Flavour, a product of the integration of distinct physiologically defined sensory systems that combine taste, aroma and trigeminal sensations , is a key determining factor for the acceptance of food and beverages by consumers . Flavour is a dynamic process; a series of events that involves the interaction of the product and the consumer . Characteristic flavour and aroma profiles of food and beverages arise from the composition and concentration of the volatile compounds present. Volatility refers to the ease of evaporation of a compound into the air, which allows for the volatile constituents to enter and move within the nasal or oral cavities where they can bind to olfactory receptors and elicit an olfactory response . Also contributing to potential flavour are non-volatile compounds, some of which while not odour-active can be broken down to release volatile odorants. These non-volatile constituents are known as aromatic precursors, and their potential is realized when the flavour compound—the aglycone—is released from its glycosidically bound form . A glycoside is a compound composed of both a carbohydrate (glycone) and non-carbohydrate (aglycone) residue within the same molecule. When the glycone portion of the glycoside is a glucose molecule, the compound is known as a glucoside . The ratio of glycosidically bound aroma compounds to their free aroma compound counterparts is dependent on the matrix. For example, in Muscat and Riesling wine grape varieties, this ratio ranges between 1 and 5, whereas in the Gewürztraminer variety, bound compounds are up to 15 times more abundant than the free forms . Thus, glycosidically bound compounds constitute a considerable reservoir of potential aromatic compounds that may enhance food and beverage flavour if their release can be realized. Glycosides can be separated from their sugar moieties through the use of heat, acid and enzyme hydrolysis , with the enzymes responsible for the latter known as glycoside hydrolases or glycosidases .
Juice and wine are two products that have been extensively studied with respect to glycosidically bound aroma precursors . For instance, monoterpenes are important aglycone compounds that contribute to the flavour, aromatics and typicity of many grape varieties and their wines . They exist in both a free volatile form, which can directly contribute to the aroma and flavour, and in a potentially volatile, glycosylated form, which are bound to either mono or disaccharides [5, 11]. Other examples of grape berry-derived aglycones that contribute to flavour include terpenolds, C13-norisoprenoids and benzene derivatives . These examples elicit mostly pleasant aromas that contribute to the aroma profile and have a low perception threshold .
Of the three methods by which glycosides can be separated, enzymatic hydrolysis is regarded as the most efficient, and considerable research has focused on its application to flavour enhancement . Acid hydrolysis of glycosides occurs at a slow rate and may induce terpenol rearrangements [13, 15]. Thermal hydrolysis has the potential to increase the breakdown of glycosides by 33%; however, high temperatures can also rearrange the natural structure of the glycosides . Generally, glycosidases are best characterized as the group of enzymes that act on disaccharides, oligosaccharides and polysaccharides  to specifically cleave the glycosidic bond present between the aglycone and the glycone of the glycoside structure [6, 9, 18]. They are able to catalyze the hydrolysis of both S- and O-glycosidic linkages.
There is some evidence that human saliva may be involved with the release of non-volatile aroma compounds from glycoside structures. Saliva is a complex, heterogeneous, dilute aqueous solution that contains secretions from both the major and minor glands, including numerous inorganic salts, microorganisms, crevicular fluid, sloughed cells/food debris and a diverse selection of organic compounds such as proteolytic enzymes and mucous glucoproteins [19, 20]. In the mouth, flavour release from food and beverages is a dynamic process influenced by factors such as release rates of odorants, matrix compounds, breathing, mastication and saliva flow [2, 20]. For non-volatile constituents, including aroma precursors, saliva acts as a carrier of the compounds to the taste buds located throughout the oral cavity, which is necessary to elicit a taste response . Further, odour perception can be influenced by the length of time odour compounds persist after food or beverages are swallowed, by remaining in the oral cavity. Saliva plays a role in the intensity and duration of this aftertaste; it acts as a transport system by extracting odorants from materials that are not swallowed .
Most research to date on salivary enzymes, including β-glucosidase, has focused on their role in the diagnosis of periodontal disease  and responses to its treatment . Few studies appear on the possible activity and prevalence of β-glucosidase in human saliva. Menguy et al.  suggested that approximately one in five people possess β-glucosidase in their saliva; however, the sample size of their study was quite small. Later, Arnaldos et al.  modified a spectrophotometric assay for strawberry callus protein for potential application in determining β-glucosidase in human saliva.
The optimal pH for β-glucosidase activity is approximately 5.5, with an activity range of 4.3–7.0 and an optimal temperature of 45 °C . Varying activity of β-glucosidase within human saliva could represent a potential source of individual variation in the perception of flavour. Individual differences in flavour perception have been strongly linked to variation in food/beverage preferences, as well as habitual diet-related nutrition and health outcomes [27–31]. β-Glucosidase activity may increase the concentration of free volatile compounds available in the oral cavity through the hydrolysis of glycosidically bound non-volatiles, potentially enhancing flavour and overall perception of the food or beverage consumed.
Existing analytical methods for measuring β-glucosidase activity often require expensive, specialized equipment such as that used for high performance liquid chromatography (HPLC) . Experimental methods and data analysis can be time-consuming with such techniques and require highly trained personnel. Further, current enzymatic approaches require extended incubation times and are highly temperature dependent . The first objective of this study is to optimize an enzymatic method for measuring human salivary β-glucosidase activity that is rapid, simple and uses ubiquitous laboratory equipment (a spectrophotometer). A secondary objective is to apply this method to determine the prevalence of salivary β-glucosidase activity in humans. This research informs wider goals of fully eliciting the determinants of human flavour perception and sources of individual differences.