**1. Introduction**

Lactitol, a sugar alcohol, is not found in nature, and its synthesis requires lactose in solution, hydrogen gas, and solid catalyst. The first attempts of lactitol synthesis were made about 100 years ago. Since then, the synthesis of lactitol has evolved into a highly efficient process with a projected production of 1.9 million metric tons by 2022 [1]. In a nutshell, the synthesis consists in the incorporation of a hydrogen ion into the carbonyl group of lactose. Such incorporation involves a set of multiple elementary reactions known as Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetics. The hydrogenation has consensually thought to occur by adsorption, reaction surface, and desorption of the reactants. A number of kinetics models suggest that the surface reaction is the predominant step [2]. Within the surface reaction, the reaction between two adsorbed species is catalyzed by a transition metal supported in an inert material. Over the years, several catalytic systems (metal and support) have been investigated in terms of their physical and chemical properties. An important feature of the catalytic hydrogenation is the multiphase nature of the reaction, where liquid, solid, and gas are brought into contact for a given time. Upon completion of the reaction, lactitol is separated from the slurry by centrifugation and crystallization. In crystalline form, lactitol can exist in four crystal forms, depending on the crystallization protocol [3]. Each type of crystal is characterized by its melting point and solubility. The most common structure of lactitol is the monohydrate form, and therefore it is the most studied one.

Lactitol is best known as a nutritive sweetener, whose relative sweetness is between 30 and 40% comparable with that of sucrose [4]. More importantly, regulatory agencies such as European labeling and FDA consider a caloric value of lactitol as 2.4 and 2.0 kcal g−1, respectively, which correspond to a reduction of 48–40% with respect to sucrose [5]. The molecular structure of lactitol offers stability over a wide range of pH and temperature, making it a suitable candidate for the synthesis of biopolymers, hydrogels, and surfactants. Over the last past decades, lactitol has emerged into a multipurpose ingredient from low-caloric sweetness to coating material in chewing gums.

This chapter summarizes relevant advancements over the 100 years of lactitol history. Section 2 provides a historical overview of lactitol, highlighting some of the most significant milestones. Section 3 discusses an overview of the catalysts used for the hydrogenation of lactose. Section 4 addresses some chemical and physical properties of lactitol. Finally, a summary of current and potential applications of lactitol is discussed in Section 5.
