Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem A review, analysis and extension of water activity data of sugars and model honey solutions Balaji Subbiah, Ursula K.M. Blank, Ken R. Morison ⁎ Department of Chemical and Process Engineering, University of Canterbury, Private Bag 4800, Christchurch, New Zealand ARTICLE INFO Keywords: Honey Glucose Fructose Maltose Water activity Hydration number ABSTRACT Water activity is a physical property measured in the food industry which helps predict shelf life and microbial activity. Honey normally has a water activity less than 0.6, but this can vary with the amount of crystallization in solution. The aim of this work was to obtain relationships, as fundamental as possible, that can be used to predict the water activity of solutions with compositions similar to honey. Water activity measurements of aqueous sucrose solutions have been well analysed in literature using hydration theory. The analysis based on hydration numbers was easily able to show the quality of data previously published, and hence relationships were proposed for the hydration numbers of glucose, fructose, maltose and glycerol. A model was proposed in this study, to predict the water activity of food systems containing high concentrations of sugars and some electrolytes. The model was analysed and validated using mostly literature data supplemented with new experimental data. 1. Introduction Water activity is a very useful measurement for the prediction of shelf life of food products and it is very relevant to honey. In nature, bees reduce the water activity of honey by hydrolysis of sucrose and evaporation. Beuchat (1983) stated that some yeast can grow at a water activity of 0.62, but most moulds require a minimum water activity of at least 0.75. Honey normally has a water activity less than 0.60. The main contributors to low water activity in honey are fructose, glucose and various disaccharides. Because of the complexity of honey it is easier to study model honey solutions, so, for example, Rüegg and Blanc (1981) used a dry-basis composition of 48% fructose, 40% glu- cose, 10% maltose and 2% sucrose with various amounts of water to model honey. Prediction of the water activities of such solutions re- quires reliable data and relationships for binary solutions. Water activity in aqueous sugar solutions is affected by water-water, water-sugar and sugar-sugar interactions which are all concentration and temperature dependent (Starzak, Peacock, & Mathlouthi, 2000). Water molecules are preferentially attracted to some solutes which are then referred to as being hydrated. Hydration of sugars is generally attributed to hydrogen bonding between water and hydroxyl groups on the sugars, but the orientation and availability of these groups, and hence hydration, depends on the type of sugar and its self-association (Suggett, 1975). The hydration number can be defined as the average number of water molecules that are bound to each solute molecule so that they do not contribute to water activity (Scatchard, 1921). Burakowski and Gliński (2012) proposed a more general definition: “the average number of water molecules that are affected by interac- tions between the solute and solvent and cause an observable effect on a physical property of the solution”. Various techniques have been used to determine the hydration of sugars in solution. Each technique measures a different physical prop- erty and hence there is no expectation of a unique hydration number. All the techniques use a common conceptual model of a hydration shell that alters the size, compressibility or mobility of the water molecules within it, or of the hydrated solute. Branca et al. (2001) used the viscosity of sucrose, maltose and trehalose to determine the change volume fraction caused by hydration of the solute. Furuki (2002) re- lated the change in heat of fusion of disaccharide and oligosaccharide solutions to the proportion of unfrozen water and hence to hydration number. Burakowski and Gliński (2012) reviewed the calculation of hydration from measurements of the speed of sound which depends on the compressibility of the solution. Shiraga, Ogawa, Kondo, Irisawa, and Imamura (2013) used the change in refractive index in the ter- ahertz frequency range to obtain another estimate of number of hy- drated water molecules with slower dynamics than the bulk water. For this paper, water activity will be the physical property used. Water activity is a representation of the colligative properties: va- pour pressure, freezing point depression, boiling point elevation, and osmotic pressure, all of which are unique and quantitative measures of https://doi.org/10.1016/j.foodchem.2020.126981 Received 3 March 2019; Received in revised form 16 February 2020; Accepted 3 May 2020 ⁎ Corresponding author. E-mail address: ken.morison@canterbury.ac.nz (K.R. Morison). Food Chemistry 326 (2020) 126981 Available online 07 May 2020 0308-8146/ © 2020 Elsevier Ltd. All rights reserved. T