I've consumed olive oil almost every day of my life and that spans a rather long period of time now. At any given moment, there are some four or five bottles of olive oil in my kitchen, each with its own set of uses (this is not rocket science but rather, for the most part, the expression of the belief that if you want to make a dish that was developed in some given place, it makes sense to use as much as possible ingredients from that given place).
I must say that I've never had an experience with olive oil such as you describe here. Rancidness, as suggested above, should not result in a massive increase in bitterness and there is presumably something else going on; indeed, bitterness is most associated with young olives and young oil and bitterness tends to diminish a bit with time.
The oil in question, Filippo Berio, is one of the very large producers in Italy (along with, for example, Carapelli and Bertolli) that is to my mind quite serviceable but not especially interesting. These oils are all pan-Mediterranean blends; that is, they buy decent oil from wherever they can (especially Spain and North Africa, also Turkey, etc.) at a decent price and blend it together and hope you will not read the fine print on the bottle (or can) and just notice that they are Italian-based or more specifically Tuscan-based. I use Berio and Bertolli only of the regular grade, i.e. "olive oil", not extra-virgin and not light and flavoured, just plain olive oil. That's one of my basic oils (along with peanut) for large-scale frying or deep-frying. Extra virgin olive oil has a far more pronounced flavour and is expensive and thus less suited to those uses.
I assume that you've used Berio extra-virgin oil before and have liked it. If it is, however, the case that you haven't used that brand or don't habitually use extra-virgin oil, then perhaps your reaction is just to the more pronounced flavour of Berio (and that particular batch may have included some oil or oils that were more bitter than most). In general, if you want mild flavoured oil, those large-scale Italian producers fit the bill well. More expensive, more subtle are generally Provençal and northern Italian (that is, for the most part Ligurian) oils. I've also found that some of the Middle Eastern oils can be remarkably mild in flavour (read: bland) but there are various problems involved in finding good Lebanese or Tunisian oils; I like them a lot when they're good (read: neither just bad nor bland) and once in a while I find one that is quite good.
For my day-in day-out cooking I use primarily olive oils from central and southern Italy, and there especially southern Lazio (the ancestral zone) and Sicily, and also Greece; I've gone on record elsewhere on the board as being a big fan of olive oil from Crete but there are some from the Peloponnese and elsewhere in Greece that I like very much. Of late I've also been purchasing regularly an oil from Spain -- Andalucía, to be more precise -- which is both fairly cheap and quite good for lots of basic applications.
All in all, one must simply try a lot of oils and figure out what ones and, more generally, what kinds of oils one prefers. It is clearly true that most people in this country prefer relatively mild-flavoured oils (hence all the "light" versions one sees springing up, which have only less flavour, not fewer calories). I love olive oil and enjoy very much a wide range of styles and flavour profiles (bitter, spicy, fruity, etc.).* And only for deep-frying for large-scale pan frying do I use a blander (though the crucial factor is generally more the cost ), non-extra-virgin oil.
In general, if you don't like at all a bitter element in your olive oil, the large producers of blends are the way to go for basic oils. Beyond that, bitterness varies with the age of the olives from which the oil is made, the age of the oil itself (a 'new' oil will mellow a little before it ultimately goes rancid) but also with the type of olive and where precisely the olives are grown.
Antonius
* In Italian, one speaks of oils being 'fruttato verde' and 'fruttato dolce' with the former having piquant, bitter and astringent elements, along with the fruity flavour(s) of the olives; 'dolce' or 'sweet' oils aren't, of course, actually sweet but rather just not piquant or astringent and lack a bitter element. Both kinds have their uses and I'm personally fond of oils with the 'green' characteristics.
Alle Nerven exzitiert von dem gewürzten Wein -- Anwandlung von Todesahndungen -- Doppeltgänger --
- aus dem Tagebuch E.T.A. Hoffmanns, 6. Januar 1804.
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Na sir is na seachain an cath.