Review pubs.acs.org/JAFC Generation of Acetoin and Its Derivatives in Foods Zijun Xiao*,† and Jian R. Lu*,§ † Centre for Bioengineering and Biotechnology, China University of Petroleum, Qingdao 266580, China § Biological Physics Laboratory, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom ABSTRACT: Acetoin is a common food flavor additive. This volatile compound widely exists in nature. Some microorganisms, higher plants, insects, and higher animals have the ability to synthesize acetoin using different enzymes and pathways under certain circumstances. As a very active molecule, acetoin acts as a precursor of dozens of compounds. Therefore, acetoin and its derivatives are frequently detected in the component analysis of a variety of foods using gas chromatography−mass spectrometry. Because of the increasing importance of these compounds, this paper reviews the origins and natural existence of these substances, physiological roles, the biological synthesis pathways, nonenzymatic spontaneous reactions, and the common determination methods in foods. This work is the first review on dietary natural acetoin. KEYWORDS: acetoin, derivatives, key enzymes, spontaneous reactions, food additive, flavouring agent ■ INTRODUCTION example of a bacterially fermented product that is also rich in Acetoin (3-hydroxy-2-butanone) is a pale to yellowish liquid acetoin. As for yeast-fermented products, acetoin exists in with a pleasant yogurt creamy odor and a fatty butter taste. almost all alcoholic beverages and is indispensable in fl 3 With FEMA No. 2008, it is a substance generally recognized as contributing to their avors. Natural acetoin has also been fl safe (GRAS) and mainly used in food industries to enhance the detected in fruits, vegetables, and ours, contributing to their fl flavor of their products (visit http://ntp.niehs.nih.gov/ for distinct natural avors. As a very active molecule, acetoin forms fi toxicity and related information). As the threshold for taste is various derivatives, which can also be frequently identi ed − affected by various conditions such as temperature, solution, during gas chromatography mass spectrometry (GC-MS) and personal physiological and psychological status, acetoin is analysis. perceptible at different levels in different foods and drinks. For We have previously discussed acetoin metabolism in bacteria 4 example, the sensation threshold of acetoin in different types of focusing on bacterial physiology and catabolism. Very recently, beers is quite different, ranging from 6 to 50 ppm by different we also reviewed the main approaches for enhancing authors.1 In contrast, its reported levels of use in foods could be fermentative acetoin production using bacterial strains with 5 hundreds of parts per million (Table 1). industrial potential. However, acetoin-related metabolic Acetoin can be biosynthesized in a variety of dietary mechanisms in eukaryotes including humans are fundamentally materials. For example, in yogurt and cheese, lactic acid different from those in prokaryotes. For example, acetoin esters bacteria convert lactose and citrate to important metabolites and glycosides were mainly found in higher plants, but the including acetoin and its analogue diacetyl, which add a strong biological mechanisms remain unclear to date. Given its buttery and cheesy flavor to the products. Vinegar is another increasing importance in foods and our daily life, we present here the first review disseminating the knowledge of the dietary Table 1. Acetoin Average Maximum Use Levels in Foods2 natural acetoin. We will outline the natural origins of the dietary acetoin, introduce the key relevant enzymes and use levels use levels food category (ppm) food category (ppm) nonenzymatic spontaneous reactions of acetoin in food-related higher species, and, where relevant, incorporate a basic baked goods 750 gravies 0.029 discussion to cover the main acetoin derivatives. Improvement beverages, alcoholic 3.1 hard candy 84.89 in our scientific understanding in this area will help us to better beverages, 17 imitation dairy 100 nonalcoholic control and use the dietary acetoin compounds in foods and breakfast cereals 0.67 meat products 24.27 drinks. cheese 10 milk products 0.03 chewing gum 0.42 other grains 400 ■ COMMERCIAL PRODUCTION OF ACETOIN condiments and 8 reconstituted 200 relishes vegetables Being highly active, acetoin is also very useful in chemical confection and 100 seasonings and 90 synthesis in addition to its importance in foods and drinks. For frosting flavours example, acetoin acts as the precursor for a range of fats and oils 750 snack foods 98 frozen dairy 50 soft candy 50 Received: March 22, 2014 fruit juice 0.03 soups 0.05 Revised: June 26, 2014 gelatins and 81 sweet sauce 98 puddings Accepted: June 26, 2014 Published: June 26, 2014 © 2014 American Chemical Society 6487 dx.doi.org/10.1021/jf5013902 | J. Agric. Food Chem. 2014, 62, 6487−6497 Journal of Agricultural and Food Chemistry Review compounds and as the chelating agent of alkoxides. As a transport and store. It can become easily lost during handling, bioactive compound, it shows great potential in microbiology, resulting in environmental pollution and compromising safety. botany, and pest control as well.5 Most of these issues can be resolved by using paraldehyde but Because acetoin has broad usages, it is valuable to study its require the extra step of paraldehyde depolymerization, production. Although there is industrial natural acetoin increasing the complexity and decreasing the yield.8 available from several small fermentation plants, current commercial acetoin is mainly produced by chemical synthesis ■ NATURAL ORIGINS OF ACETOIN from fossil-based raw materials and then added into our foods In contrast to the organic chemistry routes, however, acetoin 5 as a flavor enhancer. There are currently three main routes that can be generated by various natural or “green” routes. Generally can be used for the synthesis of acetoin: route A, exploiting the speaking, natural acetoin has three origins, microorganisms, transformation processes from structural analogues such as plants, and animals, and exists widely in some fermented diacetyl and 2,3-butanediol; route B, utilizing the hydrolysis products, fruits, crops, insects, mammals, and so on. Table 2 processes from halogenated butanone; and route C, seeking a gives some examples of the natural origins of acetoin. direct synthesis from aldehyde. Route A. The main disadvantage of using diacetyl as the raw Table 2. Some Natural Origins of Acetoin material for the production of acetoin is the high cost as diacetyl or related compounds are fine chemical products. material origin of acetoin There are also additional issues associated with high equipment artisanal Spanish cheese9 microbial fermentation requirements and more stringent reaction conditions and French blue cheeses10 microbial fermentation controls. If over-reduced, acetoin can be further converted to Chinese vinegars11 microbial fermentation 2,3-butanediol, decreasing the reaction yield. Similar issues European vinegars12 microbial fermentation would occur if 2,3-butanediol is used as the raw material. For beers13 microbial fermentation these reasons there has been no report of industrial uptake of wines from different countries3 microbial fermentation this route.6 young red wines14 microbial fermentation Route B. In this route butanone is usually used as the starting Garnacha Tintorera-based sweet wines15 microbial fermentation material, involving the first step of halogenation to make 3- sherry wines16 microbial fermentation halogenated butanone and the second step of hydrolysis to palm wine17 microbial fermentation make acetoin (Figure 1, top). Methyl ethyl ketone is a pawpaw18 fruit, plant synthesis dekopon19 fruit, plant synthesis durian20 fruit, plant synthesis banana21 fruit, plant synthesis lychee22 fruit, plant synthesis coconut23 fruit, plant synthesis oil palm23 fruit, plant synthesis guar bean24 vegetable, plant synthesis ash gourd25 vegetable, plant synthesis corn tortillas26 corn, plant synthesis Figure 1. Industrial chemical synthesis routes of acetoin. rice cakes27 rice, plant synthesis honeys28 flower, plant synthesis Avicennia marina29 flower, plant synthesis commonly used raw material. Route B thus has much reduced sweet olive30 flower, plant synthesis raw material cost. In addition, it has milder reaction conditions, red clover31 flower, plant synthesis lower equipment specifications, and no special catalysts kiwi fruit flowers32 flower, plant synthesis required either. It is thus not surprising to see its extensive Solomon’s lily33 flower, plant synthesis uses in industry. However, the hydrogens on the α-position of summer chafer34 insect, animal synthesis butanone tend to have greater reactivity. Apart from the target rhinoceros beetle35 insect, animal synthesis halogen-substituted ketone, other reactions also occur to cockroaches36 insect, animal synthesis produce 1-substituted butanone, 3,3-dihalobutanone, and liver homogenate and perfused liver of rats37 mammal, animal other derivatives, causing the yield reduction of the 3- synthesis halogenated product. Furthermore, under the alkali hydrolysis blood and urinary from normal male students after mammal, animal ethanol ingestion38 synthesis in step 2, byproducts such as 1-hydroxy-2-butanone make 39 fi ffi 7 human sweat mammal, animal separation and puri cation more di cult. synthesis Route C. Acetaldehyde is used as the starting material, and the product is produced