fermentation Review Sweet Dreams (Are Made of This): A Review and Perspectives on Aspartic Acid Production Holly Appleton and Kurt A. Rosentrater * Department of Agricultural and Biosystems Engineering, College of Engineering, Iowa State University, Ames, IA 50011, USA; [email protected] * Correspondence: [email protected] Abstract: Aspartic acid, or “aspartate,” is a non-essential, four carbon amino acid produced and used by the body in two enantiomeric forms: L-aspartic acid and D-aspartic acid. The L-configuration of amino acids is the dominant form used in protein synthesis; thus, L-aspartic acid is by far the more common configuration. However, D-aspartic acid is one of only two known D-amino acids biosynthesized by eukaryotes. While L-aspartic acid is used in protein biosynthesis and neuro- transmission, D-aspartic acid is associated with neurogenesis and the endocrine system. Aspartic acid production and use has been growing in recent years. The purpose of this article is to discuss various perspectives on aspartic acid, including its industrial utility, global markets, production and manufacturing, optimization, challenges, and future outlook. As such, this review will provide a thorough background on this key biochemical. Keywords: bio-based; bio-chemicals; bio-materials; fermentation; synthesis Citation: Appleton, H.; Rosentrater, 1. Industrial Utility K.A. Sweet Dreams (Are Made of In addition to its biofunctionality, aspartic acid has wide application in the food, This): A Review and Perspectives on beverage, pharmaceutical, cosmetic, and agricultural industries [1]. L-aspartic acid is used Aspartic Acid Production. as a nutritional supplement in both functional foods and beverages, but its primary use Fermentation 2021, 7, 49. https:// is in combination with the amino acid phenylalanine which together make aspartame, doi.org/10.3390/fermentation7020049 an artificial sweetener [2]. Aspartic acid is also used to bolster immune function and as a natural combatant to depression [1]. Its ability to aid in energy production, fatigue Academic Editor: Diomi Mamma resistance, RNA and DNA synthesis, and liver detoxification give it broad clinical use [1]. Additionally, it is used as an intermediary substrate in the manufacture of pharmaceuticals Received: 29 January 2021 and organic chemicals, serving as the building block molecule for active pharmaceutical Accepted: 26 March 2021 Published: 29 March 2021 ingredients [1]. Aspartic acid’s utility stretches further upon consideration of its derivatives including acetyl aspartic acid, used as an active ingredient in anti-aging cosmetics that target wrinkling, skin lifting, and loss of firmness [3]. It is also used to produce polyaspartic Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in acid, a fertilizer synergist which increases both nitrogen absorption and crop yields [4]. published maps and institutional affil- Polyaspartic acid hydrogels are a type of biodegradable superabsorbent polymer which iations. have exceptional water-holding abilities and are used in the production of many modern amenities including diapers, feminine products, and engineered tissue [5]. The range and depth of aspartic acid’s applicability, in particular the L-configuration, has placed it on the Department of Energy’s Top Value Added Chemicals from Biomass list [2]. Copyright: © 2021 by the authors. 2. Global Markets Licensee MDPI, Basel, Switzerland. This article is an open access article The global aspartic acid market is a highly fractionated market meaning it consists distributed under the terms and of several small company players rather than large conglomerates, yet it is growing with conditions of the Creative Commons significant potential for industrial relevance [6]. According to a 2015 report by Grand Attribution (CC BY) license (https:// View Research, the global aspartic acid market is projected to reach $101 million with a creativecommons.org/licenses/by/ market demand of 60.6 kilotons by 2022 which represents a compound annual growth 4.0/). rate of 5.6% [6]. As of 2014, the baseline year of said report, polyaspartic acid represented Fermentation 2021, 7, 49. https://doi.org/10.3390/fermentation7020049 https://www.mdpi.com/journal/fermentation Fermentation 2021, 7, 49 2 of 12 22.6% of the total aspartic acid market volume making it the largest market segment, seconded by aspartame [6]. Both aspartic acid derivatives are anticipated to increase in demand as polyaspartic biodegradable polymers replace polyacrylic acid in agriculture, water treatment, and the petrochemical industries and as food and beverage trends shift towards added sugar labeling and health-conscious, convenience foods [6]. Of all aspartic Fermentation 2021, 7, x FOR PEER REVIEW 2 of 15 acid market sectors, the medical sector is projected to grow the most as is attributed to the American healthcare system, which is housed in the largest regional market, accounting for 39.0% of total aspartic acid volume as of 2014 [6]. Internationally increased demand Viewfor asparticResearch, acid the is global also expected aspartic toacid increase market in is the projected form of to greater reach aspartame $101 million demand with a for marketcarbonated demand beverages of 60.6 inkilotons Asia Pacific by 2022 [6]. which represents a compound annual growth rate of 5.6% [6]. As of 2014, the baseline year of said report, polyaspartic acid represented 3. Production and Manufacturing 22.6% of the total aspartic acid market volume making it the largest market segment, se‐ condedThere by aspartame are three [6]. main Both methods aspartic to acid produce derivatives aspartic are acid: anticipated protein extraction, to increase chemical in de‐ mandsynthesis, as polyaspartic and enzymatic biodegradable conversion polymers [2]. The replace hydrolysis polyacrylic of protein acid for in extraction agriculture, methods wa‐ terproduces treatment, an and abundance the petrochemical of amino acids industries from which and as the food L-aspartic and beverage acid must trends be separated.shift to‐ wardsChemical added synthesis sugar labeling requires and high health temperature‐conscious, and convenience pressure and foods results [6]. inOf a all racemic aspartic mix- acidture, market producing sectors, both the L-medical and D-isomers sector is projected thereby requiring to grow the the most additional as is attributed processing to the steps of optical resolution and racemization to achieve the preferred L-isomer [1]. Thus, enzy- American healthcare system, which is housed in the largest regional market, accounting matic conversion is the currently favored route of production. The enzymatic conversion for 39.0% of total aspartic acid volume as of 2014 [6]. Internationally increased demand process exists in two forms: simple enzyme-substrate interaction (hereafter referred to as for aspartic acid is also expected to increase in the form of greater aspartame demand for “enzymatic conversion”) or whole-cell enzymatic conversion, i.e., fermentation. Table1 carbonated beverages in Asia Pacific [6]. summarizes various economic and technical aspects of the production of aspartic acid. Stereospecific, industrial production of L-aspartic acid currently utilizes a one-step re- 3. Production and Manufacturing action of fumaric acid, in the presence of high concentrations of ammonia, to L-aspartic acid viaThere L- aspartate are three ammonia-lyase, main methods an to enzyme produce also aspartic referred acid: to protein as “L-aspartase” extraction, (Figure chemical1 )[ 7]. synthesis,L-aspartase and canenzymatic be purified conversion and immobilized [2]. The hydrolysis in a gel of matrix protein for for continuous extraction production methods producesor overexpressed an abundance in bacterial of amino cells acids bound from which to polyurethane the L‐aspartic carriers acid must [7]. be Production separated. via Chemicalimmobilized synthesis enzymatic requires conversion high temperature or bacterial and fermentation pressure and utilizes results the in same a racemic enzyme mix and‐ ture,substrate; producing however, both L enzymatic‐ and D‐isomers production thereby is requiring favored for the its additional high product processing concentration, steps of productivity,optical resolution minimal and byproducts, racemization and to the achieve ease ofthe downstream preferred L processing‐isomer [1]. [2 Thus,]. Yet, enzy fermen-‐ matictative conversion production, is the albeit currently less productive, favored route has beenof production. around since The theenzymatic 1950s when conversion research processand development exists in two into forms: the simple production enzyme of aspartic‐substrate acid interaction began [8]. (hereafter referred to as “enzymaticWhile conversion”) several species or ofwhole bacteria‐cell includingenzymatic select conversion, Pseudomonas, i.e., fermentation. Bacillus, and Table Proteus 1 summarizeshave been various identified economic as producers and technical of aspartic aspects acid, ofE. the coli productionand Cornybacterium of aspartic glutamacium acid. areStereospecific, nearly exclusively industrial used production by industry of [ 1L,9‐aspartic]. Fumaric acid acid currently is the primaryutilizes a substrate one‐step in reactionL-aspartic of fumaric acid production; acid, in the however, presence maleate, of high concentrations a less expensive of feedstock, ammonia, can to L be‐aspartic used in a acidtwo-step via L‐ aspartate reaction which ammonia uses‐lyase, maleate an isomeraseenzyme