I have developed this technique over about a year, it is absolutely insane and no reasonable human would go through the trouble I do to make syrup. In this guide there are several optional shortcuts that will make the entire process much easier. However, if you do choose to go for the maximum effort version, you will be rewarded with a delicious, intense, honey-like syrup that tastes like fresh fruit.
Obtain fruit juice. (I would recommend a minimum of 2L, but preferably more like 4-5L for a worthwhile yield of syrup.)
The easy option is of course to buy fruit juice. I have done this technique with store-bought apple juice, and orange juice. I have also done this technique with a mixture of store-bought and fresh juice. Obviously slightly better results would come from juice that you could buy freshly pressed, but that might be expensive.
The harder version is of course to make your own fruit juice. There are a few primary advantages to this. Firstly, it allows you to make a syrup out of a fruit that may not be commonly available as a store-bought juice. Secondly the juice will of course be much fresher than anything you can buy in a store, although completely avoiding oxidation is essentially impossible. Be careful if adding anything like ascorbic acid/vitamin C to your fruit during juicing, since it will be significantly concentrated in the following steps.
Clarify your fruit juice. Clarifying is an important step in this procedure, it will remove pectin as well as any very small particles of sediment/pulp. Removing pectin is important because after concentration we want a thick, but free-flowing, syrup. Pectin can also inhibit some flavors and clarified juices tend to taste brighter. For this you will need some form of pectinase enzyme at a minimum, for citrus such as grapefruits you may also need additional clarification agents. With grapefruit in particular there are some additional steps you can take to ensure that the juice is not overly bitter. For more information please check out Dave Arnold’s website for information on agar clarification Also note that for certain fruits such as apples, it may be helpful to use a pectinase before juicing to increase your yields. But afterward you will still need to filter/separate your juice.
The easiest way to actually filter/separate your clarified juice from the particulates is time. After adding pectin and in the other clarification agents, simply let your juice sit at room temperature for several hours or in the fridge overnight, and skim off clear juice from the top. If foam develops, you may need a syringe or similar device to extract clear juice from the middle of your container.
A significantly faster option is the previously mentioned agar clarification, which is not only highly recommended for something like grapefruit juice but can also speed up the process with any juice. The downside of course is that it requires agar, a food grade nylon filtering bag (or something similar), and may result in a lower yield.
Now the easiest option in terms of effort is actually the most difficult option in terms of cost and equipment. Clarifying juice is extremely easy if you happen to have access to a centrifuge such as the Spinzall. If you do happen to have access to one, it might be easiest to combine juicing and clarification into a single process. Especially for soft fruits such as berries.
Adjust the sugar content of your juice. You will need a hydrometer or a refractometer to determine the sugar content a.k.a. the Brix of your clarified juice. I then recommend you add some amount of plain white sugar, as this will increase your overall yield. The easiest way to determine how much sugar to add is with an online calculator used in homemade wine making.
The next step is concentration so you can make that significantly easier if you add a good amount of sugar. Theoretically you could add enough sugar for your juice to be 50 Brix, then you have simple syrup that could remain in the fridge and be done.
My preference is to increase the sugar content of the juice to 20 Brix. This slightly increases my yields since most juices are between 10-15 Brix, but it also makes it easy to calculate my final yield after concentration.
Concentrate your juice mixture. The degree of concentration will entirely depend on how much sugar you added, and what your target concentration is. For something that will last several weeks in the fridge, and be readily usable for cocktails, I would go for 50 Brix as a minimum. My preference for a final concentration is roughly 80 Brix, which will behave more like honey. I’m not a food safety expert and I encourage you to do your own research, but in my experience, syrups at 80+ Brix are very shelfstable.
The easiest way to concentrate your syrup is of course boiling it down. Once you have a mixture that reaches 108-112°C, you should be at between 80-85% sugar based on candy making procedures. If your target concentration is lower than that, it may be difficult to determine when you are done based on temperature. While relatively fast, the disadvantage of this process is that it introduces cooked fruit flavors. Depending on your planned uses, this might not be an issue. As an additional precaution for preservation, you can put the syrup resulting from boiling through a standard jam/jellies canning procedure.
My preference for evaporation takes significantly longer. I generally evaporate my syrup slowly using sous vide. The fruit juice mixture is in an open tray/basin resting inside a larger basin with the water and immersion circulator. I usually have my circulator set to 62°C, although depending on your setup it may need to be a bit warmer. You want the mixture itself to be at 60°C minimum, this will prevent unwanted fermentation or spoilage. At this temperature the mixture evaporates very slowly. Going from a 20 Brix all the way to 80 Brix takes around 24-36 hours total. The advantage of this technique is that the final syrup has a flavor much closer to the fresh fruit juice, rather than a cooked jam or preserve. Of course if you’re not super concerned about preserving a fresh flavor, you can also evaporate sous vide but at a slightly higher temperature.
Enjoy your fruit syrup! Depending on what choices you made you should have a decent yield. When going from 20 Brix to 80 Brix, you should get 250g of final syrup for every 1 kg of starting mixture. If your target concentration and added sugar are different, you will need to recalculate your estimated final product. I find measuring the mass of the final syrup is a good way to determine when I’m done, since my sugar measuring instruments don’t work at high concentrations.