Here is my go to pesto recpie: Serious Eats Pesto.
It has been a hit but lately whenever I make it, it has a bitter aftertaste to it. It's not as pungent or umamied, but bitter and leafy.
I use the same proportions (but not the same brands of olive oil or same basil leaves). What in the processing or ingridients might be responsible for the bitterness?
immediate consumption
You make genoese pesto
(indeed the linked recipe is ok for genovese pesto and not simply a genoese pesto-like green sauce)
for an almost immediate consumption.
In such a case each of the ingredient can be responsible for a bitter taste or note. I would suggest checking each of them for their own taste. Some`hints are given by commenters.
Pine nuts can be sweet and well kept, or rancid (they have fats/oil). They can be perfect as those called "from Pisa" or less valuables. Check them and find the best from your local stores.
Extra virgin oil can be of extremely bitter taste. Extra virgin mostly refers to the extraction, not to the organoleptic characteristics, of course. But this should be clear for you as you probably use oil for salad etc.
Pecorino sardo (romano as by official definition) can be more or less salty and quite pungent. Traditionally, when it goes into pesto formulation, is very old and harder than parmigiano to give an idea. This can be overly tasty to many. In this case reduce its amount and increase correspondingly that of parmigiano reggiano (or grana padano).
Now comes the core: basil. You cannot easily taste it alone but relies on parfum. It should not be minty or herbaceous at all. The best leafs are small and pale green.
Here in Genova the basil is collected when the plants are young and less than 12 - 13 cm tall. Not sure about how many varieties of it can exist as obviously I stick to what is growing in my neighbourhood but for sure the tallness of the plants is s crucial point as for the biochemistry of the plant change after a certain level of maturity. Note that this has chemical basis. I.e. the essential oil composition change after a certain size.
However, rather than bitter a too big basil plant imparts a mint and/or herbaceous taste.
I suggest tasting the oil (if not done) as it seems to me the first suspect ingredient.
Related to oil: if you are using a blender (OP doesn't but let me stay general) be sure to work by very short pulses as for blending at high speed can microscopically rise the temperature to "burns" ingredients. This is felt by some as a bitter/burned note indeed.
Later serving
In case you make pesto for a later consumption: this is somehow detrimental as for oxidation is basically unavoidable. You will always see pesto getting darker and less scented to some extent.
Unfortunately pesto is a delicious condiment only when is almost perfect. That is why long storage pesto items are disgusting as compare to other sauces in the supermarket shelves.
Nevertheless a home made pesto can be used within days if immediately frozen. Many families do that. A nice trick is to use ice cubes racks so that the needed portions can be used at times.
In this case is clever to skip the cheese while blending. Grated cheese is very prone to oxidation or perhaps microorganisms attack (not sure exactly) and gets pungent and bitter very quickly. So for postponed consumption better to prepare a cheese-free pesto and mix-in the cheeses just at the last moment, carefully working with a fork.
Extra notes:
once I read here that somebody warm up pesto in a quick cooking fashion. NO and NO.
pesto "has an ingredient" that is never mentioned because does not enter the mortar (or mezzaluna or blender) step. Just before serving the pasta, a very little nut of butter and drops of cooking water shall be added to each portion in order to get a more pleasant creamy texture of the sauce.
Buon appetito from Genova.
Where do you live? What is the source of your basil?
As the comments point out, there could be more than one source of your pesto bitterness. However, since you imply that you have been successful before, my theory is that your basil is the most likely culprit.
When I grow basil (and other soft herbs), I notice that as the season progresses, especially near the end of the growing season, most soft herbs take on a more bitter/astringent note. I would pay attention to the main component of your pesto as your source of the issue.
Pesto made with basil leaves of plants already with flowers will be bitter. Use only leaves before blooming begin.
Dont use entirely just extra virgin olive oil... use half EVOO with a more neutral oil or just use all olive oil (not extra virgin).
EVOO in large amounts can be "bitter" sometimes.
