Applications of Deep Eutectic Solvents in the Recovery of Bioactive Compounds from Brewer Spent Grains

Figures & data

Table 1. Composition of brewing residues reported in recent studies.

Sample	Composition (%w/w dry basis)							Total phenolic compounds (mg GAE/g dry weight)	Reference
	Cellulose	Hemicellulose	Lignin		Protein	Lipid	Ash
			Insoluble	Soluble
BSG	14.0 ± 0.2	32.0 ± 0.6	20.8 ± 0.2		22.1 ± 0.5	6.2 ± 0.3	3.32 ± 0.06	0.365^a, 0.109^b	Alonso-Riaño et al.^[Citation8]
BSG	12.31 ± 1.52	26.28 ± 0.69	3.48 ± 0.43		17.87 ± 1.34	6.72 ± 0.05	2.33 ± 1.17	-	Silva et al.^[Citation9]
craft BSG	38.1 ± 0.2^c				18.7 ± 0.1	6.3 ± 0.01	3.2 ± 0.1	0.23	Jin et al.^[Citation10]
craft BSG	18 ± 2	26 ± 2	13.5 ± 0.5	4.30 ± 0.06	17.7 ± 0.1	5.90 ± 0.01	2.90 ± 0.02	2.72 ± 0.04^a, 1.6 ± 0.2^b	Alonso-Riaño et al.^[Citation11]
BSG	14.73^d				24.83	6.60	3.25	2.33^e	Farcas et al.^[Citation12]
BSG	15.2 ± 0.50	25.1 ± 0.70	7.0 ± 0.50	5.5 ± 0.30	21.2 ± 0.20	-	2.3 ± 0.10	-	Rojas-Chamorro et al.^[Citation13]
Malt bagasse	19.62 ± 0.36^d				23.48 ± 0.63	6.31 ± 0.64	4.30 ± 0.02	1.79 ± 0.03	Saraiva et al.^[Citation14]
Light BSG	-	-	1.52 ± 0.01		20.12 ± 0.1	4.22 ± 0.14	3.23 ± 0.06	0.83 ± 0.01	Kumari et al.^[Citation15]
Dark BSG	-	-	2.54 ± 0.25		22.69 ± 0.07	6.75 ± 0.98	1.97 ± 0.12	4.88 ± 0.59

GAE: gallic acid equivalent, ^aWater extractable polyphenols, ^bEthanol extractable polyphenols, ^creported as total dietary fiber, ^dreported as crude fiber, ^emg GAE/g fresh weight.

Table 2. The improvement of food products application with brewing by-products.

Raw material	Food products	Improvement of nutritional and sensory properties	Reference
BSG	Cookie	Substitution of BSG improved the protein, arabinoxylan, and phenolic acid contents. The added BSG increased the antioxidant activity (ABTS and DPPH) of the cookies (p < .05). The cookies produced with 10–20% BSG inclusion presented a high overall acceptability score similar to the control cookies, which were assumed to be acceptable.	Heredia-Sandoval et al.^[Citation61]
	Muffin	Fortified muffins with BSG enhanced the chemical composition of final products including protein and total dietary fiber contents. Total phenolic content and free radical scavenging activity were also increased compared to control. The addition of 15% BSG was observed to retain the brightness of muffins and presented a slightly higher mean score for appearance and color.	Shih et al.[24]
	Chip	New chip products containing different levels of BSG showed no significant difference in texture, color (L* and a* values), and flavor. Consumers have preferred the texture of 40% BSG substitution over 8%.	Garrett et al.[7]
	Pasta	The addition of BSG and fermented BSG can improve the nutritional profile of pasta; having higher protein and fiber contents than pasta without BSG. The strong gluten network formation was enhanced with BSG and the pasta firmness was slightly increased.	Neylon et al.[58]
Malt bagasse	Hamburger	The replacement of fat in hamburgers with malt bagasse (MB) increased protein and fiber contents, while reduced fat content. The antioxidant activity (ABTS) was increased according to the increase of MB (p < .5). The inclusion of MB had no influence on the color of hamburgers in terms of lightness and yellowness, the difference between control and hamburger with MB was not detected by consumers.	Saraiva et al.^[Citation14]
Spent malt rootlet	Biscuit	The addition of spent malt rootlets flour (MRF) in biscuits increased the content of polyunsaturated fatty acids (PUFAs) significantly (p < .5) and linoleic acid was the most represented. Monounsaturated fatty acids (MUFAs) content was highest in biscuits with 25% MRF. The presence of limonene was detected according to the amount of MRF added. The biscuit with 15% MRF was favored by the panelists in terms of texture and overall acceptability.	Chiş et al.^[Citation60]

Table 3. Comparative properties of ILs and DESs.^[161,162].

	Ionic liquids (ILs)	Deep eutectic solvents (DESs)
Chemical component	Mixtures of organic heterocyclic cations with organic or inorganic anions.	Mixture of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs).
Intermolecular interaction	Ionic interaction.	Hydrogen bond.
Melting point	Both ILs and DESs exhibit low melting points below 100°C. DESs, however, have significantly lower melting points than each individual component.
Density and viscosity	The density and viscosity of the solvents are influenced by their composition and temperature.
Polarity	Polar	Can be polar (hydrophilic) or non-polar (hydrophobic) depending on the properties of the parent components
toxicity	Less biodegradable, and higher toxicity than DESs.	Stable, biocompatible, more biodegradable, and have lower toxicity than ILs.

Table 4. Preparation of DES or NADES for bioactive compound extraction.

Sample	Target compound	DES/NADES			Water content	Solid:Liquid	Preperation method	Extraction method	Result	Reference
		HBA	HBD	Molar ratio
Cistus creticus	Polyphenolic and volatile compounds	trisodium citrate	Glycerol	1:15	20% (v/v)	1 g:30 mL	Stirred (400 rpm)at 70℃	Conventional method: stirred (500 rpm) at 50℃ for 3 h	TPC extraction was achieved by using DES (ChCl:glycerol) with PEF pre-treatment.	Palaiogiannis et al.^[Citation87]
		ChCl	Glycerol	1:2
		glycine	Lactic acid	1:5				PEF (duration 1000 µs/sample, frequency 1 hz,)
		sodium butyrate	Glycerol	1:6
Sour cherry pomace	Polyphenols	ChCl	Malic acid	1:1	20% (w/w)	300 mg:4 mL	Stirred (650 rpm)at 50℃	Conventional method: stirred (650 rpm) at 40℃ for 30 min	DES based on ChCl:malic acid showed high efficiency for anthocyanin extraction (62.33%).	Popovic et al.^[Citation90]
			Urea					UAE at 40℃ for 30 min
			Fructose					MAE at 90W up to 15s
		ChCl	Malic acid	1:1			Using ultrasound in an ultrasonic bath at 50℃, 37 khz, 30 W	Conventional method: stirred (650 rpm) at 40℃ for 30 min
			Urea					UAE at 40℃ for 30 min
			Fructose					MAE at 90W up to 15s
		ChCl	Malic acid	1:1			Microwave irradiation using microwave oven at 180 W within 30s	Conventional method: stirred (650 rpm) at 40℃ for 30 min
			Urea					UAE at 40℃ for 30 min
			Fructose					MAE at 90 W up to 15s
Spent hops	Xanthohumol	ChCl	Glycerol	1:2	5% (w/w)	1:10, 1:20, 1:40, and 1:50 (w/w)	Stirred in an oil bath at 80°C for 20-40 min	Stirred (500 rpm) at 60°C for 1 h in an oil bath	The highest xanthohumol yield (2.30 mg/g spent hop) was obtained using ChCl:propylene glycol (1:2) with 5%wt water, solid:liquid 1:50 (w/w), and 3:1 antisolvent/DES (v/w) at 60 °C for 1 h.	Grudniewska andPopłoński^[Citation97]
			Ethylene glycol		10% (w/w)
			Propylene glycol		5% (w/w)			Extracted with methanol, acetone; stirred (500 rpm) at room temperature for 1 h
			Lactic acid		5% (w/w)
Tartary buckwheat	Rutin	ChCl	1,2-propanediol	1:1	0-50% (w/w)	40 mg:1 mL	Stirred at an elevated temperature(⩽100 °C)	UAE at 40℃ for 1 h, 20 khz, 200 W	ChCl:glucerol showed the highest extraction yield of rutin, 9.6 mg/g (at 20% water, w/w).	Huang et al.^[Citation98]
			Glycerol
			Glucose	2:5
			Sucrose	1:1
			Xylitol	1:2
			Sorbitol	2:5
		Glycerol	L-proline	3:1
			L-alanine
			Glycine
			L-histidine
			L-threonine
			L-lysine	4.5:1
			L-arginine	4.5:2
White ginseng	Ginsenosides	Glycerol: L-proline: Sucrose		9:4:1	added small amount of water	100 mg:1.9 mL	Freeze dried for 24 h or until the mixture reached a constant weight.	UAE for 30 min	DES based on glycerol: L-proline: sucrose in combination with UAE has been shown to be significantly more efficient in a short extraction time (30 min).	Jeong et al.^[Citation94]
								Heated at 70 °C for 1 h
								HRE at 60℃ for 1 h
Olive leaves	Phenolic compounds	ChCl	Acetic acid	1:2	20% (v/v)	0.036 g:1.8 mL	Submerged at 60–80℃ in a water bath, then stirred at 600 rpm	Stirred (800 rpm) at 50℃ for 3 h	ChCl:acetic acid (54.1°C, 50.0% water addition) extracted 15% more phenolic compounds than ethanol.	de Almeida Ponteset al.^[Citation99]
			Malonic acid	1:1
			Malic acid	1:1
			Citric acid	2:1
Foxtail millet bran	Phenolic compounds	ChCl	Oxalic acid	1:1	30% (v/v)	1 g:20 mL	Magnetically stirred at 80℃	UAE at 50℃ for 30 min, 250 W	-The optimized RSM conditions were ultrasonic power at 247 W for 31 min, 29% (v/v) water, and a temperature of 61°C. -Betaine:glycerol and glycerol:citric acid showed the highest TPC.	Zheng et al.^[Citation100]
			Lactic acid	1:2
			Glycerol	1:3
		L-Proline	Glycerol	2:5
			Lactic acid	1:1
		Glycine	L-lactic acid	1:5
		Betaine	Glycerol	1:2
		Glycerol	Citric acid	1:2
			Sodium acetate	3:1
Moringa oleifera L. leaves	Phenolic compounds	ChCl	Malic acid	1:1	30% (v/v)	0.1 g:1 mL	Magnetically stirred at 80℃	UAE at 40℃ for 30 min, 320 W	-The optimal RSM conditions were 40 °C for 15 min, 37% water, power at 144 W, and a solid:liquid ratio of 1:80 (w/v). -L-proline:glycerol showed the highest TPC.	Wu et al.^[Citation101]
			Glycerol
			Oxalic acid
			Xylitol
			Xylitol	1:2
			Levulinic acid
			Ethylene glycol
		Betaine	Levulinic acid	1:2
			Glycerol
		Citric acid	Glycerol	1:2
		L-Proline	Levulinic acid	1:2
			Glycerol	2:5
			Lactic acid	1:1
Soy products	Isoflavones	ChCl	D-glucose	2:1	30% (v/v)	1:1, 1:2, 1:3, 1:4, 1:5, 1:7.5 (mg/µL)	Stirred (600 rpm) at 70℃ for 15-30 min	UAE at 60 °C for 60 min, 440 W	The optimized RSM conditions were NADES based on ChCl:citric acid (1:1) at 60 °C for 60 min, 30% water, solid:liquid 1:3, and power 616 W.	Bajkacz and Adamek^[Citation102]
			L-tartaric acid	1:1
			Citric acid	1:1
				2:1
				1:2
			Saccharose	2:1
			Glycerine	1:1
				2:1
			D-xylose	2:1
		Urea	ChCl	1:1
			Citric acid	2:1
			L-tartaric acid	2:1
		Glycerine	D-glucose	2:1
			L-tartaric acid	2:1
			Citric acid	2:1
			ChCl: Citricacid: Glycerine	1:1:1
				2:2:1
Scutellaria baicalensis	Flavoniods	Citric acid	β-alanine	1:1	20, 30, 40, 50 and 60%(w/w)	100 mg:2 mL	Stirred at 50℃	Incubated at 40℃ for 1 h, ultrasonicated for 30 min at room temperature, and centrifuged for 20 min	The highest flavonoids yield was obtained with citric acid:β-alanine (1:1) with 50% water (w/w) and citric acid:proline (1:1) with 40% water.	Oomen et al.^[Citation103]
			Glucose
			Xylitol
			Proline
Rosemary (Rosmarinus officinalis L.)	Phenolic compounds	ChCl	Glycerol	1:2	10% (w/w)	150 mg:2.85 mL	Stirred at 50℃	UAE at 40±1℃ for 120 min, 50-60 khz	The highest TPC was obtained from ChCl:1,2-propanediol (62.21±3.85 mg GAE/g). ChCl:glycerol presented the highest value of DPPH.	Barbieri et al.^[Citation104]
			Oxalic acid	1:1
			Lactic acid	1:3
			1,2-propanediol	1:2
Wine lees	Anthocyanins	ChCl	citric acid		50-90% (w/w)	0.1 g:1 mL	Stirred at 80°C for 2-6 h	UAE at 35℃ for 15-45 min, 190-380 W, 37 khz	The optimized RSM conditions were an extraction time of 30.6 min, ultrasound power of 341.5 W, and 35.4% (w/w) water.	Bosiljkov et al.^[Citation105]
			oxalic acid
			malic acid	1:1
			glucose	2:1
			fructose	1.9:1
			xylose	2:1
			glycerol	1:2
Apple pomace	Phenolic compounds	ChCl	Lactic acid	1:1	50% (v/v)	2.5, 5 and 7.5% (w/v)	Stirred at 60℃	UAE at 60℃ for 2 h, 37 khz, 140 W	Sugar-based NADES (glucose:fructose:water) were the most effective NADES, followed by acidic NADES.	Deniz et al.^[Citation106]
				1:6
				1:9
			Malic acid	1:1			Stirred at 70℃
			Citric acid	1:1			Stirred at 80℃
				3:1
			ChCl: Sucrose:Water	1:1:11						Stirred at 50℃
			Glucose: Fructose: Water
			Fructose: Sucrose: Water
			Glucose: Sucrose: Water
		ChCl	Ethylene glycol	1:2			Stirred at 80℃
			Glycerol
			Urea
Apple pomace	Flavonoids and phenolic compounds	ChCl	Glycerol	1:2	30%	1:30 (w/v)	kept at 80℃ for 3 h in a shaking water bath	UAE at 40℃ for 40 min, 20 khz, 400 W	ChCl-based DES with glycerol, lactic acid, and citric acid performed better extraction than the conventional method. The highest TPC value was achieved using sonication at 40℃ for 40 min, 30% water, and a solid:liquid ratio of 1:30.	Rashid et al.^[Citation106]
			Lactic acid	1:3
			D-glucose	3:2
			Urea	1:2
			Citric acid	1:1
		Urea	Glycerol	1:2
		Tartaric acid	Glucose	1:1
Chinese wild rice (Zizania latifolia)	Flavonoids and phenolic compounds	ChCl	Glycerol	1:2	30% (w/w)	50 mg:2 mL	Magnetically stirred at 80℃	UAE at 50℃ for 20 min, 25 khz, 200 W	The optimal conditions were DES (choline chloride:1,4-butanediol) with UAE at a temperature of 51.2℃ and a solid:liquid ratio of 37.0 mg/mL.	Zeng et al.^[Citation107]
			1,4-butanediol	1:6
			D-fructose	1:1
			D-glucose	3:2
			Lactic acid	1:2
			DL-malic acid	1:1
			Urea	1:2
Helichrysum arenarium L.	Phenolic compounds	ChCl	Lactic acid	1:1	NADES:water, 3:1 (w/w)	500 mg:15 mL	Stirred at 80℃	UAE at 50℃ for 60 min (1st extraction) and 20 min (2nd extraction)	The optimized RSM conditions were an UAE time of 85 min, 38% water, and a molar ratio of ChCl:lactic acid of 1:4.	Ivanović et al.^[Citation108]
			Malic acid
			Malonic acid
			1,4-butanediol	1:6
			1,2-propanediol	1:2
			Glycerol
			Ethylene glycol
			ChCl: Fructose: Water	2:1:1
			ChCl: Maltose: Water	3:1:2
			ChCl: Glucose: Water	2:1:1
		ChCl	Urea	1:2
Kinkeliba (Combretum micranthum G. Don)	Phenolic compounds	ChCl	Lactic acid	1:2	20%	1 g:20 g DES	Stirred at 80℃ for 2 h	UAE at 25℃ from 30 min, 40 khz, 296W	ChCl:lactic acid showed the highest TPC values, followed by ChCl:acetic acid and ChCl:tartaric acid.	Zannou et al.^[Citation109]
			Tartaric acid					HAE: homogenized at 100 rpm, 25℃ for 30 min.
			Acetic acid					ME: at 25℃ for 12 h
Elderberry flower (Sambucus nigra)	Polyphenol	Glycine	L-lactic acid	1:5	DES:water 55, 70, 85% (w/v)	1 g:35 mL	Heated at 80℃ for 120 min	Pretreatment with ultrasound at ambient temperature (23 ± 1℃) for 5-40 min, 50 hz, 550 W. Extracted by stirring (500 rpm) at 50℃ for 150 min	The optimized RSM conditions were operated with stirring at 200 rpm, DES:water (85% w/v), and solid:liquid 1:60 (g:mL).	Kaltsa et al.^[Citation110]
				1:7
				1:9
				1:11
				1:13
Spent hops	Proteins	ChCl	Ethylene glycol	1:2	10 %wt	1:50 (w/w)	Stirred at 80℃ in an oil bath (20-40 min)	Extracted with trichloroacetic acid, acetone, phenol; centrifuged at 4℃	High protein yields were obtained from ChCl:ethylene glycol and ChCl:lactic acid, which varied between 40 and 64%.	Grudniewska and Pastyrczyk^[Citation111]
			Propylene glycol		5 %wt	1:40 (w/w)
			Glycerol			1:50 (w/w)
			Lactic acid			1:40 (w/w)
Microalgae (Chlorella pyrenoidosa)	Lipid	ChCl	Acetic acid	1:3	-	0.2:4.0 (w/w)	Stirred (≤800 rpm) at 80℃ for 2 h in water bath	Pre-treatment with DES at room temperature (28℃) for 5 and 10 min: manually shaken, stirred, and stirred with an ultrasonic homogenizer. Extracted with solvents (methanol and n-butanol)	The highest lipid yields (19.25%) were obtained using pretreatment with an ultrasonic homogenizer-assisted DES (ChCl:acetic acid, 1:3).	Ngatcha et al.^[Citation112]
Microalgae (Dunaliella salina)	Lipids and carotenoids	ChCl	Oxalic acid	1:1	*	20 mg:100 µL	Stirred at 80℃	Pre-treatment with DES: magnetically stirred (150 rpm) at ±29℃ for 24 h, followed by extraction with ethyl acetate and ethanol solvent	The total lipid recovery was 67.41%, achieved using a ChCl:urea-based DES without adding water to the system.	Asevedo et al.^[Citation113]
					20% (v/v)
			Urea	1:2	*
					20% (v/v)
			Glycerol	1:2	*
					20% (v/v)
Microalgae(Nannochloropsisgaditana)	Fatty acids(omega-3eicosapentaenoic acid)	ChCl	Lactic acid	1:2	-	0.5 g:45 g	Stirred at 80℃	Extracted at temperatures of 60 and 70℃ for 1.5 and 3 h	The fatty acid recovery percentage was higher (118%) with ChCl-ethylene glycol.	MorenoMartínezet al.^[Citation114]
			Ethylene glycol
			Glycerol
		Sodium acetate	Lactic acid	1:3
Dipsacus fullonum leaves	chlorogenic acid derivatives, flavone glycosides and iridoid glycosides	ChCl	Lactic acid	1:1-1:4	0-60%	-	Stirred at 80°C for 30-60 min	Shaken at room temperature for 1 h, then shaken in an ultrasonic bath at 40℃, 350 khz, 640 W for 30 min	-RSM and proposed optimized conditions: ChCl:lactic acid (1:2.4), 35% water, and a solid:liquid ratio of 1 g:27 mL.-The highest content of the extract was derived from flavone glycosides (0.47–2.28 mg/g), followed by chlorogenic acid (0.38–1.65 mg/g) and iridoid glycosides (0.17–0.92 mg/g).	Saar-Reismaa et al.^[Citation115]
		Choline acetate	Lactic acid	1:2	30%
		ChCl	Glycerol	1:1	30%
		Betaine	Lactic acid	1:2, 1:3	30%
		Menthol	Formic acid	1:1	*
			Acetic acid	1:1
			Lactic acid	1:1
			Formic acid, acetic acid, lactic acid	1:3 (1:1:1)
Sugarcane bagasse	Lignin	ChCl	Glycerol	1:2	-	1 g:10 mL	Stirred (300 rpm) at80-90℃	Heat at temperature range from 80-120℃ in sand bath for 2 h	The optimized conditions for lignin extraction were ChCl:glycerol at 135°C for 9 h of operation time and solid:liquid 1:23.41 (w/v), with obtained lignin yields of 70.31±1.91%.	Varilla-Mazaba et al.^[Citation116]
			Urea
Hemp bast	Cellulose fiber	ChCl	Urea	1:2	-	1:10 (w/v)	Heated at 80-90℃ for 1-2 h in an oil bath.	MAE at 100℃, 1000 kW for 5 min	The higher yield of cellulose (83.27%) was obtained from ChCl:urea.	Ahmed et al.^[Citation117]
			Glycerol
			Citric acid
		Sulfamic acid	Urea

*DES without water added, PEF-pulsed electric fields, MAE-microwave-assisted extraction, UAE-ultrasound-assisted extraction, HRE-heat reflux extraction, ME-maceration extraction, HAE-homogenate-assisted extraction.

Table 5. Applications of DES or NADES for extraction of BSG.

Target compound	DES/NADES			Water content	Solid:Liquid	Preparation method	Extraction method	Result	Reference
	HBA	HBD	Molar ratio
Lignin	ChCl	Lactic acid	1:5	5, 10, 15, 20, and 50% (v/v)	1:10 (w/w)	Stirred (100 rpm) at 60°C for 10–15 min	Extracted at temperatures 60, 90 and 120°C for time 1, 3 and 5 h	Lignin yield was 54.4% with purity of 77.2%,and TPC was 88.8 ± 9.2 mg GAE/g lignin.	Cassoni et al.^[Citation146]
Lignin	ChCl	Lactic acid	1:10	-	1:20 (w/v)	Stirred at 80°C for 2 h	Magnetically stirred at 60 or 80°C for 24 h in an oil bath	Lignin extraction yields of 39.3% and 34.5% were obtained from DES ChCl: lactic acid at80 and 60°C, respectively.	Provost et al.^[Citation147]
		Glycerol	1:2
Protein	Sodium acetate	Urea	1:2	with and without 10 wt% of water	-	Stirred at 80°C	Extracted at 80°C (90°C for sodium formate:urea mixtures) for 1, 2, 4 and 20 h.	High extraction yields of 79% and 74% were obtained with sodium acetate: urea in molar ratios of 1:2 and 1:3, respectively, and also with potassium acetate: urea (1:3) (78%).	Wahlström et al.^[Citation153]
			1:3
	Potassium acetate		1:2
			1:3
	Sodium formate		1:2
			1:3
Phenolic compounds	ChCl	Ethylene glycol	1:2	20–70% (v/v)	10% (w/v)	Stirred (500 rpm) at 80°C for 30–90 min	MAE at 65°C for 20 min, maximum power 700 W	The optimum condition of microwave-assisted DES extraction was observed with ChCl:glycerol at 100°C for 13.30 min and 37.46% (v/v) water in the DES, which contained TPC of 2.89 mg gallic acid/g BSG.	López-Linares et al.^[Citation160]
		Lactic acid
		Glycerol
		1,2-propanediol

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