Cheese Whey for Producing Renewable Biofuels

Cheese Whey for Producing Renewable Biofuels: Comparison

Please note this is a comparison between Version 2 by Vivi Li and Version 1 by Carlos Saul Osorio-Gonzalez.

Agro-industrial residues such as bagasse, pomace, municipal residues, vinasse and cheese whey are an environmental problem around the world, mainly due to the huge volumes that are generated because of the food production to satisfy the nutritional needs of the growing world population. Among the above residues, cheese whey has gained special attention because of its high production with a worldwide production of 160 million tons per year. Most of it is discarded in water bodies and land causing damage to the environment due to the high biological oxygen demand caused by its organic matter load. The environmental regulations in developing countries have motivated the development of new processes to treat transform cheese whey into added-value products such as food supplements, cattle feed and food additives.

bioenergy
cheese whey
bioethanol
biohydrogen
biomethane
biodiesel

1. Introduction

Dairy is a worldwide industry and the main waste generated by the milk transformation is whey. The world production of this waste in 2020 was 183 million tons ^[1]. Since governments in various jurisdictions around the world acted, except for some developing countries, it is currently illegal to dispose of untreated cheese whey in water bodies ^[2]. In the second half of the 20th century, community action groups, environmental agencies and processors equally recognized and highlighted the environmental damage caused by the release of untreated cheese whey. Essentially, when cheese whey is released into water bodies or directly into the soil, contributes to eutrophication in the water body and increases the acidity (depending on the discarded cheese whey type) in the soil ^[3]. The principal compound of cheese whey is lactose (44–46%), leading to a high biological and chemical oxygen demand (30 to 50 mg/L, and 60 to 80 mg/L, respectively) that contributes to the eutrophication ^[4]. The increase in acidity in the soil is highly dependent on cheese making process due to factors such as type of cheese (fresh, mozzarella, cottage), curd process and milk source have an effect. For instance, cheese whey obtained from the curding process utilizing organic acids (acid cheese whey) hold pH values between 3.5 to 4.5, and higher than 5.6 when curt-enzymes processes are used. Likewise, it has been reported that secondary cheese whey holds pH values ≤ 3 ^[1]. The high untreated amounts that are discarded and the pollution caused by this dairy residue have led governments from all around the world to demand industries focus on the clean production of goods and services. Likewise, secondary cheese whey resulting mainly from cottage cheese production has been used directly as feedstock to produce biofuels. However, it has been reported that the substrate has some limitations to be used in the microbial process. Some of these limitations is the increase in acidity (≥3), high dissolved oxygen (80 gL⁻¹), high biological demand (30 gL⁻¹) and low solid content (8 gL⁻¹). The above nutrient limitations and the relatively small production in comparison with the other cheese whey sources are the main barriers to complete exploitation and better approach to this residue [5,6]^[5][6]. This situation has obligated cheese companies to create solutions to decrease their cheese whey loads by re-designing their processes and/or valorizing their by-products ^[7].

The exploitation of cheese whey to produce different goods has increased in recent decades. Some of the conventional goods and products are lactose, organic acids and protein fractions. During the past years, the research on the use of this residue has been intensified to produce chemical compounds for industrial sectors such as pharmaceutical, cosmetic and bioenergy [8,9,10]^[8][9][10]. Figure 1 shows some of the products as well as the sectors in which cheese whey is currently used as feedstock to produce added-value compounds.

Figure 1. Biorefinery concept of cheese whey.

Biorefinery concept of cheese whey.

Several technologies have been studied to treat cheese whey with the main purpose to decrease the organic load. Figure 2 shows conventional processes to treat cheese whey. Physical treatments for cheese whey have been mainly focused on membrane technologies such as diafiltration, microfiltration, ultrafiltration, nanofiltration, electrodialysis and reverse osmosis. These technologies are particularly used to recover and remove specifically lactose and proteins. Cheese whey treatment through chemical processes consists of the use of chemical catalyzers to convert lactose into different sugar isomers such as galactose, glucose, or D-tagatose. One of the most common chemical reactions to treat cheese whey is the Lobry de Bruyn Alberda van Ekenstein (basic or acid). The isomerization is carried out using soluble catalysis with sodium hydroxide (NaOH), potassium hydroxide (KOH) or calcium hydroxide (Ca(OH)₂), among others [11]. Physicochemical processes are mainly focused on the use of coagulants and flocculants such as aluminum sulfate (Al₂(SO₄)₃, ferric chloride (FeCl₃) and ferrous sulfate (FeSO₄) [12]. Finally, biological processes to treat cheese whey mainly through aerobic and anaerobic fermentation. The use of these alternatives is the most reliable biological-base process in terms of the cheese whey transformation/elimination [13,14,15].

Figure 2. Conventional process to treat cheese whey.

2. Cheese whey-based biofuels

2.1 Bioethanol

Bioethanol production through fermentation has emerged as a potential alternative to replace fossil fuels such as gasoline. This renewable biofuel not only has application in the energy industry but is widely used as a replacement for chemical or grain-based ethanol in the cosmetic, pharmaceutical, food and beverage industries ^[11]. It has been reported that bioethanol production from corn and sugarcane has been produced extensively by the United States and Brazil, respectively. Nevertheless, the use of the above two feedstocks increases the total production cost and compromises food security due to the high land use for these crops ^[12]. In this sense, different feedstocks such as different lignocellulosic biomass, starches, food wastes and agri-food residues have been used for bioethanol production. The use of cheese whey as a substrate to produce bioethanol through fermentation is economically competitive in comparison with substrates such as sugarcane, corn and lignocellulosic biomass. In addition, it is a residue, and its valorization represents several advantages in terms of sustainable development, such as a decrease in waste, and organic carbon recycling ^[13].

2.1 Biomethane

Methane is the one of most abundant biogas fractions produced by anaerobic di-gestion of organic residues, including cheese whey. As mentioned above, anaerobic di-gestion is a well know technology to produce methane. However, several challenges come with each specific feedstock that is used as a carbon source. These challenges can be classified into three main categories, microbiological, chemical, and operational, making anaerobic digestion one of the most complicated biological processes. Moreover, this technology is highly recommended to treat wastewater and residues with high biological oxygen demand, such as cheese whey ^[14]. Additionally, requirements related to the installation and operation of anaerobic biodigesters such as technology, energy consumption and space are relatively low. Nevertheless, depending on the re-actor type and feedstock the total cost can vary considerably. Likewise, the reactor type plays a key role during biogas production and classified the anaerobic digestion process into two different systems: low-rate system and high-rate system.

2.2 Biohydrogen

Methane is the one of most abundant biogas fractions produced by anaerobic di-gestion of organic residues, including cheese whey. As mentioned above, anaerobic di-gestion is a well know technology to produce methane. However, several challenges come with each specific feedstock that is used as a carbon source. These challenges can be classified into three main categories, microbiological, chemical and operational, making anaerobic digestion one of the most complicated biological processes. Moreover, this technology is highly recommended to treat wastewater and residues with high biological oxygen demand, such as cheese whey ^[14]. Additionally, requirements related to the installation and operation of anaerobic biodigesters such as technology, energy consumption and space are relatively low. Nevertheless, depending on the re-actor type and feedstock the total cost can vary considerably. Likewise, the reactor type plays a key role during biogas production and classified the anaerobic digestion process into two different systems: low-rate system and high-rate system.

2.3 Microbial lipids for Biodiesel

Biodiesel is one of the most popular biofuels produced due to is environmentally friendly and its net greenhouse emissions are lower in comparison with the produced from fossil fuels. Microbial lipid-base biodiesel production is one of the most promising biofuels due to its advantages (non-toxic, biodegradable, renewable, no sulfur content, high lubricity) in comparison with fossil diesel ^[15]. Microbial lipid-base biodiesel production is a potential alternative using low-cost residues such as cheese whey with high carbon content as a feedstock ^[16]. Table 1 shows a some of the research in biofuel production using cheese whey as a substrate.

Table 1

. Produced biofuels using cheese whey as substrate.

Substrate	Strain	Ethanol Concentration (g/L⁻¹)	Volumetric Productivity (g L⁻¹ h⁻¹)	Reference
Cheese whey (permeate)	K. marxianus URM 7404	8.90	0.66	^[17]
Cheese whey (powder)	S. cerevisiae	23.80	nd	^[18]
Fresh cheese whey	K. marxianus URM 7404	25.81	2.57	^[17]

Substrate	Inoculum	Bioreactor	Methane Yield	Reference
Cheese whey powder + vinasse	Sludge from a poultry slaughterhouse	AnSBBR	11.5 molCH₄ kg COD⁻¹	^[19]
Fresh cheese whey	Sludge from the wastewater treatment plant	SBR	340.4 L CH₄ kg⁻¹ COD_feed	^[20]
Cheese whey powder	Sludge from the wastewater treatment plant	Anaerobic batch reactors	0.266 L CH₄ g COD_consumed	^[21]

Substrate	Strain	Biohydrogen yield	Biohydrogen productivity	Reference
Cheese whey (permeate)	Microbial consortium	3.60 mol H₂/mol of lactose	140.02 mmol H₂/L day	^[22]
Fresh cheese whey	Clostridium sp.	6.35 mol H₂/mol lactose	139 mL/g/h	^[23]
Hydrolysed cheese whey	Microbial consortium	1.93 mol H₂ mol⁻¹ of sugars	5.07 L H₂ L⁻¹ day⁻¹	^[15]

Substrate	Strain	Total lipid (g/L⁻¹)	Lipid Accumulation (%)	Reference
Fresh cheese whey	M. circinelloides URM 4182	1.06	22.5	^[24]
Deproteinized cheese whey	C. oligophagum JRC1	5.64	44.12	^[25]
Ricotta cheese whey	M. isabelline 1757	4.49	37	^[26]
Substrate	Strain	Ethanol Concentration (g/L⁻¹)	Volumetric Productivity (g L⁻¹ h⁻¹)	Reference
Cheese whey (permeate)	K. marxianus URM 7404	8.90	0.66	[17]
Cheese whey (powder)	S. cerevisiae	23.80	nd	[18]
Fresh cheese whey	K. marxianus URM 7404	25.81	2.57	[17]

Substrate	Inoculum	Bioreactor	Methane Yield	Reference
Cheese whey powder + vinasse	Sludge from a poultry slaughterhouse	AnSBBR	11.5 molCH₄ kg COD⁻¹	[19]
Fresh cheese whey	Sludge from the wastewater treatment plant	SBR	340.4 L CH₄ kg⁻¹ COD_feed	[20]
Cheese whey powder	Sludge from the wastewater treatment plant	Anaerobic batch reactors	0.266 L CH₄ g COD_consumed	[21]

Substrate	Strain	Biohydrogen yield	Biohydrogen productivity	Reference
Cheese whey (permeate)	Microbial consortium	3.60 mol H₂/mol of lactose	140.02 mmol H₂/L day	[22]
Fresh cheese whey	Clostridium sp.	6.35 mol H₂/mol lactose	139 mL/g/h	[23]
Hydrolysed cheese whey	Microbial consortium	1.93 mol H₂ mol⁻¹ of sugars	5.07 L H₂ L⁻¹ day⁻¹	[15]

Substrate	Strain	Total lipid (g/L⁻¹)	Lipid Accumulation (%)	Reference
Fresh cheese whey	M. circinelloides URM 4182	1.06	22.5	[24]
Deproteinized cheese whey	C. oligophagum JRC1	5.64	44.12	[25]
Ricotta cheese whey	M. isabelline 1757	4.49	37	[26]

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