Clarifying doubts for those who stockpile food Is reheating these 11 dishes really risky? After viral online warnings not to reheat mushrooms, chicken, eggs, and vegetables, Mahidol University's Institute of Nutrition provides clear answers and safe storage and reheating advice to prevent diarrhea.
In today’s fast-paced life, especially for working adults and condo residents, buying ready-made meals or cooking large portions to store in the fridge for 2-3 servings and reheating in the microwave has become the most convenient and time-saving option.
Recently, widely shared online information warned that "11 foods you must not reheat again as they risk producing toxins and causing cancer." This has caused concern for many. What is the truth? Is eating reheated food still safe?
Information clarified by Associate Professor Dr. Sitima Jittinand, Institute of Nutrition, Mahidol University, who categorizes the 11 foods in question into 4 groups to clarify from scientific and nutritional perspectives as follows:
ConcernsComplex proteins in mushrooms, chicken, and eggs supposedly transform when reheated into “egg toxin” or poisons causing stomach pain, digestive problems, and heart issues.
Answer This is not true. Reheating cooked protein-rich foods does not cause proteins to become toxic. Any stomach pain or diarrhea results from improper storage, such as leaving cooked food at room temperature too long or insufficient refrigeration, allowing microorganisms to grow and produce heat-resistant protein toxins, causing food poisoning when consumed.
ConcernsThese vegetables have high nitrate levels which, when repeatedly heated, release nitrites that could become carcinogenic.
Answer Partly true but not as dangerous as feared. Repeated heating softens and breaks down vegetable tissues (like in mixed vegetable stew), releasing substances including nitrate.
Theoretically, excessive nitrate intake converts to nitrites binding with red blood cells, causing acute oxygen deficiency (symptoms include bluish skin, fatigue, headache), but practically, typical vegetable intake per meal is insufficient to cause health harm. Moreover, fresh produce sold must meet standards limiting nitrate levels.
ConcernsUncooked rice contains bacterial spores that multiply after cooking and survive reheating. Potatoes left at room temperature can develop botulinum toxin.
AnswerCooking heat destroys disease-causing microbes, but spores and spoilage microbes require higher temperatures than household cooking to fully destroy. Thailand’s hot climate favors microbial growth. Thus, cooked rice or dishes should not be left at room temperature longer than 2 hours. Uneaten food must be refrigerated immediately to slow spore and microbe growth.
ConcernsRepeated frying turns oil into toxins causing disease.
Answer This is true. Repeated high-temperature frying degrades oil into "polar compounds" and carcinogens like PAHs (Polycyclic Aromatic Hydrocarbons), increasing heart and vascular disease risk.
Advice
At home, do not reuse frying oil more than twice. If oil darkens, foams excessively, or becomes sticky, discard and replace immediately. Do not dilute old oil by adding new oil.
Eating “Reheated food” remains safe and does not harm health if proper hygiene principles are followed:
(Source: Article “Is reheated food safe?” by Associate Professor Dr. Sitima Jittinand, Institute of Nutrition, Mahidol University)