title_body: What acids are there in coffee?
upvoted_answer: Note:  I leave out a lot of the details of espresso shot extraction and just mention the major points of troubleshooting so don't flame me =p
  
  Acidity is always tricky when you talk coffee because there are differences between perceived acidity and actual acidity.  Most coffees are slightly more acidic than milk, and they are in a very tight pH range.  When I talk to customers and try to describe acidity, I typically describe a spectrum where on one end, you have acidity, and the other end you have bitterness.  
  
  If you use identical beans, you will typically get the exact same amount of acids in your cup of coffee.  These acids are primarily Citric and Malic acids and they extract at relatively low temperatures compared to the other compounds in coffee which is to say, you can't use water temperature to change acid extraction without brewing at such a low temperature as to basically not brew coffee.  The caveat to that is cold brew methods which essentially trade heat for time and are known for being very low acidity.
  
  You mention better flavor and less bitterness in your espresso.  This indicates to me that rather than being concerned about acid extraction, we need to look on the other end of the spectrum and look at over extraction.  If you think about a coffee ground like a sponge filled with flavors, the first thing to come out is the acids.  This is why when you brew weak coffee it is thin, and tastes acidic.  The longer you squeeze the sponge so to speak, the more compounds come out, the last being the melanoidins that contribute body to the coffee.  Just like too much acid in weak coffee, if you extract too much out of the ground you will start getting bitter flavors.
  
  The first thing is determining if the flavor defect is in fact acidity, or bitterness.  The biggest identifier for me is if it is overbearing acidity, I can feel it on the sides of my tongue (that's the sour receptors).  If instead you are dealing with bitterness, or charry flavor that is going to require the exact opposite to fix it.
  
  Here is a post where I was troubleshooting a particularly difficult espresso shot that gives a lot of the detailed mechanics of how to pull a shot, and what might be going wrong.  Coffee SE Espresso Thread
  
  Lets say your problem is bitter espresso shots.  The first thing to check is if your beans are overroasted.  Presuming your input beans are good, and presuming the machine is functioning as intended.. let's go through how to troubleshoot.  Bitterness means overextraction which could be caused by a large number of factors, the top being too many grounds, coffee ground too fine, tamp technique, or even inserting the portafilter into the machine too early.  First thing is first, time your shot.  You should get roughly 2 fl oz of extract (2 shot glasses) in between 25-30 seconds.  Is it taking longer than this?  If so, this could be contributing to bitterness.  To remedy, reduce the amount of grounds in your portafilter, or grinder more coarse, both of which will speed up the extraction.  An additional complication, is if you tamp the grounds using too heavy a tamp, you can cause the extraction to run slow.  
  
  On the flip side, if you decide the espresso is too acidic, you do exactly the opposite, namely, tighten the grind, make sure you are applying enough pressure during the tamp, or even add a gram or two of grounds.
  
  This is a reasonably simplified solution to troubleshooting espresso, but I hope it gives some insight into acidity vs bitterness, how to begin troubleshooting espresso shots etc.