Sure enough, the citric acid is rapidly being consumed as the pH has shifted to 3.7 from 3.5 in just 24 hours.That's about 10 of the 35 grams I added.
I needed another 200 grams of sugar to bring the og to the desired 1090, but have run out. Since I now have 5 litres of red gjc, I decided to use that instead, to maintain the balance of grape and sugar. 300 g gjc is about the equivalent of 200 g of sugar. The pH of the gjc is 3.2. Adding it to the brew has put the pH back to 3.5, and also added another £2 to the total cost which is now £35, the same as a Cellar 7 kit. It also strongly suggests a very low proportion of gjc in the kit. Theoretically, it may be possible to assess the proportion of gjc to sugar /water/glucose syrup by measuring the acidity of the mixture, before diluting. The water and sugar dilutes the acid, so half the acidity of pure gjc means a 50/50 mix, assuming that no extra acid has been added. This should be done by titration, since, having diluted a sample of my pure gjc with 3 parts water, the pH remained unchanged at 3.2! My gjc has a reading of 24 ppt (as sulphuric). If diluted with 4 parts water, this would be under 5. Therefore a similarly diluted sample of the kit concentrate with a reading of 2 ppt would have less than 50% gjc. This Cantina kit has no added acid. Judging by the amount of acid required to bring it up to it's present 5 ppt, it seems the gjc content is less than 50%. Another way of measuring is density. A Cellar 7 kit also weighs 7 kg, but when made up to 21 litres, the sg is 1090, whereas the Cantina is only 1078, which means it's concentrate contains a higher proportion of water.
The sg is now 1070. Taking account of the extra sugars, this is a drop of 20, which suggests the claimed 4 day fermentation may well be valid if starting at the original 1078.