The Collaborative International Dictionary of English v.0.48:

Sweeten \Sweet"en\, v. t. [imp. & p. p. Sweetened; p. pr. &
   vb. n. Sweetening.] [See Sweet, a.]
   1. To make sweet to the taste; as, to sweeten tea.
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   2. To make pleasing or grateful to the mind or feelings; as,
      to sweeten life; to sweeten friendship.
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   3. To make mild or kind; to soften; as, to sweeten the
      temper.
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   4. To make less painful or laborious; to relieve; as, to
      sweeten the cares of life. --Dryden.
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            And sweeten every secret tear.        --Keble.
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   5. To soften to the eye; to make delicate.
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            Correggio has made his memory immortal by the
            strength he has given to his figures, and by
            sweetening his lights and shadows, and melting them
            into each other.                      --Dryden.
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   6. To make pure and salubrious by destroying noxious matter;
      as, to sweeten rooms or apartments that have been
      infected; to sweeten the air.
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   7. To make warm and fertile; -- opposed to sour; as, to dry
      and sweeten soils.
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   8. To restore to purity; to free from taint; as, to sweeten
      water, butter, or meat.
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The Collaborative International Dictionary of English v.0.48:

Sweetening \Sweet"en*ing\, n.
   1. The act of making sweet.
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   2. That which sweetens.
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WordNet (r) 3.0 (2006):

sweetening
    n 1: something added to foods to make them taste sweeter [syn:
         sweetening, sweetener]
    2: an improvement that makes something more agreeable [syn:
       enhancement, sweetening]
    3: the act of adding a sweetener to food

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "sweetening":
   ambrosia, artificial sweetener, bait, blackstrap, bribe,
   calcium cyclamate, cane syrup, carrot, clover honey, comb honey,
   corn syrup, cyclamates, edulcoration, encouragement, fillip, honey,
   honeycomb, honeydew, honeypot, incentive, incitement, inducement,
   interest, invitation, lure, maple syrup, molasses, nectar, payment,
   percentage, persuasive, profit, provocation, reward,
   saccharification, saccharin, sodium cyclamate, sorghum,
   stimulation, stimulative, stimulus, sugar, sugar-making,
   sugaring off, sweetener, sweets, syrup, treacle, whet