They said two, three or four reheats can push brewed coffee toward greater bitterness and acidity as chlorogenic acids break down into caffeic and quinic acids.
Doctors say people with reflux, dyspepsia or sensitive stomachs may see worse symptoms because caffeine can relax the lower oesophageal sphincter and harsher compounds may stimulate gastric secretion.
The problem is repeated heating rather than microwaves alone; experts suggest brewing smaller batches, using insulated tumblers, avoiding coffee on an empty stomach and reheating only once briefly.
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