Influential Factors In Yoghurt Making

Introduction

Choice of Milk

The final firmness of yoghurt is heavily influenced by the concentration of milk solids, especially protein. Virtually any type of mammalian milk (and some plant milks) can be used, as all those bacteria need is a source of lactose (or other carbohydrates will do). However, the most luxurious result will be achieved with some lovely fat-containing grass-fed milk. In order to make a high-protein, lower-fat, Skyr-like yoghurt, use skimmed milk and then strain it. This will concentrate the proteins.

Homogenized Milk

Homogenized milk tends to give a smoother consistency and a more stable product as it prevents fat separation during fermentation. This can result in a layer of grease on top of the yoghurt, although this can be removed by skimming after heating. Homogenization enhances formation of cross-structures, which can improve texture, although this is not necessarily noticeable to the home yoghurt maker.

UHT Milk

Although the fermentation community tends to be more inclined towards fresh produce (and rightly so), UHT milk can be used, if that is all that is available. As UHT milk is homogenized and has been heat-treated, the proteins are already denatured, and it actually makes decent yoghurt without having to be heated first. Sometimes, home yoghurt makers add dried milk powder, which further increases the protein content and makes the resulting yoghurt firmer.

Raw Milk

Yoghurt can be made from raw milk without heating it. The end product will have a runnier consistency – the proteins will not have been denatured, so coagulation will not be as efficient – but it will retain its raw nutritional properties. Use the freshest raw milk you can lay your hands on – bacterial counts in raw milk can be extremely high, which will provide too much competition for the starter, and could result in some ‘off’ flavours. If you are using a starter that you have made yourself, or you have purchased an heirloom culture, remember to keep some of this separate, as unknown microbes in the raw milk could permanently alter its balance. Of course, you can heat/denature, which effectively pasteurizes it, for improved texture.

Choosing a Starter Culture

LAB need to be present to sour the milk to make yoghurt and there are a few ways to achieve this.

Shop-Bought Yoghurt Starter

Pouring smooth, stretchy cheese curds into a metal container

Common practice these days is to purchase a favourite live yoghurt and use a spoonful of it to ‘seed’ your own batch. The probiotic potential may be increased, for a wider range of microbes in your finished product, by combining starters from different commercial yoghurts.

You can usually use a spoonful of your own resulting batch of yogurt three or four successive times before it stops working. The reason it ‘wears out’ is because commercial strains of LAB are selected to be good at working in very specific factory conditions; they might not replicate with such vigour in a home environment.

Shop-bought yoghurt, with Bifidobacterium sp., Streptococcus thermophilus and Lactobacillus bulgaricus.

Single-Use Sachets of Yoghurt Cultures

Sachets of yoghurt starter culture that contains a particular combination of strains are available to buy. These are usually labelled as ‘single-use’, but you will probably be able to get successive batches from one sachet.

Yoghurt cultures look like skimmed milk powder – they almost are, but they also contain dried microbes that can be easily reconstituted.

Heirloom Cultures

Successful continuous yoghurt production requires an heirloom culture. These are largely uncharacterized communities of microbes that work together, and have evolved together – rather like kefir grains without the grain. Because they occur naturally and have developed a symbiotic rhythm, they can keep going indefinitely, as long as you look after them: they are quite demanding and ideally need to be propagated weekly, to keep the microbes alive. A neat trick is to arrest their growth by popping them in the freezer – put spoonfuls into ice-cube trays.

When making yoghurt, add the culture at about 2.5–5%; too much of a starting inoculum can lead to rapid acid production, which can adversely affect the formation of the curd.

A Starter From a Friend

If you know someone who has an heirloom culture up and running, why not ask for a spoonful? Just like kefir grains or sourdough starter, a yoghurt starter can easily be donated.

Your Own Starter From Scratch

It is perfectly possible (and really exciting) to make your own yoghurt starter from natural sources, although this is not without risk, as you will not be sure what microbes you are growing. The yoghurt pH of 4.6 and temperature of 46 degrees should, however, be suitable for the growth of thermophilic LAB.

Making a yoghurt starter with (front to back) chilli stems, lentils (control), dogwood and, at the back, shop yoghurt.

All of the following are said to contain the right sort of LAB to make yoghurt:

  • Cornus mas (dogwood) stems;
  • chilli stems;
  • ants’ eggs;
  • goat droppings.

Dogwood and chilli stems can both be used to make yoghurt starters. The method is simple: place chilli stems and/or sections of dogwood stem into heated and cooled milk then incubate just as you would normal yoghurt. After 4 hours, you may already have achieved a set. In my own experiment, the flavours were interestingly different: the chilli stem starter was much more flavoursome than the dogwood. I tried just a quarter of a teaspoon in the first instance to check that there were no ill effects for 24 hours, before scaling up and offering it to the family.

Individual jam jars used to make set yoghurt in an Instantpot. Alternatively, it could have been made en masse for straining. You may also read about drying.

Making Thermophilic Yoghurt

Equipment

Elegant breakfast arrangement with milk and fruit bowl

Making your own yoghurt is very cost-effective. You can easily make a couple of litres at a time, which will keep successfully in the fridge for at least a couple of weeks, if not more. If you have a yoghurt maker, simply follow the instructions that come with the machine. As with any other branch of fermentation, a lack of gadgetry should not hold you back, although life will be easier with a thermometer, which you should be able to pick up for less than a fiver.

A thermometer really does make life easier.

When making thermophilic yoghurt (heat-requiring), to keep the milk and microbes warm you could use a large thermos flask. Alternatively, use any lidded container or wide-mouthed bottle wrapped in a towel and kept in an insulated cool box or an airing cupboard, although it might take a few hours longer to set this way. You can also put your oven on its lowest heat setting.

The equipment you have will help you decide what vessels to use for your yoghurt. If you are using a yoghurt maker, it is decided for you; if using an ‘instant pot’, you can either make it in one large volume, or fill individual jars so it sets in the jar.

‘Scalding’ the Milk

The scalding is the stage of yoghurt preparation that is most likely to put people off, as it does involve an element of pot-watching. There are four reasons for heating the milk to almost boiling temperature:

  1. Even if it has been previously pasteurized (usually at 72°C for 15 seconds), milk will still contain spores and Paenibacillus sp. A higher/longer heat treatment will reduce their numbers so they will not interfere with the yoghurt-making process.
  2. The high temperature causes denaturation of the whey proteins (90% denaturation gives optimal yoghurt texture) and increases their cross-linking.
  3. Holding the milk at just below boiling for 30 minutes causes evaporation, increasing the proportion of milk solids and improving the texture, although only if it is heated in an open pan. Although it is not essential, stirring prevents a skin forming and can contribute to the formation of a silky-smooth yoghurt.
  4. Scalding the milk removes dissolved oxygen that can affect the performance of the starter cultures.

Despite all these reasons, if you are going to strain your yoghurt to make it thicker anyway, and are prepared to compromise a little on texture, this step is not essential for pasteurized milk or UHT.

You can choose to scald your milk either fast and short or slow and long; both methods will work. An impatient person might use 95°C for 5 minutes, but many commercial machines do a 30-minute 85-degree heating. Generally, the shorter higher temperature tends to be more successful in terms of a firmer curd. If you heat for longer, or boil the milk, it is not a disaster, but you might be able to taste the difference in the finished product.26

Cooling the Milk

Rapid cooling of heated milk to inoculation temperature may help to give a firmer set, although there are divided opinions on this. Perhaps it is to do with reduced dissociation of whey proteins as they cool rapidly. This can be achieved by standing the scalded milk pan in a sink of cold water and whisking. first thing first, read up on drying and dehydrating.

Incubation Temperature and Time

A high incubation temperature speeds up the process of yoghurt formation but results in a weaker structure and greater whey separation (syneresis), which is what happens when liquid gets excluded from the gel structure. A lower incubation temperature results in slow protein aggregation, with more cross-protein bonds and small pores. This is better for trapping liquid, but is slower. Some people start off at a higher temperature of about 45°C to allow the yoghurt bacteria to get going, and then allow this to fall to around 37 degrees. Many commercial yoghurt makers will include this stage in their set programmes. Most people settle for around 42 degrees for about 8 hours.

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