Industrial production of lactic acid and its applications

Abstract

Lactic acid is one of the most commercially useful hydroxycarboxylic acids. Its applications range from bulk production of products, like Poly Lactic Acid (PLA), in industries to simple household applications such as food containers. Many cheap materials, such as starchy and cellulosic materials, and renewable materials, such as agricultural wastes, can be used as raw materials for lactic acid production. Microorganisms belonging to bacteria and fungi are actively involved in the production of lactic acid from the provided raw materials. This article discusses various raw materials, microorganisms, fermentation methods involved in the production of lactic acid, and also the applications of lactic acid in different fields.

Full text

42
Review Article: IJBR
Dec 2018: Volume 1, Issue 1
Industrial production of lactic acid and its applications
Battula Savithra Krishna*, Gantala Sarva Sai Nikhilesh1, Besetty Tarun1,
Narayana Saibaba K V1, R Gopinadh1
Department of Biotechnology, GITAM Institute of Technology, GITAM University
(Received 20 December, 2018 Accepted 26, 2018)
Corresponding author E-mail address: bskrishna1999@gmail.com
Key words: Lactic acid, Solid State Fermentation, Bacteria, Fungi, Renewable raw materials.
Abstract
Lactic acid is one of the most commercially useful hydroxycarboxylic acids. Its applications
range from bulk production of products, like Poly Lactic Acid (PLA), in industries to simple
house hold applications such as food containers. Many cheap materials, such as starchy and
cellulosic materials, and renewable materials, such as agriculture wastes, can be used as raw
materials for lactic acid production. Microorganisms belonging to bacteria and fungi are actively
involved in production of lactic acid from the provided raw materials. This article discusses
various raw materials, microorganisms, fermentation methods involved in production of lactic
acid, and also the applications of lactic acid in different fields.
1. Introduction
Lactic acid is of earnest importance as it is
not only used as a raw material for the
production of various products by
fermentation, but it is also used in food,
pharmaceutical and textile industries
(Young-Jung Wee et al. 2006). It was
initially considered as a milk component by
Scheele when he first discovered it in 1780.
It was later named as Acide lactique by
Lavoisier in 1789 (H. Benninga 1990). CE
Avery was the first person to commercially
produce lactic acid in 1981 in Littleton, MA,
USA (Vickroy 1985).
Besides its applications in food preservation,
pharmaceutical and chemical industries, it is
also a widely used flavoring agent, acidulant
and inhibitor of bacteria. Its water retaining
capacity makes it useful as a moisturizer in
cosmetic formulations. Due to presence of
hydroxyl and carboxyl groups in its
structure, it can be converted into useful
chemicals like esters, bio solvents, etc. (Gao
C et al. 2011). Lactic acid is also used as a
monomer in the production of biodegradable
PLA (Poly Lactic Acid) (Datta et al. 1995)
and it is used for medical purposes as in case
of prostheses, surgical sutures, etc. (Wee et
al. 2006). Due to its versatile applications,
lactic acid is deemed as one of the most
important hydroxycarboxylic acids and it is
in high demand (Lopes MS et al. 2012).
Around 370,000 metric tons of lactic acid
was produced in the year 2017 (C.
Rodrigues et al. 2017) and the demand of
lactic acid is rapidly increasing every year at

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43
the rate of 5-8% (Yadav et al. 2011;
Jamshidian M et al. 2010).
Various fermentation approaches for the
production of lactic acid include batch, fed-
batch, and continuous batch fermentations.
Batch and fed-batch cultures yield high
concentrations of lactic acid when compared
to continuous cultures. Though continuous
cultures yield low lactic acid concentrations,
the productivity is high (Hofvendahl et al.
2000). Microorganisms that are capable of
production of lactic acid are of two groups:
Bacteria and fungi (Litchfield 1996).
Although fungal fermentation has a slight
advantage over bacterial fermentation as
filamentous fungi need only a simple
medium to produce lactic acid, by the usage
of glucose aerobically (Tay et al. 2002), it
requires vigorous aeration (Yin et al. 1997).
Lactic acid can be also produced by
chemical synthesis. The chemical synthesis
of lactic acid is mainly done by hydrolysis
of lactonitrile by strong acids, which
produces a racemic mixture of L- lactic acid
and D- lactic acid. Various other processes
for production of lactic acid by chemical
synthesis include oxidation of propylene
glycol, nitric acid oxidation of propylene,
etc. But none of the chemical synthesis
processes (except hydrolysis of lactonitrile)
are economically and technically feasible.
The major advantage of fermentation
process over chemical synthesis is that it
requires cheap raw materials like starchy
waste, molasses and other materials rich in
carbohydrates (Anuradha et al. 1999).
The recent trend in usage of lactic acid is in
the production of Poly Lactic Acid (PLA).
PLA could be potential replacement for
fossil fuel based plastics, but its production
cost should be reduced to half of its current
price in order to achieve that (Lopes MS et
al. 2012; Abdel-Rahman et al. 2013). If this
is made possible, the demand of lactic acid
would rise even higher.
2. Raw Materials for lactic acid
production
In order to produce large amount of lactic
acid by fermentation at low cost, cheap raw
materials are required. Besides being cheap,
the raw materials should also have the
properties such as ability to produce high
yield, negligible or no formation of by-
product, high productivity, and less
contamination so that not much pre-
treatment is required (John et al. 2009). The
use of refined materials, such as
carbohydrates, may reduce the product
purification cost considerably, but it is not
economical as it results in high production
costs (K. Hofvendahl et al. 2000). The
following are some of the cheap raw
material sources for the production of lactic
acid: Starchy materials such as corn, potato,
rice, and wheat starch, and cellulosic
materials such as wood, cellulose are mainly
used as raw materials as they are in available
ample amount and cheap (K. Richter et al.
1998; K.V. Venkatesh 1997; C. Åkerberg
and G. Zacchi 2000). Starchy materials have
α (1,4) and α (1,6) linked glucose in the
structure (Richter et al. 1994; K. Hofvendahl
et al. 1997). Cellulosic materials have β
(1,4) glucan, lignin, arabinan, galactan and
xylan (Litchfield et al 1996; Hofvendahl et
al. 2000).
Apart from starchy and cellulosic materials,
renewable sources such as agricultural
residues, which are rich in carbohydrates are
used. Agricultural residues are abundant as
about 3.5 billion tons of agricultural residues
are produced per annum, however,
availability is not the only criteria, price and
purity also matter. The presence of
cellulosic residues in the agricultural
residues results in low protein content and
poor digestibility. So, the utilization of
agricultural residues is limited (John et al.
2007). The following are some of the
agricultural residues used for lactic acid

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44
production: corn cob (Vickroy 1985),
cassava bagasse (Rojan et al. 2009), beet
molasses (Ch. Kotzamanidis et al. 2002;
Yekta Göksungur, Ulgar Güvenç 1999),
lignocellulose (Karel et al. 1997),
cellulose(K.V. Venkatesh 1997), sugarcane
press mud (S. Xavier,B. K. Lonsane 1994),
and carrot waste(Ashok Pandey et al. 2001).
It is reported that waste paper is a good
source as well (Yáñez et al. 2005; Park et al.
2004).
Whey and molasses, which are industrial
wastes, are commonly used as substrates for
lactic acid production. Whey contains
lactose, protein, fat and mineral salts as it is
a by-product of dairy industry.
Microorganisms such as Lactobacillus
helveticus (A.W. Schepers et al. 2002; U.
Kulozik and J. Wilde 1999) and
Lactobacillus casei are used in production of
lactic acid from whey (A.O. Büyükkilci and
S. Harsa 2004; T. Pauli and J.J. Fitzpatrick
2002). Molasses, on the other hand, contains
large amount of sucrose as it is a waste
product from sugar manufacturing process.
For the production of lactic acid from
molasses, microorganisms like Lactobacillus
delbrueckii and Enterococcus faecalis are
used (Monteagudo et al. 1997; Göksungur
and Güvenç 1999; Wee et al. 2006). For any
fermentation process, it is essential to
provide nutrients to the fermentation media.
Though it results in high production costs,
yeast extract is the most commonly used
nutrient source for the production of lactic
acid (Vickroy 1985; Hofvendahl et al.
2000). H. Oh et al. attempted to use corn
steep liquor as an alternative for yeast
extract as it results in significantly low
production costs and succeeded. Malt sprout
extract, and whey permeates can also be
used as nitrogen sources in lactic acid
production (Suzanne F. Dagher et al. 2010).
3 Microorganisms for lactic acid
production
Microorganisms play a pivotal role in the
production of lactic acid and they must be
readily available and cheap. Lactic acid
producing microorganisms are classified
into bacteria, fungi, and yeast. Most of the
lactic acid production is done industrially by
the use of lactic acid producing bacteria (Cui
et al., 2011; Kleerebezem and van
Loosdrecht, 2007; Nancib et al., 2009).
However, the fungal species of Rhizopus
have their own advantage as they make use
of glucose aerobically to produce lactic acid.
But, the production rate of fungal
fermentation is low due to mass transfer
limitations (Park et al. 1998). The Genetic-
engineering techniques are exploited to
improve the lactic acid yield and optical
purity by various microbial producers
(Okano et al. 2010).
3.1 Bacteria
Lactic acid producing bacteria are classified
into four main categories, which are, Lactic
Acid Bacteria (LAB), Escherichia coli,
Corynebacterium glutamicum, and Bacillus
strains (Litchfield et al. 2009; Budhavaram
and Fan 2009). Out of these, Lactic acid
bacteria are most commonly exploited.
Choosing a proper strain is very important
because factors such as yield, productivity,
purity and nutrition requirements are
dependent on it (Rodrigues et al. 2017).
Some of the limitations of lactic acid
production by bacteria include low yield due
to formation of by-product, requirement of
nutrient rich medium, high risk of cell lysis,
necessity of mixed strains for development
of phage-resistant strains to prevent
bacterio-phage infection (Rodrigues et al.
2017).

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45
Lactic acid bacteria can produce lactic acid
by anaerobic glycolysis with high yield and
productivity. They are present in dairy
products, meat, and in plants. Different
bacteria grow at different conditions. In
general, the optimal pH range for the growth
of bacteria is 3.5–9.6 and the optimal
temperature is 5–45 °C (Rodrigues et al.
2017). Based on the fermentation end
product, Lactic acid bacteria are grouped
into two types: homofermentative and
heterofermentative. Homofermentative
lactic acid bacteria glucose exclusively into
lactic acid by Embden-Meyerhof pathway
(Yun et al. 2003). Hence, homo fermentative
LAB are used in commercial production of
lactic acid. Some of the homo fermentative
lactic acid bacteria used in the production of
lactic acid are Lactobacillus delbrueckii,
Lactococcus lactis, Lactobacillus casei,
Lactobacillus helveticus, and Lactobacillus
acidophilus (Rojan et al. 2007).
Heterofermentative LAB produce less yield
due to formation of by-products.
Lactobacillus pentosus (Bustos et al. 2004),
Lactobacillus bifermentans, and
Lactobacillus brevis (Cui et al. 2011) are
some of the examples of heterofermentative
bacteria. Enterococcus mundtii (Abdel-
Rahman et al., 2011) and genetically-
engineered Lactobacillus plantarum (Okano
et al., 2009) have the capability to convert
pentose sugars into lactic acid by
homofermentative process. One of the key
reasons for usage of lactic acid bacteria in
industries is because it does not have any
adverse health effects (Rahman et al. 2013).
The properties such as high acid tolerance
and the ability to be engineered for selective
production of D-or L-lactic acid make lactic
acid bacteria commercially useful
(Rodrigues et al. 2017).
3.2 Fungi
Though majority of the lactic acid
production activities are performed by lactic
acid bacteria, some fungal species, such as
Rhizopus, with their amylolytic enzyme
activity, can convert starch into L(+) lactic
acid (Wee et al. 2006). Some other
advantages of fungal fermentation over
bacterial fermentation include low-cost
downstream process, low nutrient
requirements, and formation of fungal
biomass, which is an important by-product
(Zhang et al., 2007). Fungal fermentation
uses chemically defined medium, and so, the
purification of products is simple. This is a
major advantage in food industry (John et al.
2007). Generally, ethanol and fumaric acid
are the common by-products formed by
fungal fermentation (Litchfield, 2009; Vink
et al. 2010). The organisms of genus
Rhizopus are deemed as the best fungal
source for lactic acid production by fungal
fermentation (Rojan et al. 2007). Besides
Rhizopus, other fungi, like, the organisms of
genus Monilia and Mucor are also used in
lactic acid production (Prescott and Dunn,
1959). The main drawback of lactic acid
production by fungi is that the lactic yield is
reduced as the carbon is utilized for the
production of by-products besides lactic
acid. The limitations of production of lactic
acid by fungi also include mass transfer
limitation -which results in low production
rate, and requirement of vigorous aeration as
it is an aerobic process (Wee et al. 2006;
Park et al. 1998).
3.3 Yeasts
All the fermentation processes require
abundant amount of nutrient supply. In
many of the fermentation processes, not
only lactic acid, yeast is used as the key
nutrient source. The major advantages of
using yeast as nutrient source include their
tolerance against low pH (1.5), which
prevents the regeneration of precipitated
calcium lactate, thereby reducing the cost of
neutralization by neutralizing agents such as
calcium carbonate, and their ability to grow

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46
in mineral media (Rahman et al. 2013). The
yield of lactic acid produced by fermentation
with wild-type yeast as nutrient source is
low. With the advent of genetic engineering,
genetically modified yeasts capable of
producing high yield of lactic acid have
been created and they are being used
successfully (Bianchi MM et al. 2001). The
yeast of species Saccharomyces, Candida,
Zygosaccharomyces, and Pichia are
genetically modified to produce high yield
of lactic acid (Rahman et al. 2013). The
main drawback of using yeast as a nutrient
source is that it leads to increase in
production costs. However, corn steep liquor
(H. Oh et al. 2005), rice bran, and wheat
bran can be used as alternatives for yeast
(Yun et al. 2004).
4. Fermentation methods for lactic acid
production
Fermentation of lactic acid, like any other
fermentation process, is dependent on
factors such as raw materials used, nutrients
present in media, and the microorganisms
used. Three different methods of
fermentation are practiced, namely, Batch
fermentation, Fed-batch fermentation, and
Continuous fermentation.
4.1 Batch fermentation
In batch fermentation, all the required
materials such as carbon source, nitrogen
source and other components are added prior
to beginning of the fermentation process. It
is the most commonly practiced
fermentation process as it is simple to
perform. The major advantage of batch
fermentation is that it prevents
contamination to a good extent when
compared to the other methods as it is a
closed system, and so, high concentrations
of lactic acid is produced (Hofvendahl and
Hägerdal, 2000; Rahman et al. 2013). The
drawbacks of batch fermentation include
low productivity due to substrate inhibition
or product inhibition, and as the amount of
nutrients provided is limited, low cell
concentrations are obtained (Kadam et al.
2006; Yun et al. 2003). Batch fermentation
is mainly of two types, which are, Solid
State Fermentation (SSF) and Separate
Hydrolysis and Fermentation (SHF).
Solid State Fermentation (SSF) is a process
that occurs with no water or negligible
amount of water. Natural raw materials such
as wheat bran, rice bran, barley, fruit pulps,
sugarcane bagasse are used as carbon source
in this process (Pandey and Ashok 2008).
This process is used for the production of
pharmaceutical products, industrial
chemicals, feed, and fuel. Soccol et al.
(1994) were able to produce 137.0 g/l of
lactic acid at the rate of 1.38 g/l/h by using
Rhizopus oryzae by SSF. The usage of in
solid state fermentation process produced
the lactic acid yield of 0.97 g/g of recycled
paper (Marques et al. 2008). The advantages
of using solid state fermentation method
include high productivity, single reaction
vessel, rapid processing time, and less
enzyme loading (Abdel-Rahman et al.,
2011). In separate hydrolysis and
fermentation process, the raw materials are
first pre-treated and the unnecessary
compounds, such as lignin in the case of
lignocellulosic biomass, are eliminated.
Then, the raw materials are subjected to
enzymatic saccharification and the
hydrolysate formed is subjected to
fermentation (J. Choudhary et al. 2016). As
SHF is preceded by such a hefty process for
pre-treatment of raw materials, the real
productivity decreases (Rahman et al. 2013).
It was reported by Marques et al. (2008) that
the same Lactobacillus rhamnosus, which
produced lactic acid yield of 0.97 g/g of
recycled paper by SSF produced only 0.81
g/g when done by SHF method.

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47
4.2 Fed-batch fermentation
In Fed-batch fermentation, all the required
raw materials such as carbon source,
nitrogen source, and other required
components are added during fermentation
process at regular intervals of time without
removal of fermentation broth (Ding and
Tan, 2006). This type of fermentation is
especially useful to maintain low substrate
concentration by supplying nutrients to the
fermentation culture which in turn reduces
substrate inhibition (Rahman et al. 2011).
Ding and Tan reported that lactic acid
production by Lactobacillus casei using fed-
batch fermentation was found to be more
efficient in production of lactic acid than
other methods. They used 1% yeast extract
and glucose as raw materials and obtained
maximum lactic acid concentration of 210
g/l and L-lactic acid concentration of 180 g/l
at the rate of production 2.14 g/l/h, and the
yield of about 90.3% of L-lactic acid was
obtained. Dong-Mei Bai et al. (2003) used
Lactobacillus lactis for the production of L-
lactic acid and obtained about 97% yield of
L-lactic acid at the rate of 2.2 g/l/h.
4.3 Continuous fermentation
Continuous fermentation involves addition
of fresh medium to the fermenter while
withdrawing the already existing broth at the
same rate. Due to this, the concentrations of
substrates and products is maintained
constant (Rahman et al. 2013). The
advantages of continuous fermentation
include prevention of end-product
inhibition, less frequency to shut down, less
decrease in productivity during lag phase,
inoculation of culture is done once only,
high product yield can be obtained, saves
time and involves less labour work.
Continuous fermentation suffers from a few
drawbacks such as contamination,
requirement of field operator with expertise,
and it is expensive to perform. Shibata et al.
reported the production of lactic acid at the
rate of 1.56 g/l/h using Enterococcus
faecium by continuous fermentation (Shibata
et al. 2007). Ahring et al. reported the
production of lactic acid using Bacillus
coagulans (strain AD) at the productivity of
3.69 g/l/h using continuous fermentation
(Ahring et al. 2016).
5. Applications
Lactic acid is a hydroxycarboxylic acid with
wide range of commercial and household
applications. In the food industry, it is used
as a preservative, flavoring agent, pH
regulator, to increase shelf life, and to
control pathogens, among other applications.
As a food preservative, it inhibits putrefying
bacteria, which prevents spoilage of food.
Lactic acid, in the form of sodium or
potassium lactate, is used to extend the shelf
life of meat, poultry & fish. Due to its mild
acidic taste, lactic acid is used as an
acidulant in pickled vegetables, baked
goods, salads, and beverages. In case of
dairy industry, lactic acid is an excellent
acidification agent due to its natural
presence in dairy products, combined with
the dairy flavour and good antimicrobial
action of lactic acid. It is also used to
enhance savoury flavours. Lactic acid is
used in chocolates and sweets for the
flavour, as well as for obtaining the correct
pH. Other advantages of adding lactic acid
in chocolate and candy production include
ease of handling, low inversion rate, and
production of clear candies (Wee et al. 2006;
Corbion Purac).
In pharmaceutical industry, lactic acid is
used as an electrolyte in various
parenteral/intravenous solutions. These
parenteral solutions are prepared to
supplement bodily fluids. In addition to this,
lactic acid comes into play in pH-regulation,
chiral intermediate and metal sequestration
(Ramzi A et al. 2015). Lactic acid is also

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48
involved in mineral preparations, tablets,
prostheses, controlled drug delivery system,
surgical sutures, and in preparation of
dialysis solutions for dialysis processes like
Continuous Ambulatory Peritoneal Dialysis
using artificial kidney machines (Wee et al.
2006). Lactic acid, along with its salts, acts
as an intermediate in the manufacture of
pharmaceuticals, to adjust the pH of
preparations. Pharmaceuticals contain L (+)
lactic acid as the D (-) isomer is not
metabolized by the human body. Salts of
lactic acid such as Calcium, iron, sodium,
and other salts are used in pharmaceutical
industry because of their anti-tumour
activity (Ramzi A et al. 2015).
In the chemical industry, lactic acid is used
as neutralizer, cleaning agent, descaling
agent, pH regulator, and antimicrobial agent.
Lactic acid is an excellent remover of
polymer and resins due to its high solvency
power (Wee et al. 2006). Due to presence of
two reactive functional groups, that is, a
carboxylic group and a hydroxyl group,
lactic acid can undergo a variety of chemical
reactions that yield useful chemicals. The
following are the chemicals that lactic acid
gets converted into when it undergoes
various reactions:
Lactic acid has numerous applications in the
field of skin care and cosmetics. Studies
have found that lactic acid plays an essential
role as a skin brightener and it also aids in
removal of brown spots on skin (M Ochi et
al. 2004). Lactic acid acts as a moisturizer,
anti-acne agent, humectants, anti-tartar
agent, pH regulator, skin-lightening agent,
and skin-rejuvenating agent. Lactic acid acts
as a moisturizer due to its water retaining
capacity. Lactic acid promotes collagen
production, and hence it prevents wrinkles
and fine lines on skin and keeps the skin
firm. So, it acts as an anti-aging tool
(Bouwstra and Ponec 2006). Lactic acid also
has applications in manufacture of oral
hygiene products (Martinez et al. 2013). The
recent advent in lactic acid application is
Poly Lactic Acid (PLA). It is a
biodegradable plastic and has numerous
applications in day-to-day activities such as
food packaging, containers, trash bags,
protective clothing, etc. (Södergård and Stolt
2002; Vink et al. 2003).
6. Conclusion:
Lactic acid is widely used in
pharmaceutical, food, chemical and
cosmetic industries due to its easy
availability of raw materials, high
productivity, and low cost of production. It
is commercially produced in batch and
continuous methods. However, batch
fermentation method gives high
concentration, continuous fermentation
gives more productivity. In batch
fermentation lactic acid is produced by
bacterial genus Lactobacillus and fungal
genus Rhizopus. Genetically produced yeast
species Saccharomyces, Candida,
Zygosaccharomyces, and Pichia are found to
produce high yield of lactic acid but cost of
production is high. Batch fermentation is
carried out by Solid State Fermentation
(SSF) is found to be more effective than
Separate Hydrolysis and Fermentation
(SHF). Refined starch and cellulose
materials are commonly used as raw
materials for lactic acid production, however
the present trend of research is towards the
use of renewable resources such as
agricultural waste materials: corn cob, corn
stalks, bagasse, beet molasses etc. In this
study it is revealed that Fed-batch
fermentation using lactobacillus produces
high yield of lactic acid compared to other
methods. There is a lot of scope for the
production of lactic acid using renewable
materials as raw material sources.

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49
Table 1: Raw materials used in lactic acid production by fermentation and their yield
Raw Material
Microorganism
Lactic Acid
Yield
Reference
Corn
Enterococcus faecalis RKY1
63.5 g/L
H. Oh et al 2005
Wheat
Lactococcus lactis ssp. lactis ATCC 19435
106.0 g/L
K. Hofvendahl et al. 1997
Enterococcus faecalis RKY1
102.0 g/L
H. Oh et al. 2005
Molasses
Lactobacillus delbrueckii NCIMB 8130
90.0 g/L
C. Kotzanmanidis et al. 2002
Cellulose
Lactobacillus coryniformis ssp. torquens
ATCC 25600
24.0 g/L
R. Yáñez et al. 2003
Whey
Lactobacillus helveticus R211
66.0 g/L
A.W. Schepers et al. 2002
Lactobacillus casei NRRL B-441
46.0 g/L
A.O. Büyükkilci et al. 2004
Rice
Lactobacillus sp. RKY2
129.0 g/L
J.S. Yun et al. 2004
Wood
Lactobacillus delbrueckii NRRL B-445
108.0 g/L
A.B. Moldes et al. 2001
Apple pomace
Lactobacillus rhamnosus ATCC 9595
32.5 g/L
Gullón et al. 2008
Glycerol
E. coli AC- 521
56.8 g/L
Hong et al. 2009
Lignocellulose-
derived sugars
Enterococcus mundtii QU 25
129 g/L
Abdel-Rahman MA et al.
2015
Raw sweet
potato
Lactobacillus paracasei and Lactobacillus
coryniformis
198.32 g/L
Nguyen et al. 2013

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50
Field
Applications
Food industry
 Preservatives
 Acidulants
 pH regulators
 Mineral fortification
 Bacterial inhibition
Pharmaceutical industry
 Dialysis solution
 Mineral preparations
 Prostheses
 Surgical sutures
 Controlled drug delivery systems
 Parenteral/intravenous solution
Chemical industry
 Neutralizers
 Chiral intermediates
 Green solvents
 Cleaning agents
 Descaling agents
 pH regulators
Cosmetic industry
 Moisturizers
 Anti-acne agents
 Humectants
 Anti-tartar agents
 pH regulators
 Skin-lightening agents
Poly Lactic Acid
 Food containers
 Protective clothing
 Trash bags
 Rigid containers
Table 2: Applications of lactic acid in various fields (modified after Wee et al. 2006; Vink et al. 2003)
Reaction
Chemical produced
Hydrogenation
Propylene oxide
Decarboxylation
Acetaldehyde
Dehydration
Acrylic acid
Reduction
Propanoic acid
Condensation
2,3-pentanedione
Self-esterification
Dilactide
Table 3: Chemicals that lactic acid gets converted into via various reactions (Varadarajan et al. 1999)

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51
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