Abstract

Introduction

Because of today’s lifestyle, the number of people suffering from diabetes mellitus and obesity is increasing. The desire of the customers to regain their health created a whole market of non-sugar and non-caloric or non-nutrient foods. An important part of this market is the production of sugar alcohols, the so-called polyols. The applications vary from food over cosmetics to pharmaceuticals. Whereas polyols like sorbitol, xylitol, mannitol, lactitol, and maltitol are already established and widely used as sugar alternatives for quite a while, erythritol is still developing its whole potential (Billaux et al. 1991; Goossens and Roper 1994). As the production of erythritol is more difficult than of the other polyols, intensive research was performed to optimize its production in terms of improving erythritol concentration, productivity rate, and/or yield. Reports on erythritol reflect the scientific and commercial history of erythritol production. Early studies collected data and information about naturally producing organisms. Later investigations focused on media and cultivation optimization as well as metabolic pathway engineering to increase the amount of produced erythritol. Then, research split into two directions. One focused the discovery of alternative, suitable organisms, like bacteria or filamentous fungi, to open up the range of optimization parameters. The other research direction focused on metabolic pathway engineering or genetic engineering to improve yield and productivity as well as to allow the use of inexpensive and abundant substrates. This review will present the history of erythritol production-related research from a more commercial viewpoint moving towards sustainability and fundamental research.

Erythritol

Erythritol ((2R,3S)-Butan-1,2,3,4-tetrol) belongs to the family of sugar alcohols also known as polyols, which are formed due to hydrolysation processes of the aldehyde or ketone group in various carbohydrates (Billaux et al. 1991). The chemical structure of erythritol and the other sweeteners discussed in this review are provided in Fig. 1. Polyols are naturally abundant in fruits and vegetables, like grapes and mushrooms as well as in fermented foods like soy sauce (Bernt et al. 1996; Shindou et al. 1988; Yoshida et al. 1986). The most valuable properties of these sugar alcohols are their sweetness and low calorie content combined with being non-cariogenic (Mäkinen 1994). For an overview on these properties, see Table Table1.1. Within the sugar alcohols, erythritol plays a somehow extraordinary part. It consists of only four carbon atoms and has therefore the smallest molecular weight of all sugar alcohols, which is associated with slightly different physical and chemical properties. Erythritol is also a symmetrical molecule and therefore exists only in one form, the meso-form (Fig. (Fig.1).1). It forms anhydrous crystals with a moderate sweetness of 60–80% of sucrose (Goossens and Gonze 1996) (Table (Table1).1). However, as an advantage, it can be mixed with more intense sugars due to the absence of any aftertaste (Barbieri et al. 2014; Bernt et al. 1996; Moon et al. 2010). But due to the high production costs of erythritol compared to more intense sweeteners, it is not primarily chosen for its sweetness synergy. As a more important feature, erythritol can improve the mouth feeling and can mask certain unwanted aftertastes such as astringency and the irritant effect of intense sweeteners (de Cock 2012). When dissolved, erythritol exhibits a strong cooling effect due to its high negative heat of solution (Park et al. 2005). Along with the artificial sweetener sucralose, it is the only polyol that is non-caloric, providing no energy to the body. The majority of erythritol cannot be metabolized by the human body and is excreted unmodified into the urine without changing blood glucose and insulin levels (de Cock 2012; Efsa Panel on Dietetic Products and Allergies 2011; Grabitske and Slavin 2008). The latter is a stand-alone property of erythritol among the commonly used polyols and allows its usage as sweetener in specialized food for diabetics or people suffering obesity (Wheeler and Pi-Sunyer 2008). It also means that a severe disadvantage of other polyols, namely sorbitol and xylitol, leading to diarrhea is eliminated (Bernt et al. 1996; de Cock 2012). Only a little amount, less than 10%, undergoes a reversible metabolic reaction like the dehydration to d- or l-erythrulose (Moon et al. 2010; Park et al. 2005, 2016). Finally, erythritol is also a free radical scavenger with the ability to potentially exercise its anti-oxidant activity while circulating the body before it is excreted into the urine (de Cock 2012; den Hartog et al. 2010). For an overview on the biological effectiveness and reported side effects of the sweeteners discussed in this review, see Table Table22.

Fig. 1

Chemical structures of sweeteners discussed in this review, namely a erythritol, b sorbitol, c xylitol, d mannitol, e maltitol, f lactitol, g sucrose, and h sucralose

Applications

Although it was firstly isolated in 1852, it took until 1990 for erythritol to become present on the Japanese market as a new natural sweetener (Boesten et al. 2015). The range of applications for erythritol is still growing. It can currently be found on its own, or in combination with other polyols in foods, cosmetics, and pharmaceuticals.

To date, the use of erythritol in foods has been approved in more than 60 countries, including Europe, the USA, Japan, Canada, Mexico, Brazil, Argentina, Turkey, Russia, China, India, Australia and New Zealand (Boesten et al. 2015; de Cock 2012). Within the food sector, erythritol is mainly utilized as sweetener to balance the finished product with regard to its sensory characteristics, such as flavor, color, and texture. Erythritol can therefore be used to produce no-sugar added, reduced-sugar, or sugar-free alternatives. Erythritol as sugar replacement can be found as tabletop sweetener, in beverages, chewing gum, chocolate, candies, and in bakery products (de Cock 2012). Due to its mild sweetness, it allows a volume-for-volume replacement of sugar, whereas for example, sucralose that has a much higher sweetness needs fillers and even then has a noticeably different texture in baked products. With regard to sucralose, it also needs to be considered that it is a chemically synthesized substance that does not naturally occur in nature. As a consequence, it currently accumulates in the environment due to the lack of sufficient natural degradation mechanisms (Lubick 2008).

Polyols are commonly used within the personal care industry like the cosmetic or toiletries sector. They are more and more incorporated as excipients in the manufacture of care products like toothpaste, mouthwashes, creams and lotions, make-up, perfumes, or deodorants. Due to its humectant function as well as its pleasant taste, its sweetness and its non-cariogenic properties, erythritol can be used as base for toothpaste and mouthwash recipes (EPA European association of polyol producers 2017). It gives toothpastes the required viscosity and humectancy. Additionally, erythritol inhibits the growth of Streptococcus mutans and acts as caries limiting in combination with xylitol (de Cock et al. 2016; Grembecka 2015). Further, it was found that a 3-year consumption of erythritol-containing candies by 7- to 8-year-old children resulted in reduced plaque growth, lower levels of plaque acetic acid, and propionic acid (Grembecka 2015; Runnel et al. 2013).

Erythritol can be used in a wide range of solid and liquid formulations, including granulated powders, tablets, tablet coating, consumer-friendly lozenges, medicated chewing gum, syrups, and as mentioned before, as oral care products (Michaud and Haest 2003). For pharmaceutical use, its interaction with water and its high stability in temperature and in acid or alkaline environments is a key (Grembecka 2015). Because of its properties, erythritol as excipient offers good flowability and stability, making it an ideal carrier for actives in sachets and capsules. More and more active ingredients are derived from biotechnological processes, which are often very efficient but also extremely reactive. Using the non-reducing sugar erythritol instead of lactose, which is the most commonly used pharmaceutical excipient, the unwanted reaction between the amino groups of the active and the reducing sugar can be prevented. Therefore, lactose is more often being replaced by erythritol. Besides this, non hygroscopic polyols need to be used when a very water-sensitive active has to be reformulated (EPA European association of polyol producers 2017).

Production—history and development

To extract erythritol from its natural sources, like fruits or vegetables, is not practical because of their low erythritol contents. And in contrast to the other polyols, erythritol is not favored being produced via chemical synthesis. The needed high temperatures as well as the nickel catalyst result in a cost-ineffective reaction with a low