Learn the science behind:
As a people, we seem to have managed to make alcoholic drinks from all sorts of produce. Grains are transformed into beer, or spirits (e.g. whiskey). Grapes are transformed into wine, pears into perry, and apples, into cider. Making an alcoholic drink was a great way to preserve produce (and have something safer to drink than contaminated water), by now, we mostly drink them for enjoyment.
All these processes are quite similar. They start with a way to free up the sugars (aka the food for the yeast). Next, the yeasts are ‘released’ to do their job and ‘eat’ those sugars, converting them into alcohol. We’ve discussed beer-making quite extensively before, but hadn’t delved into the more fruity drinks. Since we had an ample supply of apple juice at hand, we focused these experiments on this delicious fruit. Hence it’s time to dig into the science of apple cider!
- What is (hard) apple cider?
- It starts with apples
- Making apple juice
- Fermentation of the apple juice
- Second fermentation
- See & Test the process
- Further reading
What is (hard) apple cider?
Before diving in, let’s get some terminology sorted. As is so often the case for foods and drinks, naming conventions for apple cider differ around the world. Overall, there are two types of drinks that we’ll be discussing here:
- The juice made from an apple. This contains the sugars from the apple, it might be pasteurized or filtered, but has not otherwise been transformed. Here, we’ll refer to this as apple juice, however, some of you might refer to this drink as apple cider!
- Fermented juice from an apple, contains alcohol. When you let cider yeasts grow and thrive in the juice from the apple they’ll form alcohol (ethanol). Here in this post we’ll be referring to it as apple cider. Some of you though might refer to this as hard cider.
It starts with apples
To make apple cider: all you truly need are apples and yeast (which might already be on your apples). Don’t underestimate the importance of apples. Since the whole drink will rely on what’s in those apples, choosing a suitable apple is important (the same is true by the way when making apple pie).
When looking for cider apples, cider makers will consider the following factors:
Sweetness (aka sugar content)
For apple juice to be converted into apple cider, yeasts need to convert sugar into alcohol (and flavors). The sugar in your apples will all be converted into alcohol during fermentation. If the sugar content of your apples is very high, you might end up with too much alcohol in your cider.
Cider makers will measure the sugar content of the apple juice by measuring the specific gravity of the juice. Beer brewers use a similar method. An apple juice with a specific gravity of 1.040 is considered to be low on the sugar content scale, one with a specific gravity of 1.075 is very high.
The specific gravity of a liquid compares the density of that liquid to another liquid. It can also be called relative density. In our case the reference is water. Since apple juice contains sugar, the density of the juice is higher than that of water.Read more here
For a well-balanced drink you don’t just want to taste the alcohol. The acidity of an apple adds some ‘zing’, some sourness and freshness to the drink. Too much acid though will make for an unappetizing drink.
The main acid in apples is malic acid. As such, the acidity of apples is often expressed as total acid (TA) content. The main acid at play here is malic acid. As such, 1% acidity refers to 10g of malic acid per liter of apple juice.
Some references might refer to % of tartaric acid. This is also an acid, however, is more commonly used in wine making. These two numbers can be converted into one another.
Apple juices suitable for cider making tend to have an acidity in between 4,5-7,5%. The acidity of apple juices can vary more widely though, between as low as 1g/l or as high as 15g/l.
Tannins are a group of molecules that taste bitter and astringent. They provide some depth and interest to a drink. They are also what gives red wine their ‘body’.
Tannin can be determined using a simple chemical analysis. A low tanning apple will contain <1g/l, high tannin content apples contain around 4 g/l.
There are apple varieties that are specifically grown for making into cider. These so-called cider apples tend to be more tart and bitter because of their higher levels of tannins than regular apples. Therefore, these apples don’t tend to taste good when eaten fresh. When you transform them into a cider though, you might be looking for some of those flavors to come through.
Apart from using just one apple variety, cider makers may also use blends of apples. Choosing a specific blend can give just those properties they are looking for that you can’t find in a single variety.
Making apple juice
Once you’ve chosen your apple, the first step of making it into a cider is to transform it into a juice. You only need the juice of the apple, not the seeds, skins, etc. Extracting that juice starts by grinding the apples. While grinding you break down cell walls. These cells held onto the water, so once they’re broken the juice flows out.
After grinding, the apples are pressed to break even more cells and free up even more juices. The resulting juice won’t be clear. Instead, it will be hazy with particles floating around. In some cases, cider makers will clarify the juice at this point but most do it at a later stage, or not at all.
If you’ve ever eaten an apple you will know that if you take a bite out of the apple, and leave it out for only a little while, it will turn brown. This is caused by enzymes that initiate enzymatic browning reactions. Apart from turning brown, apple juice is also very vulnerable to spoilage by microorganisms because of the high sugar content. Therefore, manufacturers will either use the juice directly or treat it to stabilize it.
What about the rest of the apple?
Apart from the juice, you’re left with apple remainders such as peel and seeds. These are called pomace. This is full of pectin and can be used for various other applications. However, it’s not required for making apple cider.
Did you know that the pectin in apples is very helpful in setting a thick jam? It’s why jam recipes may call for apples, the pectin helps create a creamier texture.
Fermentation of the apple juice
In order to convert the apple juice into cider, the apple juice has to ferment. During fermentation the sugars in the apple are converted into alcohol and carbon dioxide (and a range of flavor molecules). Fermentation is essential for any alcoholic drink, whisky, beer, etc all start out with fermentation.
Starting with a clean slate
In order for fermentation to occur the right types of yeast need to be present. Apples contain several microorganisms, including yeasts naturally. In some cases cider makers let these naturally present microorganisms do their work at this point.
However, in a lot of cases, these may not be the right type and not give the desired flavors. Therefore, cider makers may first want to make sure that no undesirable microorganisms are present (including ones that can make you sick, pathogenic ones).
There are various ways to do this, the juice can be filtered (to take out some of that sediment which tends to contain more microorganisms) or pasteurized but the most common method is to add some sulfur dioxide (SO2). Sulfur dioxide kills all microorganisms in the juice, allowing a cider maker to start with a clean slate. However, you will have to wait several hours before adding in your specific yeast to prevent it from being deactivated by the sulfur dioxide as well!
At this point, you are either ready for the natural yeasts to do their work or you can add your chosen yeasts to the apple juice. Choosing the right type of yeast can be pretty complex. The yeast needs to thrive in apple juice for one thing and needs to make the right flavor molecules that a cider maker is looking for. The type of yeast used will definitely impact the final result of your cider. There are various yeasts out there that have been developed specifically for making apple cider. Once fermentation starts the yeast converts the sugars in the juice into alcohol, making flavor molecules along the way.
Apart from the yeast, the temperature will have a great impact on the fermentation process. Higher temperatures result in faster fermentation for instance but will give different flavors than a slower fermentation. Generally, temperatures between 15-22°C are used.
The fermentation process will last several days to weeks. By the end of it, you have an alcoholic apple cider. If left to complete fermentation entirely, the apple cider will be flat, it won’t have bubbles upon opening.
After a while, the yeast starts to slow down since there is less food (sugar) available. The cider may be transferred to a new tank for the next fermentation round. During this step, not a lot of new alcohol will be formed but the flavor of the cider will continue to change. It isn’t uncommon for cider to be aged for several weeks in a tank.
As we mentioned earlier, a good amount of tannins is important for an apple cider to have a well-rounded flavor profile. A way to add more of those interesting flavors is by aging the apple cider in wooden barrels. Most cider makers will use oak barrels that have already been used for other drinks such as wine or whisky. These barrels contain a lot of flavor molecules from both the wood as well as the drink that was in there before so will really impact the flavor of your cider.
You can also use this second fermentation or aging step to add additional flavors. One option is to add hops, which are generally added to beer. Hops have a strong and complex flavor which will enrich the apple cider after a few days of soaking. Also, hop can help preserve a cider.
Another option is to add spices such as vanilla or cinnamon to the cider at this point. These flavors will slowly infuse into the cider.
You will also find ciders to which additional fruits (e.g. blueberries) have been added again at this point. Adding fruits though will result in more active fermentation again. Fruits contain new sugars so can revive the yeasts again! If a cider maker does not want this to happen, they can force the fermentation to stop first by adding chemicals that stop the yeast. They might do this for instance when adding back some apple juice if you want to sweeten up the cider slightly but not get more alcohol again.
If you want your cider to be carbonated, you’ll need new gas to form within the bottle. This is done by adding back a little bit of sugar into the apple cider upon bottling. The yeast will again start to ferment and produce carbon dioxide. Now, since the bottles are closed, the gas can’t escape and will remain within the liquid. If too much sugar is added though the bottle might explode because of too much gas build up.
Commerical cider makers might not ferment again in the bottle. Instead, they’ll have carbonation systems (such as used for making soda) with which they can carbonate the apple cider upon bottling.
Remember that the initial apple juice is often hazy to start with? Most apple ciders though are clear and have no particles floating around. That is because they are clarified after the fermentation process. Some of the particles have settled down to the bottom of the tank (sedimentation), so those are easy to remove. Especially after quite long fermentation times, a lot of the particles will have settled out already. In other cases, a centrifuge can be used to separate these particles or you can add ingredients that will gather these particles
See & Test the process
Angry Orchard, a brand of alcoholic cider has a nice video on their website showing all the most essential steps, starting with the apples, ending with a cider (scroll down below the recipe). If you learn best by doing so, give making your own apple cider a try!
American Home Brewer’s association, The balancing act: how to make hoppy cider, Milly Shamburger, link
D. Arthey , Fruit processing, 2012, Springer science & business media, link
Cider, Methods for Cider ‘Tannin’ Analysis, link
Cidersage, How to make hard cider, link
Cider school, Cider apples, link
GEA, Cider – An apple (drink) a day…, Nov-15, 2017, link
Home cider making, Back Sweetening Hard Cider for Bottling & Kegging, Nov-26, 2017, link
Claude Jolicoeur, Acidity and pH of apple juice, link
Jolicoeur, C., The new cider maker’s handbook, 2013, link
Maria Kosseva, VK Joshi, PS Panesar, Science and technology of fruit wine production, chapter 3 & 4, link
Lallemand, Technical overview on cider production, Apr-25, 2017, link
Jessica Shabatura, Making your cider, from: How to make hard cider, link
Ben Watson, Cider, Hard and sweet: history, tradition and making your own, 3rd edition, Chapter 3 & 4, link