**3. Production of lactitol**

*Lactose and Lactose Derivatives*

coating material in chewing gums.

lactitol is discussed in Section 5.

**2. Historical timeline**

applications of lactitol.

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

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

**Figure 1** illustrates selected milestones of lactitol over the past 100 years. A comprehensive review of the technological advancements of lactitol can be found elsewhere [1]. Chemical catalysis was perhaps the first great contributor to the advancement of lactitol. In 1920, Senderens [6] hydrogenated lactose over activated nickel. Senderens' catalysis was very unstable, making unrealistic any kind of large-scale production. The stability of nickel-based catalysts became a reality with the invention of the sponge nickel by Raney in 1925 and 1926 [7]. Raney's invention consisted of crystalline particles of active nickel embedded within an inactive metal. In subsequent years, the reaction kinetics of hydrogenation was elucidated,

Early production of lactitol was aimed at research facilities, where potential applications were investigated throughout 1930–1970. In 1938, the crystalline structure of lactitol was elucidated by purification and crystallization of the

hydrogenated slurry [8]. A second anhydrous crystalline form of lactitol, dihydrate, was discovered by 1952 [9]. In subsequent years, lactitol entered the fields of nutrition, material science, and biotechnology. Fortification of infant food, synthesis of lactitol-based polyethers, sweetening agent, and animal feed are examples of

which allowed the production of lactitol at high yields and selectively.

*Selected scientific and commercial milestones of lactitol over the past 100 years. Adapted from [1].*

**36**

**Figure 1.**

## **3.1 Catalytic hydrogenation**

Lactitol is not found in nature, and it can only be produced through catalytic hydrogenation of lactose. Thus, the transition state theory of catalytic surface reactions is the foundation of lactitol synthesis. The actual synthesis consists of a sequence of elementary reactions, namely adsorption, surface reaction, and desorption [14]. Collectively, all these reactions are known as the Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetics [2]. **Figure 2** illustrates the LHHW kinetics that is formulated from a presumed elementary step. Then, the rate is derived through the different elementary steps with the assumption of one of them is the rate-determining step, while the others are achieved the equilibrium. The overall reaction rate is strongly affected by temperature and pressure since these variables determine the equilibrium of the elementary reactions.

### *3.1.1 Adsorption*

Lactose and hydrogen are adsorbed through chemisorption, where the exchange of electrons with surface sites leads to the formation of a chemical bond [15]. Lactose is adsorbed from the bulk solution, a process that overcame the interaction forces of the solvent. A molecular mechanism is responsible for adsorbing the

#### **Figure 2.**

*Illustration of Langmuir-Hinshelwood-Hougen-Watson kinetic. Adapted from [2]. Numbers 1, 2, and 3 represent adsorption, surface reaction, and desorption, respectively.*

#### *Lactose and Lactose Derivatives*

lactose and hydrogen is followed a dissociative mechanism (H2↔2H\*) due to the action of transition metals. Dissociative adsorption requires an adjacent vacant site, and the rate of attachment is proportional to the square of the vacant concentration [16]. The adsorption of reagents occurs within a very short timeframe. Once the adsorption is completed, the adsorbed molecules are in equilibrium with those molecules in the bulk phase.

## *3.1.2 Surface reaction*

Examples of reaction mechanisms occurring at the surface include duel-site, single-site, two adsorbed species, and unabsorbed species [17]. Such mechanisms have been used for hydrogenation of a number of carbohydrates including, glucose, fructose, xylose, and lactose [18].

### *3.1.3 Desorption*

The products of the surface reaction are subsequently desorbed into the reaction medium. Theoretically, the rate of desorption is exactly the opposite in sign to the rate of adsorption [19]. However, the desorption of reaction products is regarded as rapid and therefore neglected within the rate equation.
