Juices contain microorganism, some of the microorganisms, which might be dangerous, change their structure when their optimum environment changes to increase their ability to survive in a harsh environment, in our case the heat of the boiling juice, turning themselves into spores. If someone drinks the juice that contains those protected cells it would be easy for them to surpass the digestive enzymes, the gastric acids, and the immune system. Our intestines might be an optimum environment for their growth which would elicit them to germinate. This might negatively affect our microbiome, as they could compete with some of the beneficial microorganisms in our body. Long heat exposure until evaporating about 66% of the juice volume could lead to destroying them directly or indirectly which makes the boiled juice unharmful to drink/eat.
Can Boiled or Long-Term Stored Grape Juice have Adverse Effect on Health?