The effect of heat treatment, transglutaminase treatment, and high hydrostatic pressure treatment of fresh skim milk on the rheological properties of acid milk gels, formed by acidification with glucono-delta-lactone, was investigated. Acid gels prepared from unheated skim milk had long gelation times, low gelation pH, and very low final G' values. Heat treatment and pressure treatment of milk had similar effects in that both treatments decreased the gelation time, increased the gelation pH, and markedly increased the final G' of the resultant acid gels. Transglutaminase treatment of milk prior to acid gel formation also markedly increased the final G' of the acid gels; however, when transglutaminase treatment was performed during pressure treatment, a markedly higher final G' was obtained than when only pressure or transglutaminase treatment was performed. It is proposed that there is increased cross-linking of the whey proteins, and an increased cross-linking between the whey proteins and casein when transglutaminase treatment is performed under high pressure. The introduction of a higher number of intermolecular cross-links will result in increased acid gel firmness and therefore an increased final G'. (c) 2005 Elsevier Ltd. All rights reserved.