Introduction
Liquid smoke is created by condensing the smoke produced during the pyrolysis of lignocellulosic biomass in a controlled environment. The process involves four stages of thermal decomposition: the evaporation of water, and the decomposition of hemicellulose, cellulose, and lignin. Consequently, liquid smoke is recovered in a liquid form containing water and oxygenated compounds. Liquid smoke is a bio-economic product that is derived from pyrolysis and has acidic characteristics. Liquid smoke was initially used as a natural food preservative. Early research on liquid smoke as a food preservative was published in 1975 [1]. As a food preservative, liquid smoke has anti-bacterial properties toward both gram-positive and gram-negative bacteria, as well as anti-fungal activity [2]. Liquid smoke has been demonstrated to act against lactic acid bacteria, namely Pediococcus cerevisiae and Lactobacillus plantarum [3, 4]; Salmonella sp. [5]; Clostridium botulinum [6]; Aeromonas hydrophila and Staphylococcus aureus [7]; Listeria monocytogenes [8–13]; and Listeria innocua [14]. In addition, it has anti-fungal activity against Penicillium roqueforti, Penicillium camemberti, and Aspergillus oryzae [15].
Liquid smoke can maintain the quality of food, including physical properties such as color [16, 17], meat texture [18], pH [19], tenderization [20] by preventing lipid oxidation processes [21–23], and preservation of shells life [24–28], particularly in various types of meat. Fish treated with liquid smoke flavoring shows low brightness and pH, but high firmness, elasticity, color intensity, and expressible water content [25, 29]. In other marine products, the application of liquid smoke does not affect the amino acid content [30], including that of lysine [31]. In addition, liquid smoke can improve organoleptic properties and antioxidant properties [32], because of the presence of phenolic compounds [33], primarily phenol and its derivatives [34–36]. In addition, liquid smoke contains aldehydes, ketones, diketones, esters, alcohols, acids, furan and pyran derivatives, pyrocatechol derivatives, and alkyl and aryl ethers [37].
Controversy exists regarding the use of liquid smoke as a food preservative, because of the presence of by-products of the pyrolysis process, particularly PAHs such as BaP; however, research has indicated that the levels of these by-products are very low and therefore safe for consumption [38–41].
The first report on the effects of liquid smoke on health, performed in 1994, conducted cytogenic analysis indicating that liquid smoke administration does not increase the frequency of chromosome aberrations [42]. The first toxicity tests of liquid smoke with respect to health were performed in experimental animals and indicated its safety [43]. Furthermore, liquid smoke administered as a food supplement in experimental diabetic animal models has been shown to ameliorate diabetes status and prevent inflammation, according to various indicators [44].
Given the complex composition of liquid smoke, comprising primarily phenolic compounds with antioxidant and anti-inflammatory properties, several studies have investigated therapeutic effects on human health. These observed benefits have shifted perspectives on the use and economic potential of liquid smoke. The expanding applications and economic value of liquid smoke could play a crucial role in its future development, particularly in creating new health-related products. Analyzing global usage patterns is necessary to validate its high utility, economic significance, and potential in drug development. This review was conducted because of the lack of systematic reviews in this field, and because most prior reviews have focused on other aspects liquid smoke, such as production.
Materials and methods
Study design
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were followed for this systematic review. This review was aimed at analyzing whether liquid smoke from various biomass sources might have any health benefits.
Search methods and strategy
A comprehensive search of the PubMed (https://pubmed.ncbi.nlm.nih.gov), Scopus (https://www.scopus.com/search/form.uri?zone=TopNavBar&origin=searchadvanced&display=basic#basic), Science Direct (https://www.sciencedirect.com), Web of Science (https://www.webofscience.com/wos/woscc/basic-search), and Embase databases until December 2023 was conducted. The keyword “liquid smoke” was used to search available articles. In addition, the reference lists of eligible articles were searched manually to identify additional relevant publications.
Study selection criteria
The inclusion criteria for studies followed the Population, Intervention and Outcome (PIO) framework:
Type of population: all in vitro, in vivo, and in silico studies reporting the potential of liquid smoke from various biomass sources or commercial liquid smoke. The studies were required to state the source of liquid smoke
Type of intervention: health benefits, such as anti-oxidant, anti-inflammatory, anti-nociceptive, anti-bacterial, anti-fungal, and anti-viral effects
Type of outcome: the ability of liquid smoke to confer health benefits, such as anti-inflammatory, anti-diabetic, anti-periodontitis, wound healing, ulcer healing, and hepatic protection properties.
Data collection and extraction
Initially, four reviewers independently screened the titles and abstracts of articles from the databases matching the selection criteria of the study, to identify potentially eligible articles. Four reviewers eliminated duplicate articles and discussed the articles that were selected. Disagreements among reviewers were itemized, and the fifth reviewer was consulted to reach a consensus. The key information in each selected article was recorded and included in the study design, and relevant findings were tabulated.
The following information was extracted from the studies: first author’s name, year of publication, biomass source for production of liquid smoke, and health benefits. In cases of disagreement, a consensus was reached through discussion with the other authors.
Result
Study selection and screening result
Searching of the databases identified 1669 articles, 1069 of which were identified as duplicates; thus, 269 articles remained. These 269 articles were screened on the basis of their titles and abstracts, thus leaving 250 articles. Only 179 articles were available in full text, among which 89 met the inclusion criteria, and 90 articles were excluded. During the article review process, a citation search identified 7 additional relevant articles. A total of 96 articles were finally included in the systematic review ( Figure 1 ).
Liquid smoke sources
Various biomass types are used for liquid smoke production. The possible health benefits of liquid smoke produced from rice husks (Oryza sativa) [44–58], coconut shells (Cocos nucifera L.) [59–70], palm kernels ( Elaeis guineensis Jacq.) [71], cocoa pods ( Theobroma cacao L .) [72], tian op [73], hickory ( Carya tomentosa (Lam.) Nutt .) [74], apple ( Punica granatum L.), mesquite ( Prosopis pubescens Benth.), pecan (Carya illinoinensis), and oak ( Quercus robur L.) [75], as well as commercial liquid smoke [42, 76], have been reported ( Figure 2 ). Commercial liquid smoke was defined as that produced by companies and derived from single or multiple biomass sources.
The production of liquid smoke was performed by pyrolysis at 200 °C [72], 300 °C [72], 400 °C [44–56, 59–69, 72], or 340–420 °C [71]. Some studies did not describe the temperature of pyrolysis [70, 73, 75].
Toxicity of liquid smoke
Materials suitable for human use must be non-toxic. Several studies have demonstrated that liquid smoke derived from rice husks (Oryza sativa) [44, 46, 51, 54, 56], cocoa pods ( Theobroma cacao L. ) [72], tian op [73], apple, hickory, mesquite, pecan, and oak [75], as well as commercial liquid smoke [42], exhibit no toxicity in vitro or in vivo ( Supp Table 1 ).
Toxicity testing of liquid smoke derived from rice husks conducted in vitro has revealed no toxicity toward INS-1 rat insulinoma β-cells [44, 56], RBL-2H3, RAW264.7 [56], BHK-21 [46], and osteoblast cells [51, 54]. In in vivo studies, liquid smoke from rice husks (Oryza sativa) has also shown no signs of acute toxicity [45]. Oral administration of liquid smoke from tian op has not been found to affect body weight, hematological parameters, and organ weight [73]. Similarly, liquid smoke from apple, hickory, mesquite, pecan, and oak has demonstrated no toxicity or genotoxicity, while enhancing Nfr2 expression and decreasing AhR activity [75]. Additionally, a study of commercial liquid smoke has found no increase in the frequency of chromosome aberrations [42].
Anti-inflammatory properties
The anti-inflammatory properties of liquid smoke from rice husks (Oryza sativa) have been observed both in vitro and in vivo. Liquid smoke affects oxidative stress [44, 56] markers such as NO production [44, 56, 57]; iNOS genes and proteins [44, 56]; tissue gene expression of 5-LOX, COX-2, ICAM, and β-actin [56]; and cytokine genes and proteins, such as TNF-α, IL-1β, and IL-6 [44, 56]. By modulating gene expression, liquid smoke from rice husks affects the production of proteins, resulting in altered levels of TNF-α, IL-1β, IL-6, PGE2, and LTB4 [56]. Other proteins such as myeloperoxidase and β-hexosaminidase are also affected [56]. In in vivo studies, liquid smoke from rice husks (Oryza sativa) has shown anti-inflammatory properties by decreasing inflammation in the ear [56] ( Supp Table 2 ).
Anti-diabetic properties
The anti-diabetic properties of liquid smoke from rice husks (Oryza sativa) have been examined both in vitro and in vivo. The addition of liquid smoke to mouse fat has been found to ameliorate damage to liver tissue and islets of Langerhans in a model of diabetic model induced by a high-fat diet [44, 53]. Enhanced gene expression and enzyme production in the liver has been reported, including that of GOT, GTP, C6, PEPK, GCK [44, 53], GLUT2, PPAR-γ, TNF-α, IL-1β, and IL-6 [53]. Consequently, insulin release [44] and serum insulin [44, 53] increase, whereas blood glucose decreases [44, 53], and glycogen restoration is improved [44] ( Supp Table 2 ).
Anti-periodontitis properties
Topical application of liquid smoke from rice husks (Oryza sativa) has been shown to have anti-periodontitis effects by decreasing inflammatory markers and stimulating growth factors such as NF-kB [47], Nrf2 [52], IL-1β [52], TGF-β [47], FGF2 [47], and COL-1 [47] ( Supp Table 2 ). The anti-periodontitis effect is also demonstrated by liquid smoke’s ability to inhibit Porphyromonas gingivalis, a key etiological agent of periodontitis [50, 54].
Ulcer healing properties
Liquid smoke from rice husks (Oryza sativa) has been found to influence ulcer healing. Topical application of liquid smoke increases inflammatory cells, such as macrophages, lymphocytes, and fibroblasts [48]. Effects have also been observed on cytokines and growth factors, such as IL-6 [48], TGF-β [48], FGF-2, COL-1, VEGF, and PDGF [55] ( Supp Table 3 ).
Liquid smoke has also shown health benefits in humans. In an in vitro study, liquid smoke from coconut shells (Cocos nucifera L.) has been shown to be non-toxic [66], and to have anti-bacterial activity against Streptococcus aureus [70], anti-inflammatory activity [66], and anti-nociceptive properties [66, 68]. Liquid smoke from coconut shells (Cocos nucifera L.) stimulates burn wound healing properties by increasing the number of fibroblasts and wound contraction [59]. Oral ulcers, wounds occurring in the oral mucosa, have shown significant healing after topical administration of liquid smoke from coconut shells (Cocos nucifera L.), by decreasing NF-kB [61, 62] and increasing Nrf2 [64]. Effects have also been observed on the production of cytokines such as TNF-α [62, 64], IL-6, and IL-1β [64], and growth factors, such as FGF-2 and VEGF [63]. Liquid smoke also impacts inflammatory and proliferative cells, including neutrophils [65], lymphocytes [65], macrophages [62, 64], fibroblasts [63, 65], and collagen production [69] ( Supp Table 3 ).
Anti-bacterial properties
Liquid smoke also has anti-bacterial effects. Liquid smoke with such properties has been derived from various biomass sources including cajuput twigs (Melaleuca leucadendra), oil palm ( Elaeis guineensis Jacq.), palm kernels ( Elaeis guineensis Jacq.), pinecones (Pinus coulteri), ulin wood (Eusideroxylon zwageri), cinnamon ( Cinnamomum verum ), cocoa fruits ( Theobroma cacao L. ), rice husks (Oryza sativa), coconut shells (Cocos nucifera L.), and pecan shells (Carya illinoinensis) ( Figure 3 ).
Coconut shell (Cocos nucifera L.) is the most studied liquid smoke origin in the literature. The antibacterial properties of liquid smoke from coconut shells (Cocos nucifera L.) have been reported in several studies on gram-positive bacteria, such as Staphylococcus aureus [77–80], Bacillus subtilis [77], Bacillus cereus [79], Listeria monocytogenes [77, 79], Lactobacillus sp. [81], and Lactobacillus rhamnosus [82], as well as gram-negative bacteria, such as Escherichia coli [77, 79, 80, 83, 84], Pseudomonas aeruginosa [77, 78, 85, 86], Salmonella sp. [81], Salmonella typhimurium [77, 81], and Salmonella enteritidis [79].
Rice husks (Oryza sativa), one of the most common sources of liquid smoke, have antibacterial properties toward several gram-positive bacteria, such as Staphylococcus aureus [87] and Bacillus subtilis [87], and gram-negative bacteria, such as Escherichia coli [87, 88], Salmonella sp. [88], Salmonella typhimurium [57, 89], and Salmonella choleraesmus [87].
Liquid smoke obtained from cajuput twigs (Melaleuca leucadendra) has antibacterial properties toward several gram-positive bacteria, such as Listeria monocytogenes, Staphylococcus aureus, and Bacillus subtilis, as well as gram-negative bacteria, such as Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhimurium [77].
Oil palm ( Elaeis guineensis Jacq.), including shells and branches, is another source of liquid smoke. Oil palm shells ( Elaeis guineensis Jacq.) have antibacterial potential toward gram-positive and gram-negative bacteria, such as Escherichia coli [90] and Staphylococcus aureus [90, 91]. Furthermore, liquid smoke from palm kernels ( Elaeis guineensis Jacq.) has antibacterial properties toward the gram-positive bacterium Streptococcus mutants [71].
Cocoa skin ( Theobroma cacao L. ) has been demonstrated by several studies to have anti-bacterial potential against gram-positive bacteria such as Staphylococcus aureus [92, 93] and Coliform [93], and gram-negative bacteria such as Escherichia coli [92]. Cinnamon ( Cinnamomum verum ), a raw material for liquid smoke production, has been found to have anti-bacterial properties toward the gram-positive bacteria Staphylococcus aureus and Bacillus sp. [94]. Liquid from cashews (Anacardium occidentale) and pecan shells (Carya illinoinensis) has anti-bacterial activity toward Escherichia coli [95], Listeria sp., and Salmonella sp. [96].
Commercial liquid smoke also has anti-bacterial, gram-positive, gram-negative, and anti-fungal activity ( Figure 4 ). Effects on gram-positive bacteria, such as Bacillus cereus [97], Carnobacterium inhibens [98], Carnobacterium maltaromaticum [98, 99], Clostridium botulinum types A and E [6], Clostridium perfringens [100, 101], Enterococcus faecalis [98], Enterococcus malodoratus [99], Lactobacillus curvatus [98], Lactobacillus deibrueckii ssp. bulgaricus [102], Lactobacillus helveticus [102], Lactococcus lactis [98], Lactobacillus plantarum [2–4, 99], Listeria innocua [2, 12, 14, 98], Listeria monocytogenes [8, 10–12, 24, 26, 103–109], Listeria monocytogenes Scott A [9, 13], Pediococcus cerevisiae [3], Staphylococcus aureus [7, 67, 97, 100, 101, 110–113], and Streptococcus thermophilus [99, 102], have been reported.
Furthermore, antimicrobial properties toward gram-negative bacteria, such as Aeromonas sp. [7, 114], Escherichia coli [2, 97, 112, 113], Photobacterium phosphoreum [98], Pseudomonas putida [2, 98], Salmonella sp. [2, 97], Salmonella enteritidis [112], and Vibrio vulnificus [98], have also been reported.
Anti-fungal properties
Antifungal properties of liquid smoke from coconut shells (Cocos nucifera L.), cocoa shells ( Theobroma cacao L .), cinnamon ( Cinnamomum verum ), ulin wood (Eusideroxylon zwageri), palm kernels ( Elaeis guineensis Jacq.), and pinecones (Pinus coulteri) have been reported. Some anti-fungal activity has been observed toward Calathea utilis [84], Fusarium oxysporum [115], Candida albicans [80], Lasiodiplodia theobromae [116], Phytophthora infestans [117], Phytophthora palmivora [118], Colletotrichum capsica [119, 120], Pyricularia oryzae [121], and Ralstonia syzygii subsp. celebesensis [122] ( Table 1 ).
Liquid Smoke from Various Wood Sources with Anti-Fungal Benefits for Human Health
| Raw material | Anti-fungal properties | References |
|---|---|---|
| Coconut shells (Cocos nucifera L.) | Calathea utilis | [84] |
| Candida albicans | [80] | |
| Fusarium oxysporum | [115] | |
| Cocoa shells ( Theobroma cacao L. ) | Lasiodiplodia theobromae | [116] |
| Cinnamon ( Cinnamomum verum ) | Phytophthora infestans | [117] |
| Ulin wood (Eusideroxylon zwageri) | Colletotrichum capsici | [119] |
| Pyricularia oryzae | [121] | |
| Palm kernels ( Elaeis guineensis Jacq.) | Colletotrichum capsici | [120] |
| Phytopthora palmivora | [118] | |
| Pinecones (Pinus coulteri) | Ralstonia syzygii subsp. celebesensis | [122] |
The antifungal properties of commercially available liquid smoke toward fungi such as Aspergillus niger [2], Aspergillus oryzae NRRL 1989, Penicillium camemberti NRRL 877, Penicillium roqueforti NRRL 849 [15], Saccharomyces cerevisiae [2], and Phytophthora palmivora [123] have also been described.
Anti-viral properties
Anti-viral properties of liquid smoke from rice husks (Oryza sativa) toward SARS-CoV-2 have been reported. The liquid smoke components 6-octadecenoic acid and oleic acid have been shown to be potential SARS-Cov-2 inhibitors in an in silico study [49].
Hepatic protection properties
The administration of liquid smoke from rice husks has shown potential to ameliorate hepatic damage from bacteria including Salmonella [57, 58]. The addition of liquid smoke from rice husks to the diet has been found to decrease hepatocyte necrosis in mice, by decreasing catalase activity, SOD activity, TNF-α, MPO [58], and IFN and NO production [57]. Decreased hepatocyte necrosis has been demonstrated, on the basis of recovery of GOT and GPT in the liver [58]. Liquid smoke from rice husks has protective effects against not only hepatic function but also lung and kidney damage [58].
Other properties
The incorporation of liquid smoke as a food additive has indicated interactions between the constituent primary emitter pyrolysis product (EMP) and the taste receptor TA2R1, thus elucidating the mechanism through which liquid smoke enhances flavor perception when used as a food flavoring agent [76]. Other research has indicated that the addition of liquid smoke does not cause adverse effects such as inflammation or ulceration of the digestive tract, although liquid smoke is acidic [124]. However, exposure to liquid smoke derived from hickory and mesquite wood has been found to induce oxidative stress and compromise the protective function of the skin [74].
Discussion and future development
Liquid smoke has been proposed as an ingredient with health effects on humans, primarily because it has long been used as a food preservative. As a food preservative, liquid smoke improves the characteristics of foods such as meat [1, 18, 20, 21, 125–136], bacon [1, 22, 137–139], quail [140], mackerel [141–144], fish [16, 25, 27, 29, 31, 145–164], fish balls [113, 165], meatballs [166–171], sausages [23, 172–176], nuggets [28, 177], shellfish or mussels [30, 178–181], shrimp [182], prawns [183], mushrooms [184], and potatoes [17, 185].
Adding liquid smoke can maintain storage time and life [25, 27, 28, 31, 131, 144, 156, 161, 162, 164, 166, 169, 180], freshness [143], color [125, 129, 142, 147, 167], acidity [29, 126, 132, 136, 155, 158, 171, 174, 184], and sensory or organoleptic (flavor, taste, or texture) qualities [16–18, 20, 23, 25, 29, 31, 128, 129, 132, 135–138, 141–143, 146, 150, 153, 155, 157, 158, 160, 164–167, 170, 172–175, 177, 180, 183, 185]. The aroma of food containing liquid smoke is also distinctively smoky [176, 181].
In addition to various chemical properties of meat, such as total TVB-N [128, 143, 149, 160, 164, 184, 186], TBA [186], lysine [31, 141], NDMA, NPYR, and NTHZ can also be influenced [139]. Other important characteristics such as water content [125–127, 158, 163, 170, 171, 184, 187], moisture [136, 141, 163, 165, 182], protein [30, 125, 134, 141, 145, 151, 159, 165, 170, 171], and cholesterol [182] are remain stable. Total lipid [125, 140, 145, 148, 152, 159, 163, 165, 170, 182, 188] can also be maintained through mechanisms of increased lipid oxidation [131, 159, 179] or decreased lipid oxidation [21–23]. Liquid smoke addition does not affect the blood urea nitrogen and creatinine content in poultry [133]. With technologic development, liquid smoke has been transformed into edible film [144, 189], gelatin film [190], and even forms of nanoparticles [156] or nanocapsules [159] to improve its availability. Interestingly, in marine products, the application of liquid smoke can decrease the content of heavy metals [178]. The various advantages of liquid smoke may maintain nutritional content and food characteristics, thus improving human health. Other important characteristics include anti-bacterial effects against foodborne bacteria [130, 144, 146, 154, 157, 164, 166, 167, 169, 172, 174, 180, 188], such as Coliform, Staphylococcus aureus [136], Escherichia coli, Salmonella [132], and Listeria monocytogenes [31]. The anti-bacterial effects are due to the high content of phenol, acetic acid, and water in liquid smoke [1, 158, 165, 168, 174, 184].
Empirically and historically, liquid smoke has been a widely used food preservative, as supported by multiple studies; however, safety aspects remain debated, because by-products of combustion, namely PAH and BaP, are considered harmful to human health [191]. Some studies have reported that the content of this PAH in food after liquid smoke addition is low [40, 192, 193], at approximately 6.3–43.7 μg/kg [35], while other studies found no detectable levels of PAH [194]. Both PAH and BaP have been detected in some studies [38–40], at approximately 0.18–0.80 μg/kg [36], whereas in other studies, it was not found [135]. Pyrolysis temperature is associated with the PAH and BaP content; however, in some studies using higher pyrolysis temperatures of 400–500 °C, the PAH and BaP component has been found [41, 191, 195].
If an ingredient is used as a drug or therapy, fundamental questions include whether the active ingredient is contained, and what the ingredient’s the mechanism of action in the body might be. Liquid smoke contains multiple components, dominated by phenolic compounds and carboxyl components. Liquid smoke is acidic. The acidity (pH) of liquid smoke is determined by the temperature of the pyrolysis itself and the biomass type used. Lower pyrolysis temperatures result in lower pH values. Temperatures of 150–250 °C produce a pH of 2–3 [196, 197]; temperatures of 300–400 °C produce a pH pf 3–4 [198, 199]; and temperatures of 400–500 °C produce pH values of 2, 3.82–4.69 [200–202], or 4.8 [158].
Not only the acidity but also the components of liquid smoke are influenced by temperature. At temperatures of 250–600 °C, phenolic compounds are dominant (7.32–56.8%) [195, 196, 200, 203, 204]; some identified components include carbonyl, carboxylate, furan, and acidic compounds [158, 198, 200, 201, 203, 205–213]. At higher temperatures (350–550 °C), various phenolic compounds, such as 2-methoxy-4-methyl phenol, 4-ethyl-2 methoxy phenols [191, 214, 215], 2 methoxy phenol, and methoxy-4-methyl phenol, are detected [200, 214, 216–218]. Lower temperatures (150–250 °C) produce higher levels of acid (up to 51%) compared to phenolic compounds [195–197, 219]. Moreover, temperatures of 300–600 °C also produce high levels of acidic compounds (ranging from 40% to 92.30%) [195, 204, 208, 213, 220–225], 1,2-ethanediol (up to 20%) [226], and carbonyl compounds (ranging from 32% to 73%) [208, 222–224, 227].
Although the content of PAH and BaP, and the acidity, are controversial aspects regarding the safety of liquid smoke as a food ingredient, no toxicity effects have been reported. Various in vitro and in vivo studies have demonstrated that liquid smoke is safe. The first study, reported in 1994, indicated that the liquid smoke does not have any mutagenic effects and does not affect chromosome aberrations [5, 42]. In an in vitro study, liquid smoke from rice husks (Oryza sativa) has shown no toxicity to fibroblast cells [46] and osteoblast cells [51]. Other parameters, such as oxidative stress, genotoxicity, AhR, and esterogenicity, have not been detected [75]. Furthermore, toxicity tests in animals have been performed. Liquid smoke from coconut shells (Cocos nucifera L.) has been orally administered to animals, and no death resulted [43]. Moreover, liquid smoke from tian op has been tested in an animal model, and no body weight, hematological, and organ changes have been observed [73]. Liquid smoke from rice husks (Oryza sativa) has shown similar results to those of tian op. Oral administration of liquid smoke from rice husks (Oryza sativa) and cocoa ( Theobroma cacao L. ) has been shown to be safe in mice, and no signs of acute toxicity have been observed [45, 72].
The presence of PAH in liquid smoke is a concern. PAH inhalation, ingestion, and dermal contact can be toxic in the human body [228]. PAH accumulates in the body and damages the cardiovascular system, respiratory system [229], nerve cells, and liver cells [230], and may lead to cancer [231, 232], such as breast cancer [233], laryngeal cancer [234], esophageal cancer [235], and lung cancer [236]. Via the ingestion route, PAH is absorbed in the gastrointestinal tract [237], and causes hepatic toxicity and increased lipid metabolism [230]. PAH also affects oxidative stress in the human body, including MDA, GST, and LDH [238]. In carcinogenesis, PAH increases serum p53 and p21 proteins [238]. In the liver, PAH is associated with increased ALT, AST and GGT, and affects liver function [239]. At the cellular level, PAH affects cellular viability, induces free radical-like ROS production, and decreases anti-oxidant ability, including glutathione GSH levels, PSSG, and the activity of GPx, GST, and GR [240]. Together, these processes affect the physiological function of the liver ( Figure 5 ).
Possible effects of PAH on liver function by increasing oxidative stress (ROS, MDA, GST, and LDH) and inhibiting the liver enzymes such as ALT, AST, and GGT.
Health benefits of liquid smoke from coconut shells (Cocos nucifera L.) and rice husks (Oryza sativa) have been demonstrated in humans. Topical treatment with liquid smoke from rice husks (Oryza sativa) has beneficial effects in gum disease (periodontitis) [47, 52] and oral mucosa wounds (ulcer) [48, 55]. Liquid smoke from rice husks (Oryza sativa) has anti-periodontitis effects by inhibiting inflammation and stimulating healing. The mechanism is associated with inhibition of NF-kB [47] and IL-1β [52], and stimulation of Nrf2 [52] to produce growth factors including TGF-β, FGF2, and COL-1 [47]. Liquid smoke from rice husks (Oryza sativa) promotes the activity of inflammatory cells, such as macrophages and lymphocytes [48], and stimulates the production of cytokines and growth factors, including IL-6 [48], TGF-β [48], FGF-2 [55], VEGF [55], COL-1 [55], and PDGF [55]. It also enhances fibroblast proliferation, contributing to wound healing [48] ( Figure 6 ).
Possible mechanism underlying the health benefits of three liquid smoke types, from coconut shells (Cocos nucifera L.), rice husks (Oryza sativa), and palm kernels ( Elaeis guineensis Jacq.). Liquid smoke from coconut shells (Cocos nucifera L.) stimulates skin wound healing and oral mucosal healing; liquid smoke from rice husks (Oryza sativa) stimulates gum disease (periodontitis) healing and oral mucosal healing. Liquid smoke from palm kernels ( Elaeis guineensis Jacq.) has anti-bacterial properties against Streptococcus mutants.
Like liquid smoke from rice husks (Oryza sativa), liquid smoke from coconut shells (Cocos nucifera L.) has a similar health benefits in wound healing support [59, 70], and mucosal wound healing (ulcer healing) [60–65, 69]. The topical application of liquid smoke from coconut shells (Cocos nucifera L.) increases wound healing by increasing fibroblast proliferation [59] and wound contraction [59, 70], and also has anti-bacterial activity toward Staphylococcus aureus [70] ( Figure 7 ).
Possible mechanism of anti-diabetic properties of liquid smoke from rice husks (Oryza sativa), according to an in vivo model study. The anti-diabetic properties were demonstrated through food supplementation in an animal diet for 4 weeks.
For mucosal healing (ulcer healing), topical application of liquid smoke from coconut shells (Cocos nucifera L.) increases healing by inhibiting pro-inflammatory cytokines and genes, such as NF-kB [61, 62], IL-6, IL-1β [64], and TNF-α [62, 64], and stimulating growth factors supporting healing, such as Nrf2 [64], FGF-2, and VEGF [63]. The healing response also includes a decreased neutrophil response [65], as well as increased lymphocytes [65], macrophages [62, 64], fibroblasts [63, 65], and collagen [69], thus accelerating ulcer closure [60]. Liquid smoke from palm kernels ( Elaeis guineensis Jacq.) has antibacterial effects and has been used as a mouthwash to inhibit tooth decay caused by Streptococcus mutans [71] ( Figure 6 ).
Research on the health benefits of liquid smoke from rice husks (Oryza sativa) has indicated anti-diabetic properties ( Figure 7 ). Liquid smoke from rice husks (Oryza sativa), in accordance with the requirements for drug candidates, has shown low toxicity both in vitro and in vivo. In in vitro studies, liquid smoke from rice husks (Oryza sativa) has been found to maintain the viability of INS-1 rat insulinoma β-cells [44, 56], RBL-2H3, RAW264.7 [56], BHK-21 [46], and osteoblasts [51, 54]. In an in vivo study, liquid smoke from rice husks (Oryza sativa) has been found to have low acute toxicity [45].
The anti-diabetic properties of liquid smoke from rice husks (Oryza sativa) have been demonstrated in a food supplementation animal study. The addition of liquid smoke to animal diet improves diabetes status. Mechanistically, liquid smoke from rice husks (Oryza sativa) ameliorates liver damage [44, 53], and restores the damage and size of Langerhans cells in the pancreas (the source of insulin) [44, 53], thereby restoring hepatic and pancreatic function. Improvements in markers have been observed, including decreased levels of GOT and GTP; suppressed C6, PEPCK, and GCK gene and protein expression [44, 53]; and increased GLUT2 and PPAR-γ [53]. The changes these markers suppress the inflammatory state in diabetes through decreasing pro-inflammatory cytokines, such as TNF-α, IL-1β, and IL-6 [53]. Through this mechanism, insulin production and release were increased [44, 53], helping to maintain blood glucose levels [44, 53]. Additionally, glycogen stores were restored [44], and this effect contributed to a reduction in serum lipid levels and maintenance of body weight [53].
Liquid smoke from rice husks (Oryza sativa) also has anti-inflammatory properties by inhibiting oxidative stress, NO production, iNOS gene expression, and the production of pro-inflammatory cytokines [44, 56]. Furthermore, the inhibition of myeloperoxidase β-hexosaminidase, PGE2, and LTB4 stimulates 5-LOX, COX-2, and ICAM expression, thereby supporting the anti-diabetic activity [56].
Most liquid smoke has broad spectrum antibacterial effects and therefore may be developed as an anti-microbial agent. Some gram-positive bacteria, such as Bacillus cereus [79, 97] and Listeria monocytogenes [8, 10–12, 24, 26, 77, 79, 103–109], cause human illness, including gastrointestinal and diarrheal syndrome. These bacteria are food-borne pathogens [241]. Bacteria such as Staphylococcus aureus [7, 67, 77–80, 87, 90–94, 97, 100, 101, 110–113] cause skin disease and respiratory disease [242]. Streptococcus thermophilus [99, 102], Streptococcus mutants [71], and Lactobacillus plantarum [2–4, 99] are gram-positive bacteria causing diseases in the oral cavity, particularly tooth decay or caries [243–245].
Gram-negative bacteria such as Escherichia coli [2, 77, 79, 80, 83, 84, 87, 88, 90, 92, 95, 97, 112, 113], Salmonella sp. [2, 81, 88, 96, 97], Salmonella typhimurium [77, 81, 89], Salmonella enteritidis [79], and Salmonella choleraesmus [87, 112] are also foodborne pathogens that can cause human illness, such as gastrointestinal and diarrheal syndrome. All the antibacterial properties of liquid smoke are currently focused on combating foodborne pathogens. Future discovery of other anti-bacterial properties of liquid smoke is needed.
The anti-fungal effects of liquid smoke have been widely studied in various fungi causing plant diseases, such as Calathea utilis [84], Fusarium oxysporum [115], Lasiodiplodia theobromae [116], Phytophthora infestans [117], Phytophthora palmivora [118], Colletotrichum capsica [119, 120], and Pyricularia oryzae [121]. Only one fungal species associated with human disease has been found to be inhibited by liquid smoke: Candida albicans [80].
Among the fungi affected by liquid smoke, only Candida albicans causes both mucosal or skin and systemic infections in humans. Liquid smoke from coconut shells (Cocos nucifera L.), cajuput twigs (Melaleuca leucadendra) [80], eucalyptus, and Mimosa tenuiflora [246] inhibits the growth of Candida albicans. The main components of liquid smoke responsible for its anti-fungal properties are phenolic compounds, such as phenol and guaiacol [247]. Phenol has benzene rings that interact with membranes and disrupt mitochondrial balance [248]. Moreover, the antifungal effects of guaiacol might be attributable to damage to Candida albicans membranes through disruption of Ca2+ transport channels. In addition, guaiacol increases the production of MDA, CAT, POD, and SOD, thus decreasing the oxidative response [249, 250]. These mechanisms together induce oxidative stress and compromise the antioxidant defense system in Candida albicans through a toxic free radical cascade mediated by lipid peroxidation [251]. This mechanism may provide a basis for the development of liquid smoke for candidiasis treatment ( Figure 8 ).
The limitations of this review arise from a lack of information regarding the final temperature of pyrolysis in the included studies. The final temperature of pyrolysis significantly influences the composition of liquid smoke, including its phenolic compounds, acidity, and residual products such as PAH and benzopyrene. Another limitation is the lack of compound analysis provided or conducted in most of the reviewed articles. Information on compound analysis is crucial to facilitate understanding of the active components of liquid smoke responsible for therapeutic effects. Despite these limitations, this review offers comprehensive insights into the potential health effects of liquid smoke, drawing from various studies using in vitro, in vivo, and in silico methods. Future research should focus on human trials to validate the potential therapeutic applications of liquid smoke as a drug regimen.
Conclusion
The therapeutic benefits of liquid smoke, including anti-oxidant, anti-inflammatory, anti-nociceptive, anti-bacterial, anti-fungal, and anti-viral effects, have been analyzed. Furthermore, health benefits have been demonstrated, including anti-inflammatory, anti-diabetic, anti-periodontitis, wound healing, and ulcer healing effects, as indicated by various markers associated with diseases that liquid smoke may influence. These findings may increase the use value of liquid smoke as a natural product with benefits for human health. Importantly, the liquid smoke reviewed herein is a condensed form of smoke containing water and some oxygenated compounds, thus mitigating inhalation toxicity concerns. Nevertheless, although current studies suggest minimal toxicity in vitro and in vivo, further research on the long-term health risks and safety of liquid smoke in various applications is recommended.
