You could always use the usual philosophical understanding from Aristotle (a Greek pagan who died 332 years before Jesus was born).
To Aristotle, everything has a substance (what the thing IS) and attributes (which he called accidents) - what the thing looks (tastes, etc) like.
Bread may have many different (and even opposite) properties. It can be black or white. It can be hard or soft, sweet or sour, fresh or stale, etc. But it has a certain “breadness” which is the substance. We would not call crackers or pancakes “bread” even though they share in almost all physical properties. If my wife asked me to buy bread and I bring home pancake mix she would think I was nuts. But, if you compare the ingredients, they are practically the same.
In 1964, US Supreme Court Justice Potter Stewart famously remarked that he could not come up with a definition for pornography, but he would know it if he saw it. That’s because even porn has a substance, and that’s what Justice Potter would perceive.
In Catholic theology, the substance of the bread is changed. But you can’t perceive substance. No scientific test can reliably answer the question, “is this bread?” We know it instinctively and instantly, but science doesn’t know it at all.
The attributes (the things science can test) remain.