Thermal conductivity is the property of material to conduct heat. When organic acid (e.g. malic acid) is added in water, it changes the heat capacity of water due to formation of hydrogen bond. Hence thermal conductivity also changes.

You may find useful to check: B.E. Poling , J.M. Prausnitz, J.P. O'Connell, "The Properties of Gases and Liquids", 5th ed., McGraw-Hill, 2001; section 10-12.

Pure water containing no exogenous ions is an excellent insulator, but not even "deionized" water is completely free of ions. Water undergoes auto-ionization in the liquid state, when two water molecules form one hydroxide anion (OH−

) and one hydronium cation (H3O+). when organic acid is added to water, thermal conductivity will changed