Effectiveness of Available Filters for an Electric Hygrometer for Measurement of Water Activity in the Food Industry

The effectiveness of the filters available for the Novasina Hygrometer for measurement of water activity (a_w) in intermediate moisture foods, most of them suspected of being potential sources of sensor contamination, was investigated. Foods studied were apple preserve, chocolate topping, honey, milk jam, salted anchovy, intermediate moisture beef, raw ham, dry salami, smoked pork sausage, provolone and roquefort cheese, tomato ketchup, semimoist pet food, concentrated tomato paste and Worcestershire sauce. For most foods, the use of an adequate filter permitted an acceptable precision in a_w measurement; however, neither of the filters studied allowed measurement of a_w in the pet food containing propylene glycol.     

Mixed Saturated Salt Solutions as Standards for Water Activity Measurement in the Microbiological Growth Range

Present work explores the feasibility of using mixed saturated salt solutions to provide additional solutions of known water activity (a_w). The mixed saturated salt solutions studied were: K₂SO₄-KNO₃, K₂SO₄-MgSO₄, KCl-KNO₃, NaCl-KCl and NaCl-NaNO₃. The a_w of the ternary saturated salt mixtures was predicted using a theoretical model of the thermodynamic properties of mixed aqueous electrolyte solutions. Prediction of a_w compared very well with experimental determinations.     

Theoretical Prediction of the Water Activity of Standard Saturated Salt Solutions at Various Temperatures

A theoretical approach was used to predict the water activity (a_w) at 15°C and 35°C of selected saturated salt solutions used as standards in the range of microbial growth (～0.57–0.97). For this purpose a rigorous thermodynamic model to predict osmotic coefficient was used. The results agreed very well with experimental measurements at 15°C and 35°C for most salts studied, which included NaBr, NaCl, (NH₄)₂SO₄, KCl, BaCl₂, and K₂SO₄. For KNO₃ the agreement at the higher temperature was somewhat less satisfactory and the reasons for this behavior are discussed.