A Review of the Feasibility of Producing Polylactic Acid (PLA) Polymers Using Spent Coffee Ground

Abstract

Coffee is one of the most popular beverages in the world. Annual coffee consumption continues to increase, but at the same time, it generates a large amount of spent coffee grounds from the brewing process that give rise to environmental problems. An appropriate solution to manage these spent coffee grounds (SCGs) becomes crucial. Our project aims at discussing the feasibility of utilizing SCGs to synthesize polylactic acid (PLA) as a recycling application for SCGs. Producing PLA from SCGs offers the opportunity to contribute to the circular economy and sustainability due to the significant volume of coffee consumption. This approach reduces waste, provides environmental benefits, and promotes the use of renewable resources. Through repurposing SCGs into PLA, we can close the loop, minimize environmental impact, and create a more sustainable alternative to fossil-fuel-based plastics. This paper first discusses the properties and potential recycling applications of spent coffee grounds. The production of PLA from lactic acid and the potential processes for converting SCGs to lactic acid are then briefly discussed. From our review, it is feasible to utilize SCGs as the primary source for lactic acid production via bacteria fermentation and, further, to produce PLA via ring-opening polymerization. Possible ways to improve the yield and a corresponding cost analysis are also discussed.

1. Introduction

Coffee is one of the most-consumed popular beverages in the world. Arabica and Robusta are two main popular coffee species globally. In 2020, around 10.2 million tons of coffee were produced worldwide, while the world coffee consumption in 2020/2021 was 9.98 million tons [1,2]. The significance of the worldwide coffee industry can be seen in the fact that it employs over 100 million individuals in 80 nations [3]. With the influence of globalization, coffee drinking culture is not only popular in Western countries, but Asia has also gradually transformed to a major coffee drinking region. Coffee consumption in Asian countries has grown quickly over the years. The International Coffee Organization (ICO) predicted that coffee consumption would rise from 1.24 to 169.34 million bags by 2019/2020 [4]. According to the British Coffee Association [5], around 2 billion cups of coffee are consumed every day. Coffee has become an essential drink in our daily life. With such massive consumption of coffee, a large amount of associated waste products is inevitably generated. Spent coffee grounds (SCGs) are the wastes generated from the coffee brewing process. Researchers have found that one gram of ground coffee would generate 0.91 g of SCGs, while 550 to 670 g of residue coffee grounds are generated from one kilogram of coffee beans [6,7]. In the case of instant coffee, one kilogram of coffee powder creates two kilograms of wet SCGs [8]. Without proper management of the disposal of SCGs, they are generally disposed to the landfill.

In Malaysia, over 28,000 tons of residues from coffee beans, including parchment husks, coffee pulp, coffee husks, and SCGs, are produced annually, with the majority disposed of in landfills as mixed municipal waste, posing a threat to the environment due to their toxicity to plants and aquatic life [9]. As an alternative approach to reduce these consequences, SCGs can be converted into valuable products such as biodiesel, biogas, and fuel pellets through microbial degradation or recycling, in keeping with a zero-waste approach [10]. However, SCGs contain caffeine and other phytochemicals with high levels of eco-toxicity, making them unsuitable as soil amendments or fodder, as they may reduce ruminant acceptance and palatability. Environmental issues also arise from the disposal of SCGs in landfills, including the emission of greenhouse gases such as methane; soil pollution due to the release of organic residuals like caffeine, tannin, and polyphenols; as well as hazardous pathogens that can contaminate surface and groundwater [11,12,13,14]. Proper solutions to manage the ongoing production of SCGs are therefore crucial to mitigating potential environmental problems [12].

Polylactic acid (PLA) is one of the biodegradable plastics, which are synthesized by the polycondensation process of lactic acid. Lactic acids are produced through the bacterial fermentation of carbohydrates, such as corns, beets, even from agricultural wastes [15]. Our project aims at discussing the feasibility of utilizing the SCGs to synthesize PLA as a recycling application for SCGs. Producing PLA from SCGs offers the opportunity to contribute to circular economy and sustainability due to the significant coffee consumption volume. This approach reduces waste, provides environmental benefits, and promotes the use of renewable resources. Through repurposing SCGs into PLA, we can close the loop, minimize environmental impact, and create a more sustainable alternative to fossil-fuel-based plastics.

This paper discusses the properties and potential recycling applications of SCGs, the brief production process of polylactic acid, and the potential process for converting SCGs to lactic acid. In the following sections, we will provide a comprehensive overview of the potential for producing PLA from SCGs. We will begin by exploring the properties of SCGs and highlighting the drawbacks of disposing of them in Section 2. In Section 3, we will examine the current recycling applications of SCGs. Given the abundance of polysaccharides in SCGs, we propose using them as a feedstock for PLA production. In Section 4, we will review three main synthetic processes for producing PLA from lactic acid. We will discuss the production of lactic acid from SCGs and present our proposed process for producing PLA from SCGs. Section 5 will focus on potential strategies for improving the yield of PLA production and the corresponding cost analysis. Finally, a conclusion is given in Section 6.

2. Hazardous Ingredients of Spent Coffee Grounds

With the production of coffee beverages comes the creation of a significant number of coffee-derived materials (CDMs), which include coffee husk, parchment, chaff, and SCGs, having diverse physical properties and chemical compositions determined by the cultivation practices and processing technologies used [16]. Unlike other agricultural waste products, CDMs contain numerous highly hydrophobic compounds and macromolecules due to the inherent properties of coffee beans, i.e., Arabica, Robusta, Liberica, and Excelsa. SCGs, which represent a significant portion of CDMs, are non-biodegradable and produced in large volumes. They consist of approximately 38% cellulose, 7% protein, as well as carbohydrates, fats, minerals, and other ingredients. Despite their potential value, SCGs are often discarded as waste and contribute to environmental hazards when they accumulate in landfills or sewage systems [17,18].

The properties of SCGs have been studied as potential soil substrates. Results have shown that the pH is slightly acidic, with an average value of 4.3, and the electrical conductivity is 0.6 dS m⁻¹, indicating low salinity [19]. However, organic matter, total nitrogen, carbon fractionation, and cation exchange capacity data suggest that adding SCGs waste to soil may pose a risk of groundwater pollution due to high nitrogen content [18]. The physicochemical properties of SCGs, particularly total nitrogen, have significant impacts on enzymatic activities and microbial growth, which in turn influence soil fertility parameters such as microbial biomass carbon and nitrogen [20]. High concentrations of SCGs have been shown to have adverse effects on germination, seedling growth, and nitrogenase activity [21]. Therefore, it is crucial to treat or detoxify SCG agricultural waste before adding it to soil to prevent potential harm.

SCGs are a promising resource to produce bio-based poly(3-hydroxybutyrate) (P(3HB)). However, the presence of hazardous heavy metals in SCGs, including chromium (Cr), nickel (Ni), lead (Pb), mercury (Hg), and cadmium (Cd), raises concerns for human health and the environment [22]. The mobility of these heavy metals in SCGs poses a risk for potential contamination. One should not dispose of SCGs directly in a treatment plant or landfill due to their chemical properties, which makes such disposal methods impractical and cost-ineffective. Moreover, the improper disposal of SCGs can cause harm to the environment and human health. The following paragraphs provide more details on the potential harm caused by the improper disposal of SCGs. The alternative options for SCG utilization to minimize such harm and protect the environment will be discussed in Section 3.

The improper management of SCGs can have significant negative impacts on the environment and human health. Moreover, it can result in the depletion of natural resources such as land, water, and energy. When SCGs are disposed in landfills, they can release methane, a potent greenhouse gas, and leachate, a liquid waste that can pollute soil and water with heavy metals and organic compounds [23,24]. Incinerating SCGs is not an ideal solution either, as it can emit pollutants such as carbon monoxide, nitrogen oxides, and particulate matter, contributing to air pollution and respiratory problems [24]. The disposal of SCGs in water bodies can also have harmful effects. The release of nutrients from SCGs can contribute to eutrophication, harmful algal blooms, and the presence of toxic organic compounds such as caffeine and phenols, which can harm aquatic organisms [22,25,26]. When SCGs are not adequately managed and decompose, they can release methane and carbon dioxide, both of which are greenhouse gases that contribute to climate change [22]. They are further detailed in the following paragraphs.

When one disposes of SCGs directly in landfills, the organic matter in the waste breaks down and releases methane gas. Methane is a potent greenhouse gas, with a global warming potential 20 times greater than carbon dioxide. SCGs have a high moisture and oil content, which can lead to increased methane emissions during their biodegradation [12,27]. Although methane itself is odorless and difficult for us to detect, it can contribute significantly to climate change and environmental damage. The chemical formula for methane is CH4, consisting of one carbon atom and four hydrogen atoms. While carbon dioxide is the primary greenhouse gas of concern in terms of trapping heat inside our atmosphere, methane also plays a significant role through creating a barrier between the earth’s surface and atmosphere. This barrier prevents energy from escaping into space, which contributes to the warming of the planet.

The coffee industry currently relies heavily on landfills for its waste disposal, but significant amounts of coffee waste can also be found in other areas such as streets, pavements, and riverbeds. Despite being a common waste disposal option, landfills have significant drawbacks, including the potential for leaching from SCGs and negative impacts on underground water sources [28]. Additionally, caterers and cleaning staff often dispose of SCGs through pouring them down drains after brewing. This discharge contains high levels of nitrogen and phosphorus, which act as pollutants and can contribute to an increase in algae growth [18]. As algae consume oxygen that is critical for the survival of other aquatic plants and animals, this can result in an imbalance in the oxygen content of the water. Rotting algae also produces organic matter that limits light penetration and depletes the water of dissolved oxygen, posing a risk to other aquatic life.

Traditional coffee processing methods typically generate significant amounts of solid waste. Alternatively, the wet processing of coffee cherries offers a promising solution for managing coffee waste, as it generates substantial amounts of organic compounds such as fatty acids, lignin, cellulose, hemicellulose, and other polysaccharides [20]. Nevertheless, this process also generates a considerable quantity of coffee processing wastewater (CPW), which is high in suspended organic matter and organic and inorganic chemicals [18]. This wastewater has the potential to be highly polluting and must be treated before being released into the environment to prevent the contamination of underground water systems [29]. The direct discharge of untreated wastewater from coffee factories into surface waterways can also result in high levels of organic contaminants, posing risks to nearby water bodies, human health, and the aquatic ecosystem [20,22].

To minimize the adverse environmental effects of SCGs, proper waste management practices are necessary. These practices include composting, anaerobic digestion, and conversion to value-added products such as biogas, biofuels, and bioplastics. These approaches can help reduce greenhouse gas emissions, prevent water pollution, and promote a more circular economy [30].

3. Properties and Potential Recycling Applications of Spent Coffee Grounds

Spent coffee grounds (SCGs) are a rich source of polysaccharides, primarily in the form of hemicellulose and cellulose, which make up almost half of their weight [31,32]. Hemicellulose is the dominant component and is composed of mannose, galactose, and arabinose, while glucose is the primary component of cellulose. Hemicellulose is a heterogeneous polymer that contains hexoses, and sugar acids and has potential applications in the production of biofuels and chemicals [33]. Additionally, SCGs contain a significant amount of lipids, ranging from 2 to 20 wt.% [30,34], with linoleic, palmitic, oleic, and stearic acids being the predominant fatty acids in SCG oil [32]. SCGs also contain protein, caffeine, melanoidins, minerals, and polyphenols [30].

Due to the adverse environmental impact of discarding SCGs in landfills (see the discussion in Section 2), it is imperative to explore alternative ways of utilizing this waste material.

Table 1. Potential recycling usages for SCGs.

Possible Applications of Spent Coffee Grounds (SCG)
Industry	Technologies/ Process/Uses	Examples	Main Limitations	Ref.
Renewable Energy	Extraction	Biodiesel	Lead to secondary pollution Demanding condition	[8,32,36]
	Extract + Straw	Bioenergy, pellets
Environmental Remediation	Absorption	Removal of heavy metal Deodorization	Energy intensive Limited effectiveness	[15,35,37]
	Pyrolysis	Biochar
Agriculture	Composting	Fertilizers, soil improver, plant cultivation	Nutrient imbalance Not feasible for large scale industrial application	[32,35,38]
	Substrate	Mushroom growth	Allergenic potential
Healthcare Industry	Extraction of Bioactive Compound	Oils to produce soaps; Phenolic substances	High initial cost High standard for raw material	[32,39,40]
Food Industry		Food ingredient
Construction Industry	/	Bricks	Limited usage Not feasible for large scale industrial application	[35,41,42,43]
Polymer production Industry	Hydrolysis Carbohydrates, Oil	Cellulose-type polymers (Bioplastic/Lactic acid)	Complex operation	[44,45,46,47,48,49]

provides a summary of the potential recycling usages for SCGs [35], which can be broadly categorized into areas such as renewable energy, environmental remediation, agriculture, healthcare, food production, construction industries, and polymer production. We also list examples and the main limitations of each recycling usage in Table 1. To obtain a comprehensive understanding, however, kindly refer to the cited references in the table. Rather than a detailed enumeration of each usage, the following paragraphs provide a more in-depth discussion of the various ways in which SCGs can be recycled.

Studies conducted by Colantoni et al. and Silva et al. have demonstrated that SCGs possess a high calorific value, exceeding 5000 kJ/kg [8,50]. Furthermore, SCGs have a low ash content, which makes them a promising alternative energy source [8,32]. SCGs can be utilized in the extraction of oil to produce biodiesel, which could potentially offer a sustainable source of fuel.

SCGs possess excellent absorbent properties, making them well-suited for use as filters to remove heavy metals such as cadmium, copper(II), and zinc. Additionally, SCGs have a high water and oil holding capacity, which makes them suitable for conversion into biochar through the process of pyrolysis [35]. SCG biochar has been shown to be effective in absorbing heavy metals, metal ions, and pharmaceutical compounds, making it a promising material for environmental remediation [15,35,37]. However, it should be noted that the conversion of SCGs into absorbents may not be economically feasible for large-scale industrial applications.

SCGs are a potential source of fertilizer due to their high nitrogen content. However, SCGs also contain phytotoxic compounds, such as caffeine, tannins, and polyphenols, which can have adverse effects on soil fertility and plant growth when used as a raw material. To mitigate these effects, studies have suggested using SCGs as an organic amendment through combining them with other organic materials. This can reduce the phytotoxic effect and enhance soil biology and functioning [35,38]. SCGs are also rich in protein, potassium, magnesium, and phosphorus, making them a suitable material for composting and as a substrate for fermentation processes [32,35]. Additionally, SCGs have good antioxidant potential, which makes them a potential source for extracting antioxidant compounds for use in food production, cosmetics, and the pharmaceutical industry [32].

The melting point of SCGs has been measured to be around 77 °C, and they undergo decomposition and the depolymerization of oil and polysaccharides at temperatures above 200 °C [32]. Ballesteros et al. [32] have also observed that SCGs have a crystalline structure, with the cellulose component contributing to the crystalline structure and providing high tensile strength. In Japan, Starbucks has developed a method of using special lactic acid bacteria and SCGs to produce cattle feed, with the aim of improving milking efficiency [39]. Another Japanese company, SOI, has successfully turned SCGs into coffee bars called COLEHA1 through fermenting and pasteurizing the coffee paste [40]. In England, Bio-bean has upcycled coffee grounds into high-calorific-value coffee logs or pellets, which can reduce greenhouse gas emissions by up to 80% compared to sending them to landfills [36]. Despite these successes, the large-scale implementation of these recycling methods is still limited.

SCGs have shown promise as a sustainable material for use in green construction. Traditional building brick manufacturing processes generate large amounts of greenhouse gases, leading to increased interest in incorporating SCGs into bricks. Muñoz Velasco et al. found that adding SCGs to clay bricks can improve building insulation in a sustainable way, with the thermal conductivity of eco-fired clay bricks reduced by 25.7% with the addition of 11% SCGs [41]. Another method of incorporating SCGs into bricks is through alkali-activation. Chung et al. discovered that adding a small amount (1–2.5%) of SCGs as an additive to unfired clay bricks can achieve the lowest compressive strength requirement of building bricks [42]. However, excessive amounts of SCGs can induce microorganism growth and hinder the strengthening effect. Other studies have investigated the use of a novel geopolymer formed by SCGs and bagasse as a green construction material for pavement, with promising results [43]. However, the large-scale implementation of these recycling methods is still limited. In addition to their use in construction, SCGs can also be incorporated into polymer materials. Stylianou et al. found that adding SCGs to poly(butylene adipate-co-terephthalate) has a bio-reinforcing effect, making it suitable for food packaging and manufacturing industries [35]. The polymeric composite made of SCGs and polyethylene has good stability against thermal and photo-oxidative degradation, making it suitable for use in healthcare industries.

The chemical composition of SCGs makes them a valuable resource for various processes, including the production of bioplastics, lactic acids, and other materials, regardless of the type or origin of the coffee (Arabica, Robusta, Liberica, and Excelsa) [51], as shown in Table 1 (the last recycling application). SCGs contain significant amounts of cellulose and hemicellulose, making them a promising feedstock to produce cellulose-type polymers. Cellulose-type polymers that can be produced from SCGs include cellulose nanocrystals (CNCs), cellulose acetate (CA), cellulose esters, and cellulose-based hydrogels.

CNCs are nanomaterials with unique properties such as high strength, stiffness, and biodegradability, making them suitable for diverse applications such as packaging, coatings, and biomedical devices [45,46,52]. CA is a thermoplastic polymer that has high transparency, good mechanical strength, and biodegradability, making it suitable for various applications such as films, fibers, and membranes [44,47,52]. Cellulose esters, such as cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB), have good solubility, low toxicity, and biodegradability, making them suitable for different applications such as coatings, adhesives, and inks [45,48]. Cellulose-based hydrogels can be produced from SCGs via crosslinking with different crosslinking agents [49]. These hydrogels have high water absorption capacity, good mechanical strength, and biodegradability, making them suitable for various applications such as wound dressings, drug delivery, and tissue engineering [53]. While cellulose-type polymers have many potential applications, extracting cellulose and hemicellulose from SCGs can be complex and may require harsh chemicals. The yield of cellulose can be low, and some polymers may require additional processing. Moreover, the biodegradation rates for these polymers can vary, which may lead to waste accumulation. Hence, producing other kinds of polymers from SCGs is considered.

Polylactic acid (PLA) is a versatile and environmentally friendly polymer that has gained popularity as a sustainable alternative to traditional petroleum-based plastics. PLA is a biodegradable polymer which is derived from renewable resources that possess several unique properties, including biocompatibility, excellent degradation, good workability, thermoplasticity, and a transparent nature [54,55,56]. Its resistance to fats and oils make it a promising material for a wide range of applications, including biomedicine and packaging [46]. Moreover, PLA requires 22–25% less energy to produce than petroleum-based polymers [57], making it more sustainable and cost-effective.

The production of polylactic acid (PLA) from cornstarch or sugarcane has raised concerns regarding competition for food resources and land use [58], which can have negative social and environmental impacts [59]. Note that the global production capacity of bioplastics is expected to experience significant growth, reaching approximately 6.3 million tons in 2027 from around 2.2 million tons in 2022 [60]. The land area allocated for bioplastic production is expected to increase from 0.015% to 0.058%. As a result, there is a growing interest in utilizing alternative feedstocks for PLA production that do not compete with food crops. One such feedstock is SCGs, which in principle can be converted into PLA through a series of processes, thereby promoting a sustainable and environmentally friendly approach to feedstock sourcing. The conversion of SCGs into PLA reduces waste and aligns with circular economy principles through utilizing a byproduct that would otherwise be discarded. Obviously, PLA’s unique properties make it an excellent choice for converting SCGs into PLA instead of other materials, promoting sustainability and waste reduction while utilizing renewable resources.

In the upcoming section, we will present a brief explanation of the three main synthetic processes utilized to produce PLA from lactic acid. Afterwards, we will investigate the feasibility of generating PLA from SCGs.

4. Production Processes of PLA

Figure 1 shows the proposed process to produce PLA from SCGs, which will be further explained in the coming sections. The upcoming paragraphs will provide a brief discussion of both the synthetic processes used to produce PLA from lactic acid as well as the feasibility of generating PLA from SCGs. To obtain a comprehensive understanding, however, kindly refer to the cited references in each block of Figure 1. Apart from those highlighted in Figure 1, the oil extracted from SCGs may also act as plasticizer or lubricant in PLA composite fabrication. During this process, SCGs could be converted to lactic acid via bacterial fermentation and eventually transformed to PLA, which gives a circular usage life cycle for coffee waste and reduces the production cost of PLA. Based on the literature, there is a high feasibility of utilizing SCGs to produce biodegradable thermoplastics—PLA. The challenges for our research are how to perform bacterial lactic acid fermentation and ring-opening polymerization at a laboratory scale.

4.1. Producing Lactic Acid from SCGs

Lactic acid is a natural hydroxy acid, and according to Breton-Toral, Trejo-Estrada, and McDonald, it is widely used as an acidulant, seasoning, or preservation agent in the food sector and as a pH controller in the pharmaceutical business [70], as well as for PLA production. It is made biosynthetically via fermenting carbohydrates like glucose in the presence of pure cultures of lactic-acid-producing microorganisms. In a more detailed explanation, lactic acid is a chiral molecule with two enantiomers, L(+) lactic acid and D(−) lactic acid. These isomers can be obtained through the fermentation of renewable resources via selecting microorganisms that produce the desired isomer [71]. For instance, the bacteria genus Lactobacillus is known to produce D(−), L(+), and racemic mixtures of lactic acid, while Pediococcus produces pure L(+) isomer or racemic mixtures [71,72]. Leuconostoc and Oenococcus are known to produce the D(−) isomer, while Weissella can produce either the D(−) isomer or racemic mixtures [71,72]. Lactic acid is made biosynthetically through fermenting carbohydrates like glucose in the presence of pure cultures of lactic-acid-producing microorganisms. At present, the primary method for producing lactic acid involves the bacterial or yeast fermentation of carbohydrates (e.g., glucose) obtained from agricultural crops such as corn. Microbial fermentation is the most common method for industrial lactic acid production, but high substrate prices remain a challenge for large-scale manufacturing. Additionally, the use of refined sugars and starches as substrates can compete with food and feed supplies, making low-cost alternatives such as lignocellulosic biomass more attractive for long-term lactic acid manufacturing.

SCGs are a potential low-cost substrate for lactic acid fermentation, as they are produced in large quantities and could provide an alternative feedstock for PLA production [64].

SCGs can be easily obtained from nearby coffee shops and require a preconditioning process before use (see the pretreatment in Figure 1). SCGs should be dried in an oven at 105 °C until they are fully dehydrated, after which they should be refrigerated at 5–7 °C until they are used for lactic acid production. SCGs are known for their high content of carbohydrates, lipids, proteins, and minerals, and the extraction and utilization of their individual fractions have garnered a lot of interest. Carbohydrates constitute about half of the weight of a coffee bean, with hemicellulose polysaccharides such as mannans, galactans, and arabinans (30–40 wt.%) and cellulose (8–15 wt.%) making up the remaining portion. These polysaccharides can be hydrolyzed to produce fermentable sugars like glucose, mannose, galactose, and arabinose. Microbial fermentation of these sugars can lead to the production of lactic acid, acetic acid, succinic acid, polyhydroxyalkanoate (PHA), and other compounds of interest. The process of converting biomass to lactic acid involves acid hydrolysis with H₂SO₄, pH adjustment with CaCO₃/NaOH, filtration, enzymatic hydrolysis with commercial enzyme ACCELLERASE^® 1500, fermentation, isolation, and purification [61,63,64,65]. Lactic acid has been successfully produced from SCGs, as demonstrated by Wang et al., Hudeckova et al., and Kim et al. [61,63,64,65]; meanwhile, a recommended process for PLA synthesis from lactic acid will be discussed in Section 4.2.

In order to produce lactic acid from SCGs, Wang followed several steps [65]. Firstly, the solvent extraction of SCGs was carried out using hexane for 12 h, followed by evaporation of the solvent. Next, high-solids dilute acid hydrolysis was performed on the SCGs using 5.3% sulfuric acid at 100 °C for 118 min, and the resulting hydrolysate was adjusted to pH 4.9 using sodium hydroxide. Enzymatic saccharification was then performed through autoclaving the hydrolysate with sodium citrate buffer (pH 4.8), adding commercial enzyme ACCELLERASE^® 1500 and sodium azide, and denaturing the enzyme with a dry block heater. The resulting mixture was centrifuged. Finally, the SCG hydrolysate was diluted with DI water to 20 g/L, and Lipomyces starkeyi was added for microbial oil fermentation. After 7 days of incubation, the crude microbial oil was obtained through separating the cells from the fermentation broth. In their study, Wang et al. found that a 5.3% (w/w) sulfuric acid concentration and 118 min reaction time resulted in a mean yield of 563 mg/g of reducing sugar from the enzymatic saccharification of defatted SCGs, which corresponds to around 81.5% conversion of SCGs’ total carbohydrates. These findings have implications for insight into optimizing the acid hydrolysis process.

In the laboratory, the production of lactic acid from SCGs is advised to take place through a slurry process. Kim et al. used Saccharomyces cerevisiae to produce lactic acid from SCGs [63]. Adopting a whole-slurry simultaneous saccharification and fermentation method, they found that the yield of lactic acid and ethanol was 413% and 221% higher than those containing a solid fraction of SCGs [63]. Kim et al. suggested it might due to the attribution of hemicellulose in the whole slurry, as there are high concentrations of lignin in the solid fraction [63].

Potential bacteria for digesting SCGs have been investigated in El-Sheshtawy et al.’s study [62]. They have found that Kosakonia cowanii could be used as digesting bacteria for the biological production of lactic acid at the laboratory level, while coffee waste hydrolysate could be the carbon source [62]. Another study employed five species of lactic acid bacteria, namely Lactobacillus plantarum, L. rhamnosus, L. delbrueckii subsp. Bulgaricus, S. thermophilus, and B. coagulans [65]. The highest lactic acid concentrations were obtained after 48 h of inoculation at pH 7 with L. rhamnosus [64]. Another study on lactic acid production from coffee pulp utilized the thermophilic bacterium Bacillus Coagulans [68], while Wang suggested the use of Lipomyces starkeyi for microbial oil formation from SCGs, as there is no evidence of inhibition [65].

Hudeckova et al.’s study also showed that SCGs can be converted into lactic acid through a combination of dilute acid treatment and cellulase application, with Lactobacillus rhamnosus CCM 1825 identified as the most promising micro-organism for this purpose [64]. Despite the presence of inhibitory furfural and phenolic compounds in the medium, high concentrations (25·69 ± 1·45 g L⁻¹) and yields (98%) of lactic acid were achieved [65]. These findings suggest that SCGs could serve as a promising feedstock for the sustainable large-scale production of lactic acid.

After the fermentation process, the culture broth of spent coffee grounds is rich in fibers and salt ions, as the latter were used to regulate the pH during production. However, salt impurities have been found to negatively affect the polymerization of PLA, as reported by D. Pleissner et al. [61]. Therefore, downstream processes such filtration, softening, electrodialysis, chromatography, and distillation are suggested; the methods of the isolation and purification process are explained in detail by D. Pleissner et al. [61]. These processes are used for the separation of fibers and salt ions as well as the concentration of lactic acid.

Note that the reported methods for lactic acid production from SCGs are primarily derived from laboratory-scale experiments. One should select the appropriate methods based on their specific circumstances or requirements.

Table 2. Summary of methods for producing lactic acid from SGCs.

Operating Condition	Micro-Organism	Highlight/Result	Ref.
2.7% H2SO4 20 min (acid hydrolysis) + Enzymatic hydrolysis	Bacillus coagulans Lactobacillus rhamnousus	Bacillus coagulans has the ability to grow and metabolize at high temperatures. It requires simple nutritional conditions and is able to produce highly pure L(+) isomers. Lactobacillus rhamnousus will work well in lignocellulose-based substrates.	[64]
1% H2SO4 30 min (acid hydrolysis)	S. cerevisiae	Acid-pretreated SCGs perform better than water-pretreated SCGs. Acid could wash out lignin, which is not suitable for lactic acid production. Whole slurry of pretreated SCG will have a higher yield. Hemicellulose fraction in whole slurry.	[63]
5.3% H2SO4 118 min (acid hydrolysis) + ACCELLERASE 1500®	Lipomyces Stakeyi	Dilute acid hydrolysis followed by enzymatic saccharification helps the conversion of reducing sugar.	[65]
1.8% H2SO4 30 min (acid hydrolysis) + ACCELLERASE 1500® + downstream process {filtration, softening, electrodialysis, chromatography, distillation}	Bacillus coagulans	Micro- and nanofiltrations: separation of fibres and salt ions from the culture broth. Monopolar electrodialysis: created a concentrated salt stream. Bipolar electrodialysis: further purification of the salt-rich fraction obtained from monopolar electrodialysis. Anion-/cation-exchange chromatography: removal of anions and sodium from lactic acid. Distillation: final concentration step performed via water evaporation under vacuum.	[61]
5% HCl 20 min (acid hydrolysis) + downstream process {end product extraction, purification, precipitation, quantification of end product}	Kosakonia cowanii	For kosakonia cowanii to produce lactic acid, the optimum pH is 6.5 and the RPM is 150. Higher or lower values may affect productivity.	[62]

lists the necessary operating conditions and microorganisms used, and it highlights key results of the methods for producing lactic acid from SCGs. To obtain a comprehensive understanding, however, kindly refer to the cited references in the table.

4.2. Producing PLA from Lactic Acid

The primary feedstock for polylactic acid (PLA) production is lactic acid, which is typically produced via the fermentation of carbohydrates [15,66,69]. Meanwhile, the cyclic monomer in PLA is lactide (cyclic dimer of lactic acid). Figure 2 provides an overview of the polylactic acid production processes, highlighting the different steps involved in each process. There are three main synthetic processes used for PLA production. The first is direct polycondensation, which involves the direct polycondensation of lactic acid molecules to form the polymer. Since the dehydration process becomes more difficult with the increase in viscosity, it results in low-molecular-weight product formation (Mw < 50,000 g mol⁻¹) [68]. The second is a two-step polymerization process, which involves first converting lactic acid into a lactide monomer via prepolymerization (with low repeating unit), which is then polymerized to form polylactic acid (average molecular number above 20,000 mol⁻¹) [73]. The third and most widely used process is ring-opening polymerization (ROP), as it could produce a higher-molecular-weight product (Mw > 100,000 g mol⁻¹) [68]. This reaction involves the three stages of oligomerization, the de-polymerization of oligomers, and the ring-opening polymerization of lactide monomers in the presence of a catalyst [15,66,68]. PLA has been successfully produced from food waste using a recommended process developed by Hu et al. [68].

Polycondensation is a process used in polymer production where monomers are combined to form a polymer while removing byproducts such as water and alcohols. In direct polycondensation, carboxyl and hydroxyl groups are joined together, producing water molecules as a byproduct. Polycondensation is a cost-effective and straightforward method for PLA production, as it requires basic equipment and moderate temperatures (<180 °C) [68]. However, this method has several disadvantages, including low yield and low purity due to residual solvents and byproducts in the resulting polymer [68]. The byproduct is difficult to remove during the process, and the resulting PLA is typically of low molecular weight, which makes it weak and brittle in nature [66,68]. Additionally, the process generates solvent waste and pollution, which can have negative environmental impacts.

Two-step polymerization involves the production of prepolymers, or oligomers, from melted lactic acid. Through polycondensation, oligomers with molecular weights reaching the tens of thousands are produced [73]. The prepolymers are then polymerized at a temperature between the glass transition point and melting point to produce a higher molecular weight of PLA [68]. This process has the potential to yield high-purity and high-molecular-weight PLA under moderate conditions(150–250 °C) [68]. Two-step polymerization for PLA production can be time-consuming and energy-intensive, and the use of a catalyst like tin octoate can have potential toxicity and environmental impact. Impurities like lactide and water can also form during the process and negatively affect the properties of the final product.

The highest molecular weight of PLA can be achieved through ring-opening polymerization (ROP), which is widely used due to its flexibility in producing a wide range of molecular weights suitable for different purposes [68]. In this process, lactic acid undergoes oligomerization and condensation to form lactide monomers [15,68]. Different initiators can be used in ROP, resulting in different reaction mechanisms, such as anionic polymerization and cationic polymerization [15]. Common catalysts used in ROP include aluminum and tin alkoxides [15]. Lactide monomers can exist in different diastereomeric forms, including L-lactide, D-lactide, and DL-lactide [15]. Most properties of PLA made from various forms of lactide are similar, except for DL-lactide, which results in an amorphous polymer [15]. At the final step of ROP, the lactide monomers link up to form a long chain of polylactic acid via condensation, with water molecules produced as a byproduct. In general, ROP is more sensitive to impurities compared to two-step polymerization but has the feature of less negative environmental impact.

In Hu et al.’s studies, high molecular weight poly(lactic acid) (150,000 g mol⁻¹) was produced using food-waste-derived lactic acid and zinc oxide nanoparticle dispersion as a catalyst for ring-opening polymerization [69]. The lactide product was separated, purified, and then polymerized using tin(II) 2-ethylhexanoate and an initiator [69]. Furthermore, the study found that an aqueous dispersion of nano-ZnO was a superior catalyst in lactide synthesis, offering advantages in reaction rate, production yield, and chemical stability over tin(II) 2-ethylhexanoate [69]. Based on the results, Hu et al. proposed an optimal reaction procedure with a catalyst dosage of 0.6 wt.%. This protocol involves dehydration at 60–80 °C and 60 kPa for 2 h, oligomerization at 150 °C and 10 kPa for 3 h, and depolymerization at 180–220 °C and 1 kPa for 3 h [70].

5. The Feasibility Analysis

The production of PLA from SCGs involves the recycling and repurposing of coffee waste, aligning with the principles of circular economy. To assess the feasibility of this production process, it may be appropriate to utilize measuring methods proposed for evaluating circular economy practices [67]. Specifically, certain indicators designed for bio-based products are particularly relevant in investigating the feasibility of PLA production from SCGs [70]. Despite this, there may be challenges due to uncertainties in certain process details. To demonstrate the feasibility, we investigate the possible ways for yield improvement and the possible associated cost of the production process.

5.1. Possible Ways to Improve the Yield of PLA Production from SCGs

The production of PLA from SCGs faces challenges due to the low concentration of glucose and impurities like caffeine and tannins, which can inhibit fermentation process [73]. However, controversy exists around the feasibility of PLA production from SCGs, as some argue that other sources of feedstock like corn starch and sugarcane may be more efficient and economical. On the other hand, several approaches can be employed to improve the yield of PLA from SCGs [74]. Next, the pretreatment of SCGs can improve the accessibility of cellulose and hemicellulose to enzymes during hydrolysis, increasing the yield of PLA [74,75]. Furthermore, enzymatic hydrolysis is a crucial step in converting SCGs to PLA, and optimizing the conditions for enzymatic hydrolysis can improve the yield of glucose and xylose from SCGs [74,76,77]. Additionally, selecting appropriate microorganisms for fermentation, such as bacteria and yeast with a high tolerance to inhibitors, can also improve the yield of lactic acid and PLA from SCGs [78,79]. Moreover, co-culture fermentation and integration of processes like extraction, hydrolysis, fermentation, and polymerization can improve efficiency, yield, and cost-effectiveness [52,67,74,78,80,81].

5.2. Cost Analysis of PLA Production from SCGs

At the moment, it is hard to find a convincing cost analysis of the production of PLA from SCGs. Surely, the cost analysis should involve various cost factors, such as the yield of PLA and the cost of SCGs, enzymes, fermentation, and downstream processing [58,82,83,84,85]. The cost of producing PLA includes stages of extracting raw materials; using resources like seeds, fertilizers, and fuel; glucose extraction; fermentation; and polymerization [58,82,83,84,85]. Electricity, heat, process water, acids, lime, nutrients, and other chemical materials are required in these stages [5,86]. Additionally, there are costs associated with additives and waste disposal, for instance, chemicals, nutrients, and gypsum waste [58]. Furthermore, process yields, capital costs, labor costs, operating costs, and utility expenses influence the cost of PLA production [85]. Energy use during the process also plays a significant role in the cost of PLA production, especially in the refining process [85,87]. The costs associated with additives and waste disposal also depend on the choice of feedstock and the subsequent technological processing steps [58,87]. Due to the use of innovative raw materials such as SCGs and the current state of technology development, it may not be possible to provide a precise estimation of the costs mentioned above. There have been cost analyses conducted for producing PLA from corn and agricultural waste.

Table 3. A summary of results from the cost analyses of different projects.

Project	Feedstock	Min Cost per Ton (USD)	Max Cost per Ton (USD)
Manandhar and Shah (2020) [82]	Corn grains	844	1251
Sanaei and Stuart (2018) [83]	Triticale	911	1496
Wellenreuther et al. (2022) [58]	Corn grain and stover	1004	1374
Chiarakorn et al. (2011) [84]	Cassava roots	2410	2620
Kwan et al. (2018) [85]	Food waste powder	1066	3558

provides a summary of the results from these cost analyses. Note that the cost estimations presented in Table 3 can vary significantly. Through examining the various perspectives and approaches adopted in the cost analyses, it is possible to better understand the potential economic viability and feasibility of PLA production from SCGs. We therefore discuss these cost analyses one by one in the upcoming paragraphs.

Manandhar and Shah (2020) found that producing PLA from potatoes and wood chips in Maine is economically viable, using local biomass feedstocks and advanced fermentation technology [82]. This suggests that using alternative feedstocks such as SCGs for PLA production could also be cost-effective. As SCGs are readily available and abundant, using them for PLA production aligns with circular bio-economy principles. Future research on the cost analysis of producing PLA from SCGs could provide insights into the economic viability of this technology and its potential as a sustainable solution for reducing waste and producing bioplastics.

Sanaei and Stuart (2018) employed a techno-economic analysis approach, combined with a multi-criteria decision-making (MCDM) approach to identify investment opportunities in triticale-based biorefineries [83]. Through systematically identifying promising biorefinery strategies, their study considered both business-strategy-oriented and profitability-oriented criteria. Evaluating sustainability using the internal rate of return, downside internal rate of return, and resistance to supply market uncertainty, these criteria could also be applied to assess the economic feasibility and sustainability of PLA production from SCGs.

Wellenreuther et al. (2022) used a Monte Carlo analysis model to demonstrate the competitiveness of PLA production from second-generation feedstocks, such as corn stover, compared to established large-scale corn-grain-based production [58]. The use of nascent technology for incorporating innovative raw materials in PLA production can lead to high energy intensity and increased costs. However, as production processes advance and technology matures, the learning curve effect results in significant energy cost reductions, with an assumed average annual decrease of 2% in their research. This cost reduction is achieved through increased experience, knowledge, and improved processes, enabling producers to achieve economies of scale and optimize energy resource use.

Chiarakorn et al. (2011) used cost–benefit analysis to evaluate the net social benefits of producing PLA from cassava root, finding that it generated positive net benefits, with integrated PLA production further benefiting from byproduct sales and carbon credits [84]. This suggests that PLA production from alternative feedstocks like SCGs may yield similar positive results, indicating the potential of SCGs as a PLA production feedstock.

Kwan et al. (2018) proposed a techno-economic analysis to model the food waste valorization process for producing lactic acid, lactide, and poly (lactic acid), highlighting the potential of utilizing food waste in sustainable and economically viable bioplastic production [85]. This approach aligns with the principles of the circular bioeconomy, which aims to minimize waste and maximize resource efficiency. Through valorizing SCGs, waste generation can be reduced while producing valuable bio-based products. Similarly, a cost analysis of PLA production from SCGs could provide valuable insights into the economic viability and potential sustainability of this technology for waste reduction and bioplastic production.

6. Conclusions

This paper has summarized the properties of spent coffee grounds (SCGs), from their composition to their possible recycling applications. Doubtlessly, environmental issues will arise from the improper disposal of SCGs. Examples include the emission of greenhouse gases such as methane and soil pollution due to the release of organic residuals like caffeine, tannin, and polyphenols, as well as hazardous pathogens that can contaminate surface- and groundwater. Hence, it has been proposed to convert SCGs into valuable products such as biodiesel, biogas, and fuel pellets through microbial degradation or recycling. Nevertheless, most of the proposals are in the experimental stage and may not be feasible for production. We observed there is a high feasibility of employing SCGs as alternative raw material for lactic acid production. Combining insights from others, we suggested a possible flow for lactic acid production from SCGs. Through the ring-opening polymerization process, the produced lactic acid can be then converted to a green biodegradable polymer, polylactic acid (PLA). Producing PLA from SCGs offers the opportunity to contribute to the circular economy and sustainability due to the significant volume of coffee consumption. This approach reduces waste, provides environmental benefits, and promotes the use of renewable resources. Through repurposing SCGs into PLA, we can close the loop, minimize the environmental impact, and create a more sustainable alternative to fossil-fuel-based plastics. Considering that the conversion of lactic acid to PLA has been extensively studied in PLA production using other agricultural crops such as corn, lactic acid production becomes the critical factor in producing PLA from SCGs. We reviewed and summarized the methods for producing lactic acid from SCGs. One can therefore select the appropriate method based on their specific circumstances or requirements. Additionally, we have discussed possible ways of improving the yield of PLA produced from SCGs and the possible corresponding costs.