Industrial Oleochemicals from Used Cooking Oils (UCOs): Sustainability Benefits and Challenges

Introduction

Oil roasting, sauteing, and frying are ancient techniques that are frequently used for food processing and preparation in domestic and industrial environments worldwide. As the cooking oils and fats can reach high temperatures when heated (150-190 °C), contact with or immersion within hot hying oil accelerates the cooking process; in this case, the oil acts both as a heating medium and as lubricant. During this process, there is simultaneous mass and heat transfer among the oil, air and food, resulting in a complex set of physicochemical interactions that lead to the characteristic sensory profile of hied or roasted food. Due to the high temperature of the oil. the external surface of foodstuff dehydrates rapidly, losing water by steam evaporation in a bubbling action. After evaporation subsides, the characteristic porous crust of the fried food is obtained, while the internals are kept moist and soft, also acquiring a desirable odour, color, flavor and taste. This pleasing sensoiy profile is enhanced by chemical transformations such as the Maillard reaction, carbohydrate caramelization. protein gelatinization. oxidation, etc. Besides the convenience of fast cooking, the hying process also has a preventative action as a result of the thermal degradation of microorganisms, enzymes, and the reduction of the superficial water activity (Bordin et al„ 2013).

Owing to the changes in human behavior after the industrial revolution, hying became a popular way to produce fast food and snacks (primarily potato chips), mainly in urban centers. At the end of the nineteenth century, large-scale open kettles were run under batch operation, doing the transition from domestic kitchens to garages, and subsequently to industrial factories. In 1929, the development of continuous fryers (Ferry’s continuous potato chip cooker, J.D. Ferry Co.) started a new era in frying oil consumption in modem society; food frying systems were able to process from hundreds to thousands of kilograms per horn (Banks, 2007). This also boosted vegetable oil production with the consequent price reduction and higher accessibility for household users. Nowadays, the per capita world consumption of vegetable oils is around 21.2 kg/person/yr (OECD, 2020).

Department of Chemical and Environmental Engineering, Umversidad Nacional de Colombia, 111321, Bogota D.C., Colombia.

Food flying is generally performed using refined oils in order to avoid undesired off-flavors and to enhance oil thermal and chemical stability. These oils are mainly composed of triacyl glycerides of different fatty acids of an even number of carbon atoms, typically from C8 to C22, but particularly with higher content of C16 and Cl8. As presented in Table 1, the composition of refined oils and fats largely depends on their natural source, and they can contain saturated fatty acids and fatty chains with conjugated double bonds in their structure.

Table 1. Typical distribution of fatty acids in triglycerides of commercial oils and fats.

Oil or fat

Fatty acid type % wt. Saturated ■ Unsaturated ■

Carbon chain content in triglycerides, % wt.

Unsaturated C18 acids in triglycerides, °/o wt.

8

10

12

14

16

18

20

22

C18:0

C18:l

08:2

08:3

Palm

2

42

56

5

41

10

Soybean

8

91

4

28

53

6

Rapeseed

4

93

1

2

1

56

26

10

Sunflower

4

93

1

4

84

5

Palm kernel

4

5

50

15

7

18

2

15

1

Peanut

10

84

3

3

4

55

25

Cottonseed

1

15

84

6

76

2

Coconut

8

7

48

17

9

10

2

7

1

Olive oil

15

84

1

2

70

12

Com

13

87

3

31

52

1

Beef tallow

3

38

57

7

48

2

Lard

1

27

70

2

11

44

11

4

Chicken fat

1

28

64

1

6

37

20

1

Butter

3

4

12

29

41

11

28

2

Chemistry of Frying

A deep understanding of the chemical changes that occur during the frying process is fundamental in comprehending the nature of used cooking oils and defining the required upgrading processes, operating conditions, potential valorization routes, and their final uses. During oil frying, most chemical reactions occur by interaction with the carboxylic moiety and the unsaturations of the fatty' acid chains (De Alzaa et al., 2018). Water from food and the evolved steam can hy'drolyze the ester bonds, producing partial glycerides (i.e., monoglycerides (MG) and diglycerides (DG), free fatty acids (FFAs) and glycerol (G)). Due to hydrolysis, the acidity of the oil increases over time, thus, the FFA content in the oil is an important quality factor and a good indicator of the degree of use of a flying oil.

Simultaneously with esters hydrolysis, thermal and auto-oxidation reactions mainly occur at the double bonds of glycerides and FFAs. This leads to the formation of peroxides, and the subsequent rupture into volatile (e.g., hydrocarbons) and nonvolatile polar compounds (e.g., epoxides, alcohols, aldehydes, ketones, dicarboxylic acids, etc.). Therefore, while low peroxide and low volatiles contents are good indicators of the integrity of a new oil, they are insufficient to assess the quality of a used oil. This is because peroxides decompose and volatiles are removed when frying at high temperatures and for long periods, and when the oil has a high degree of reuse.

Finally, dimerization of triacyl glycerides, and polymerization of nonvolatile compounds are major decomposition processes during oil fiying (Choe and Min, 2007). These are free radicals driven reactions, and the obtained dimers, oligomers and polymers can be very large molecules, reaching up to 1600 Daltons. These polymers with a high oxygen content accelerate a further degradation of the oil and are mainly responsible for the higher viscosity, the foaming tendency at high temperatures, the undesirable dark color, and the propensity to form resin-like residues. As the polymers might still contain some ester groups, a reduction in the acid-discounted saponification value and an increase in the unsaponifiable matter content in the used oil could be surrogate of polymers presence.

Other reactions can occur between the oil and the components contained in and extracted from the foodstuff (e.g., proteins, carbohydrates, fats, flavonoids, minerals, etc.). Thus, all the chemical changes din ing frying depend not only on the oil type, but also on the characteristics of the food (nature, composition, water content, size, shape, etc.), degree of reuse, air incorporation rates into the oil, fiying temperature, kettle geometry, and the heating process (heating rate, surface area, surface temperature, operating time, etc.). Major physical and chemical changes dining oil fiying are summarized in Figure 1.

Besides the reduction of the nutritional value caused by the degradation of lipids and other components (e.g., antioxidants, vitamins), prolonged reuse of frying oils has a major impact in their edible character. Nitrogen-containing compounds extracted from food can react with oil degradation products to foim highly toxic chemicals, such as acrylamides and heterocyclic amines. In addition, highly reactive oxygenated species formed dining fiying can decrease the radical scavenging capacity, causing oxidative stress (i.e., potential carcinogenicity, mutagenicity and genotoxicity). Furthermore, dining the fiying process, trans fatty acid chains, which are linked with a large variety of deleterious health effects (Bordin et al., 2013; Pemmalla and Subramanyan, 2016), are formed. Another issue is the potential contamination with animal-derived proteins associated with different diseases (e.g., bovine spongiform encephalopathy, swine flu). Also, the potential absorption of bio-concentrated chemicals, such as persistent organic pollutants (POPs), dioxins, dioxin-like compounds, and polychlorinated biphenyls (PCBs), is of major concern. Finally, cyclic monomers formed during fiying can enhance lipids intake and the subsequent over-accumulation within the organisms (Boatella and Codony, 2000; Sanli et ah, 2011). For all these reasons, highly reused fiying oil becomes a noxious product, so it cannot be indefinitely utilized for cooking purposes, nor incorporated into animal feed (i.e., without proper refining). Consequently, used cooking oils (UCOs) turn into problematic waste that requires proper disposal.

Schematic of physical and chemical changes of vegetable oils during the fiying process

Figure 1. Schematic of physical and chemical changes of vegetable oils during the fiying process.

Impacts of Waste Fats and Oils

UCOs management and disposal is a very challenging issue that has been exacerbated by population growth and higher consumption of oils and fats. In particular, consumption of vegetable oils has steadily increased around 3% per year over the last decade (OECD, 2020), with the consequent rise in waste oil generation, primarily in heavily populated areas. UCOs are generated at household, HORECA sites (HOtels. REstaurants, CAsino and catering), and at industrial facilities. While institutional and industrial UCOs management is somehow regulated and enforced, household disposal is a major problem for cities. Few countries har e implemented laws for domestic UCOs management (e.g., Belgium), though most traditional household disposal is done by pouring UCOs through syphons or within the solid residues. In the first case, there is a cascade of negative impacts, including blockage of domestic and urban sewage pipes, sanitary server overflows, flooding during rainy seasons, damage of private/public infrastructure, proliferation of vectors (e.g., rats, cockroaches, mosquitos), bad odors, and increase in the operating costs of sewage and treatment plants, among others. When disposed of in landfills, UCOs leaching can affect the normal biodegradation processes, and can contribute to the increase of lixiviates and methane emissions. Sometimes, a more dramatic public health issue occurs, mainly in not well-regulated areas; used flying oil is illegally collected, filtered, bleached and redistributed as new oil among low-income populations.

Either from the sewage systems or from lixiviates, once UCOs reach surface or underground waters, they generate great environmental impacts on local and regional ecosystems: A supernatant lipid layer prevents water oxygenation, fresh water sand basins are contaminated with organic matter and noxious chemicals, oxygen depletion is caused by a higher chemical oxygen demand (COD), and eutrophication is boosted. While this could be considered an issue of underdeveloped and developing countries, this situation has already created problems in developed economies as well. For instance and as observed in Figure 2, in spite of the collection programs implemented in most European countries for the management UCOs, only about 6% of the domestic UCO generated is collected and properly disposed of (around 48 kt). This indicates that problems with sewage clogging are still common even in developed regions. This has prompted the implementation of household management policies that har e been effective in the Netherlands, Belgium. Austria, and Sweden, where collected household UCOs represent 30 to 65% of the generated volume (Figure 2). Comparatively, the European HORECA and industrial sectors are highly regulated, and nearly 85% of UCOs generated from these sectors are collected and reused.

To aggravate the management problem, other waste fats, oils, and greases (FOG) are also generated, namely yellow, brown, and trap greases. While UCOs are essentially produced as a residue of food processing, yellow and brown greases are blends of vegetable oils and animal fats (e.g., lard, tallow, poultry, and fish) coming from food processing and animal rendering. Typically, a yellow grease contains from 2 to 15% wt. FFA; if the FFA content is higher, it is marketed at a discounted price, otherwise it can be blended with UCOs to meet yellow grease specifications. When the FFA content is above 35% wt., the waste is considered a brown grease (Cauakci and Gerpen. 2001). Trap grease or black grease is obtained from grease collectors at industrial and HORECA locations, and from sewage interceptors and wastewater treatment plants. Normally, UCOs and yellow grease have found some secondary uses as energy sources or in the oleocliemical industry (after suitable pretreatments). In contrast, brown grease and trap grease are not generally exploited because of the high content of impurities, the large degree of chemical and biochemical decomposition, and the presence of halogenated products from sanitization and cleaning chemicals (Ward, 2012). Brown and trap greases are normally disposed of in landfills, or used for biodigestion or composting. Typical market specifications of waste fats and oils are summarized in Table 2.

 
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