Understanding Wine Chemistry

Book Preface

Introduction

From a macroscopic perspective, wine is a mildly acidic hydroethanolic solution. As shown in Table 1.1, water and ethanol represent ~97% w/w of dry table wines. Ethanol is the major bioactive compound in wine and its presence renders wine and other alcoholic beverages inhospitable to microbial pathogens. Understanding the physiochemical properties of wine will first require a review of the basic properties of water and water–ethanol mixtures. More thorough discussions of the unique properties of water, including those specific to the food chemistry, can be found elsewhere [1].

Chemical and physical properties of water

Water is a hydride of oxygen, but has unique properties compared to other hydrides of elements nearby on the periodic table, as shown in Table 1.2. For example, the boiling point of water (100 °C) is far above that of hydrides of adjacent elements on the periodic table: HF (19.5 °C), H2S (–60 °C), and NH3 (–33 °C). Thus, water exists as a liquid at room temperature, while the other hydrides exist as gases. Similarly, water also has a higher heat of vaporization, heat capacity, and freezing point than would be expected as compared to nearby hydrides.

The unique properties of water are largely due to its ability to engage in intermolecular hydrogen (H) bonding, which results in stronger molecule‐to‐molecule interactions than in related compounds.
●● Oxygen is more electronegative than hydrogen and an O–H bond is more polarized than N–H or S–H.
●● The geometry and symmetry of an H2O molecule allows for four concurrent H bonds per water molecule.

The ability of water to form strong H-bonds explains not only its higher boiling point than homologous hydrides, but also its high surface tension. A surface refers to the area in which two phases come into contact (e.g., water–air, water–oil, water–glass), and surface tension refers to the force needed to create an additional surface area between two phases, that is, to spread a water droplet on to a piece of wax paper.

Compounds that are polar (or that contain polar functional groups) and are also capable of H-bonding are referred to as hydrophilic and tend to be more soluble in water, which in wine would include most sugars and ions like K+ and SO4

Many compounds of importance to wine flavor, especially odorants, are hydrophobic and are characterized by the presence of hydrocarbon groups that are incapable of H‐bonding. A snapshot at the molecular level would show water molecules preferably forming H-bonds with each other while interacting minimally with the hydrophobic solute. This imposes order upon the system, and dissolution of hydrophobic compounds in water tends to be entropically unfavorable. Colloquially, the preference of polar solvents to solvate polar compounds rather than non‐polar compounds (and vice versa) is referred to as “like dissolves like.”