A cook I know always discards the core of the onion when chopping it. Is there any reason for this? In professional demonstrations and cooking classes, I've never seen it done. From observation, it does seem to make chopping or mincing the way I've always been taught a bit easier, as the core can be awkwardly shaped, but other than that is there anything besides personal preference to it?
There is one reason I might do this. If the core is green, that is starting to form a sprout. I cut that out as it can be bitter. Otherwise I don't know why one would do that.
The core makes you cry much more than the rest of the onion, if you're prone to tears. There's actually already a question about onions and tears, with an answer explaining this.
I'm a chef and do the same but the only reason we do it is for presentation
I was watching an Italian cooking programme the other day and the chef took away the core and explained that it is the most acidic part of the onion. Whether this is true or not I do not know.