Always read the label

The juices we are most interested in are grape (for wine) and apple (for cider). These are available in most countries. Unfortunately, the presentation (packaging, labelling) varies considerably, to conform to shoppers' expectation and also to comply with local consumer-protection regulations. We need to decipher the label to extract two vital pieces of information: Does the juice contain preservatives? How much sugar is present?

Preservatives first: Some manufacturers make a virtue of stating 'no preservatives' or words to that effect. This is helpful, but there is no legal obligation to make that statement. After all, you wouldn't expect to see a claim of 'no cyanide' just because none had been added. Also, 'preservatives' might be listed as 'stabiliser' or 'anti-oxidant', or it might say 'contains sulphites'. All are to be avoided as they will inhibit fermentation. Sometimes you will see 'with added vitamin C'. This does no harm though it is unnecessary. Ideally, you are looking for pure fruit juice with nothing added. Most juices are diluted concentrates and will have been pasteurised to stabilise them. We can live with that.

Sugar content: This information should be available but sometimes has to be decoded from the label. In Europe and the Middle East it is usually presented as grams per 100 ml (so multiply by 10 to convert to the standard g/l). In America, they often quote grams per serving, where a serving is 8 fluid ounces = 240 ml (so multiply by 4.2 for g/l). To further complicate matters, sometimes the label only quotes 'total carbohydrates' in grams /100 ml. In which case, it is safe to assume that 90 to 95% of the total carbs are sugar.

Also on the label: Some naturally unsweet juices like cranberry or grapefruit might contain artificial sweeteners. Though these will not prevent fermentation of the real sugars (including any you might  add), they do not themselves ferment and would still be present in the finished wine or cider where you might find them rather intrusive.