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I have been told that sugar adds a sour taste to the coffee.

Personally I do not put sugar at all in my coffee but I was wondering for the sake of curiosity... Also I always preach to people that do put sugar in about how it ruins their coffee, due to the sour after-taste the sugar is adding.

Is there any chemical reaction here between the sugar and the coffee? Is it a result of a mixture between bitter and sweet that creates the sour taste?

When I tried having coffee without sugar I did feel like there is this sour feeling to it, but I also sometimes taste it with espresso made from some blends.

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    Sugar produces the perception of sweetness, not sourness. If anything, sugar reduces the perception of sourness. My comments overflowed this space; continued below. Or am I misunderstanding your question?
    – hoc_age
    Commented Mar 5, 2015 at 16:17
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    Okay, I have misunderstood. Can you find a reference of sugar causing sour after-taste in coffee? That could help clarify the situation. There are counter-examples, such as this cooking.se question, about how sugar reduces perception of sourness.
    – hoc_age
    Commented Mar 5, 2015 at 16:43
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    The Question is inconsistently worded: You were told sugar adds a sour taste. You do not put sugar "at all in my coffee". You "always preach to people that do" that it ruins their coffee. Finally, "I tried having coffee without sugar". I thought you said you never use sugar in coffee?? Sure, espresso particularly can have a sour taste, but this has nothing to do with sugar that was not added to it.
    – hardmath
    Commented Mar 10, 2015 at 0:02

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Sugar adds a sweet taste to coffee, not a sour taste. Sourness and sweetness are rather separate tastes! Different flavours will to some extent interact with each other, and individual preference and taste have something to do with this, but there are particular causes of certain flavours.

Sourness is a perception of acidity. In high enough quantities, sugar can overwhelm the perception of acidity (e.g., cola has a pH ~2.5, but doesn't taste sour because the perception is overwhelmed by sugar/sweetness; see also this Seasoned Advice question on sugar reducing perception of sourness). I don't see how sugar could be the cause of sourness (i.e., sugar is not acidic). EDIT: That said, it might be the case that this is a temporal situation. Sugar reduces the perception of sourness, but it could be the case this change is temporary: For example, if the sweetness (sugar) dissipates more quickly than the sourness (acids), you might be left with "more sourness than sweetness" in the aftertaste after some time. That is, you're left with "un-maksked" residual sourness which, though present all along, is more perceptible after the sugar has dissipated. However, all of this is entirely speculation; I have no justification for this, and haven't found any supporting information.

Sweetness is perceived by the presence of sugar (and/or other chemicals that happen to interact in similar ways with sweetness perception). Sugar will also impact mouthfeel.

Bitterness is separate also; see this question on salt and bitterness in coffee.

I don't think there are any chemical changes to the sugar happening in this case.

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  • You indeed misunderstood my question... Obviously sugar adds sweetness to the coffee... I was ignoring this fact since I thought it's redundant to mention it... Sorry for not being clear about this... The thing is I have been told that it also adds sourness, and than when people try to make the coffee less bitter and more sweet, pay with adding a certain amount of sourness flavor the their coffee...
    – Aviel Gross
    Commented Mar 5, 2015 at 16:23
  • @AvielGross - okay, you've got me interested in this now. I tried a bit of sugar on my tongue to see how this affected my taste, and I gave a bit of speculation in the EDIT above as to one possible mechanism. Hopefully you'll receive an answer from someone who actually understands your question and actually understands the mechanism involved. :)
    – hoc_age
    Commented Mar 5, 2015 at 16:56
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    Inspired by my daughter's request to finish her bowl of soggy cereal this morning, I came up with the following experiment. After taking a sip of (unsweetened) coffee, take a spoonful of sugary cereal with milk. Then take another sip of (unsweetened) coffee.
    – hardmath
    Commented Mar 16, 2015 at 15:41

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