Hard Cider causing burning stomach sensation

First thoughts: During fermentation, lots of un-tasty yeast by-products are created (including fusel alcohols). Giving the fermentation time to finish allows the yeast to re-process these by-products, but you're interrupting this by cold-crashing early.

Second thought: 100 grams of sugar is enough to prime 20 litres / 5 gallons of cider. Your question states a batch size "5 litres ± 1 gal" (1 to 9 litres), so 100 grams of sugar for this is quite a lot. Maybe your cider is very gassy.

Final thought: After one day of fermentation (and a chill-time of 24 hours), the ferment will have a lot of super active yeast. Perhaps this yeast is re-fermenting once it warms up inside you.

So, what to do:

I always try to make the best possible drink. So let's do a walk-through of just that:

• Just use enough apple juice, leave the sugar out.
  • Store-bought is OK, but make sure there's no preservatives.
• Use a minimum 0.6 grams of yeast per litre
• Use a purpose-bred yeast if at all possible
  • And don't automatically reach for champagne yeast, it's too dry (IMHO) for cider
• Use bread-making yeast rather than wild yeast.
• Clean and sanitise your equipment.
• Bring your juice to 16-22°C (60-70°F)
• Add the juice roughly to get some air into it, (this adds some oxygen for the yeast).
• Add the yeast
• Keep the ferment in the 16-22°C range until complete.
• Use a hydrometer to measure the remaining sugar-density
  • OR ... Wait 2 weeks
  • Typically cider will finish around 0.999 (from a starting gravity of 1.040)
• Bottle the cider with 5 grams per litre of priming sugar, into PET bottles.
  • Wait 2 weeks.
• Back-sweeten in the glass with apple juice, to taste.

With some ciders I experience the same burning sensation after a few ciders. I haven't figured out the exact causes yet but I will share a few theories worth testing:

1) Acidity. Some apples are much higher acidity than others. Also some yeasts will produce a higher acid product than others. And as Kingsley also alluded to, carbon dioxide is also acidic so if the cider is highly carbonated, this could be a problem for some people.

2) Sulfites. If your cider is treated with sulfites before and/or after fermentation, be aware that some people are sensitive to it or claim an allergic type reaction to it. This is why commercial ciders and wines will notify whether or not they have used sulfites.

3) Alcohols. The alcohol itself could be causing a problem. And depending on the juice source, if there is haze or starches present, you could have trace amounts other alcohols besides ethanol including methanol or other higher alcohols that your body is telling you it doesn't like.

4) Wild yeast. I believe this is unlikely, however if your source juice was not treated to get rid of any wild organisms that might be present, these beasts could theoretically survive in the finished cider and could potentially wreak havoc to your digestive system. I have my doubts with this one, as I believe alcohol will kill most wild organisms, however like I said this might deserve more testing on a case-by-case basis if the juice is not pasteurized or treated with sulfites.

Theories worth testing. I have not tasted your cider so it is difficult to pinpoint which of these is closer to the right one.