Analytical Methods Application of the SPI (Saliva Precipitation Index) to the evaluation of red wine astringency Alessandra Rinaldi a,b,⇑ , Angelita Gambuti a , Luigi Moio a a Dipartimento di Scienze degli Alimenti, Università di Napoli Federico II, Facoltà di Agraria, Via Università 100, 80055 Portici (NA), Italy b Laffort, BP 17, 33072 Bordeaux, Cedex 15, France article info Article history: Received 18 April 2012 Received in revised form 29 June 2012 Accepted 3 July 2012 Available online 16 July 2012 Keywords: Astringency Precipitation Salivary proteins SDS–PAGE Red wines abstract The aim of this work was to evaluate the astringency of red wines by means of a SDS–PAGE based- method. The optimization of the in vitro assay, named SPI (Saliva Precipitation Index) that measured the reactivity of salivary proteins towards wine polyphenols, has been performed. Improvements included the choice of saliva:wine ratio, saliva typology (resting or stimulated saliva), and temperature of binding. The LOD (0.05 g/L of condensed tannin) and LOQ (0.1 g/L of condensed tannin) for the binding reaction between salivary proteins and tannins added in white wine were also determined. Fifty-seven red wines were analysed by the optimised SPI, the Folin–Ciocalteu Index, the gelatine index, the content of total tannins and the sensory quantitative evaluation of astringency. A significant correlation between the SPI and the astringency of red wines was found (R 2 = 0.969), thus indicating that this assay may be useful as estimator of astringency. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Astringency is an important sensory characteristic of food and beverages containing polyphenols. This mouthfeel is mainly due to the interactions of polyphenols with salivary proteins, causing complexes formation and their further precipitation, which leads to a reduction of the lubricating properties of saliva (Breslin, Gilmore, Beauchamp, & Green, 1993). Consequently, sensations of dryness, hardness, and constriction are felt in the mouth (Lee & Lawless, 1991). Wine astringency is generally estimated by tasting, which rep- resents an in vivo evaluation according to physiological responses. Notwithstanding, sensory analysis has some disadvantages: (i) the confusion of astringent sensation with sour and bitter tastes, (ii) the tendency of the astringency to increase on prolonged exposure, (iii) the necessity of an expert panel, (iv) the carry-over effects, (v) the influence of many factors, such as ethanol, pH and acidity on the intensity of perception (Fontoin, Saucier, Teissedre, & Glories, 2008). In addition, astringency evaluation is subject to certain sub- jectivity (Valentova, Skrovánková, Panovská & Pokorny ´ , 2002). For this reason, a chemical evaluation of astringency has gained much attention during last years. Several studies focused on methods for astringency prediction, based on polyphenol–proteins interactions and the correlation with wine astringency perceived by trained jury. Since astringency is mainly considered a tactile sensation, dif- ferent methods based on tannin precipitation were built up. The methods developed up to now to estimate the chemical astrin- gency (Llaudy et al., 2004; Mercurio & Smith, 2008) utilised differ- ent proteic standards as precipitation agent. The utilisation of saliva has not yet been considered despite its contributing to taste and mouthfeel perception (Neyraud, Heinzer- ling, Bult, Mesmin, & Dransfield, 2009) by transporting and dissolv- ing the stimuli substances, and by interacting with them (Matsuo 2000). The difficulties in measuring wine astringency using human saliva lies in reproducing the physiology involved during tasting. The perception of astringency is basically a dynamic process, that continuously changes and evolves to reach a maximum of intensity once red wine is ingested. Following the physiological course, what happens in mouth during wine tasting? When a sip of wine is made, a volume of wine is introduced in the oral cavity. According to sensory protocols, it is necessary to hold wine in the mouth for almost 8 s, during which wine constituents mix with saliva at a temperature of 37 °C. After this time, wine can be expectorated or ingested together with saliva. Astringency takes many seconds (15 s) to develop fully (Kallithraka, Bakker, Clifford & Vallis, 2001) and is perceived on soft palate, gingives, lips as well as on the oral cavity (Breslin et al., 1993). The chemical interactions of wine polyphenols with some salivary constituents and the subse- quent precipitation reduce the lubricating properties of saliva, so sensations of dryness, hardness, and constriction are felt in mouth. In a previous work (Gambuti, Rinaldi, Pessina, & Moio, 2006), the precipitation of salivary proteins after the binding reaction with 0308-8146/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2012.07.031 ⇑ Corresponding author. Address: Corso di Laurea in Viticoltura ed Enologia, Via Tuoro Cappuccini No. 6, 83100 Avellino, Italy. Tel./fax: +39 0825 784678. E-mail address: alessandra.rinaldi@unina.it (A. Rinaldi). Food Chemistry 135 (2012) 2498–2504 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem