it is recommended that the milk is heated to 180℉ (82℃) for 10-20 minutes prior to inoculation (for more reasons read this article). The long pasteurization reduces the amount of water in the milk, thickening the yogurt.
Why the correct protein balance makes the best yogurt
It is the protein in the milk which thickens it into yogurt. The protein molecules associate with each other forming a weak matrix which increases in strength as the milk acidifies. This is apparent if you incubate your yogurt for an extended period of time (for more information about the effects of long incubation check out this article).
Too much protein will give it a grainy texture as the proteins have less distance between them making it easier to associate strongly with one another and too little protein will make thin yogurt with very long weak bonds producing a thinner yogurt.
When choosing what type of milk you use to make yogurt keep a log which you can refer to when making future batches. Once you have made yogurt with a few different types of milk you will be able to refer back in your note which one made the best. This will give you a good idea as to which milk to use in the future.