Ultrahigh Pressure (UHP) oers interesting possibilities for food processing ranging from extraction of plant com- pounds, restructuring foods and rapid formation of small ice crystals. Pressures between 300 and 600 MPa can inactivate yeasts, moulds and most vegetative bacteria including most infectious food-borne pathogens. Thus, pressure is a poten- tial alternative to heat pasteurization as pressure leaves small molecules such as many ¯avour compounds and vitamins intact. In general, bacterial spores can only be kil- led by very high pressures (>1000 MPa). Bacterial spores, however, can often be stimulated to germinate by pressures of 50±300 MPa. Germinated spores can then be killed by relatively mild heat treatments or mild pressure treatments. However, in most cases a small fraction of spores can sur- vive this treatment. Consequently, there is still no practical application of UHP treatment as a sterilization process, but it is still an interesting area for further exploration. # 1998 Elsevier Science Ltd. All rights reserved This review deals with the eects of pressure on food microorganisms. For more general reviews on the eects of pressure on food, readers are referred to recent reviews [1, 2]. Increasing consumer demand for mini- mally processed additive-free, shelf-stable products prompted the exploration of physical treatments other than traditional heat treatments as potential alter- natives. These alternatives include new ways of applying heat, such as Ohmic heating or microwave heating, and non-thermal methods, such as irradiation, application of high electric ®elds, high magnetic ®elds and high pressure. Ultrahigh Pressure (UHP) treatment has been known as a potential preservation technique for almost over a century, since Hite [3] demonstrated in 1899 that microbial spoilage of milk could be delayed by applica- tion of high pressure. High pressure has been applied for many years for production of ceramics, composite materials, carbon graphite and plastics. These techno- logical developments increased the feasibility of com- mercial application in the food area. A range of pressure-treated products has been on the Japanese market for several years, including fruit preparations, fruit juices, rice cakes and raw squid. In France, pressure-treated fruit juices are available. Recently, a pressure-treated guacamole has been successfully launched on the US market. UHP causes inactivation of microorganisms and enzymes while leaving small mol- ecules, such as ¯avours and many vitamins intact [4]. Emulsions, which are sensitive to heat can be pressure- treated without aecting the stability of the emulsion. Another application of UHP is to promote the forma- tion of small ice crystals. Under a pressure of 207.5 MPa, water remains liquid at temperatures down to 22  C. Sudden expansion at that temperature to atmospheric pressure is a means of formation of very small ice crystals provided the latent heat released by freezing can be removed. Pressure stimulates many chemical reactions and can release ¯avours from plant cells. General principle of UHP Two principles underlie the eect of high pressure. Firstly, the principle of le Chatelier, according to which any phenomenon (phase transition, chemical reactivity, change in molecular con®guration, chemical reaction) accompanied by a decrease in volume will be enhanced by pressure. One would expect that temperature would have an antagonistic eect because increasing tempera- ture results in a volume increase. On the other hand, the reaction rate increases with increasing temperature according to Arrhenius' law. Secondly, pressure is instantaneously and uniformly transmitted independent of the size and the geometry of the food. This is known as isostatic pressure. 0924-2244/98/$19.00. Copyright # 1998 Elsevier Science Ltd. All rights reserved PII: S0924-2244(98)00030-2 Trends in Food Science & Technology 9 (1998) 152±158 Recent advances in the microbiology of high pressure processing J.P.P.M. Smelt Unilever Research Laboratorium, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands (tel: 31-10-4605578; fax: 31-10-4605188; e-mail: jan.smelt@unilever.com) Review