Physicochemical changes can be delayed during product storage.

The stability of food products is of paramount importance during their storage. It is possible to dry fruit and fruit products by using such methods as spray-drying or freeze-drying.

Avocado and avocado products are poorly stable products because of their polyphenoloxidase activity. Mexican researchers found that they can predict the stability of dried avocado products using sorption isotherms. Their objective was to determine the sorption isotherms of avocado at three temperatures. They found that if they know and can control the critical moisture content, such physicochemical changes as the browning or caking of avocado products can be delayed when products are stored.

In experiments, puree from mature Haas avocado--Persea americana Mill--was freeze-dried in a commercial freeze dryer, and the remaining water removed. The scientists placed the puree powder into vacuum desiccators containing phosphorus to create a totally dried atmosphere. Around 0.25 g of avocado powder was placed in bottles. The scientists placed three bottles into vacuum desiccators containing different oversaturated salt solutions to create atmospheres in a range from 0.112 to 0.904 of water activity at three temperatures: 25 C, 35 C and 45 C. They used the isopiestic method, one of the most accurate methods for determining solubility.

The equilibrium moisture content was plotted as a function of water activity. The investigators modeled the isotherms using the Brunauer-Emmett-Teller (B.E.T.), Guggenheim-Anderson-deBoer (G.A.B.), Iglesias and Chirife, and Oswin models. The avocado puree powder sorption data were adequately fitted with the Oswin and the G.A.B. models at three temperatures.

The Oswin model showed standard errors of 5.8%, 7.0% and 8.5% at temperatures of 25 C, 35 C and 45 C, respectively. The G.A.B. model showed critical moisture contents of 0.0145, 0.0124 and 0.0078 at 25 C, 35 C and 45 C, respectively. However, the standard errors were higher than that for the Oswin model at the same temperatures.

Further information. Jose Angel Guerrero-Beltran, Departamento de Ingenieria Quimica y Alimentos, Universidad de las Americas Puebla, Sta. Catarina MPrtir, Cholula, Puebla C.P. 72820, Mexico; phone: +52 (222) 229 21 26; fax: +52 (222) 229 27 27; email: aurelio.lopezm@udlap.mx.