citric acid sodium extraction system

Journal Pre-proofs Short communication New type of green extractant for oil production: citric acid/citric acid sodium extraction system Wen-Can Huang, Binglinlin Li, Xiangming Qi, Xiangzhao Mao PII: DOI: Reference:

S0308-8146(19)31949-1 https://doi.org/10.1016/j.foodchem.2019.125815 FOCH 125815

To appear in:

Food Chemistry

Received Date: Revised Date: Accepted Date:

8 August 2018 8 October 2019 27 October 2019

Please cite this article as: Huang, W-C., Li, B., Qi, X., Mao, X., New type of green extractant for oil production: citric acid/citric acid sodium extraction system, Food Chemistry (2019), doi: https://doi.org/10.1016/j.foodchem. 2019.125815

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New type of green extractant for oil production: citric acid/citric acid sodium

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extraction system

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Wen-Can Huanga§, Binglinlin Lia§, Xiangming Qia*, Xiangzhao Maoab*

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aCollege

of Food Science and Engineering, Ocean University of China, Qingdao

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266003, China bLaboratory

for Marine Drugs and Bioproducts of Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China

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* Corresponding author:

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Professor Xiangming Qi; E-mail: [email protected]

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Professor Xiangzhao Mao; E-mail: [email protected]

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§ These authors contributed equally.

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ABSTRACT

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Developing green solvents with low toxicity and low energy consumption is an

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important issue for edible oil production. In this study, a novel extraction system,

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specifically a citric acid/citric acid sodium mixture, was developed for oil extraction

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from seed crops. Peanut and pumpkin seeds were used to evaluate extraction

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efficiency and more than 70% and 57% oils, respectively, were extracted from peanut

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and pumpkin seeds at 4 ºC. After extraction, the oils floated on the surface of the

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solution and could be separated from the solvent system without evaporation. The

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extraction of edible oils was achieved without the use of toxic chemicals or

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energy-intensive equipment. This study provided a green and efficient method, and

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showed the potential of the proposed citric acid/citric acid sodium extraction system

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for production of edible oils from natural sources.

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Keywords: Citric acid, Citric acid sodium, Green extractant, Vegetable Oils,

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Extraction, Nature products, Peanut seeds, Pumpkin seeds

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1. Introduction

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Vegetable oils extracted from plants are used widely in the food, pharmaceutical,

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and cosmetic industries (Jiao, Li, Gai, Li, Wei, Fu, et al., 2014; Juliano, Cossu,

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Alamanni, & Piu, 2005; Rahate & Nagarkar, 2007). Edible vegetable oils contain a

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variety of bioactive substances and nutrients including vitamin E, phospholipids,

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choline, and omega-3 and omega-6 fatty acids as well as other polyunsaturated,

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monounsaturated, and saturated fats (Jiao, et al., 2014; Lu, Zhao, Yu, & Feng, 2015;

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Maier, Schieber, Kammerer, & Carle, 2009; Zhong, Bedgood, Bishop, Prenzler, &

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Robards, 2007). Because vegetable oils have high energy contents, some (e.g.

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soybean and corn oil) are also used as biodiesel feedstocks (Martínez, Sánchez,

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Encinar, & González, 2014; Patil & Deng, 2009).

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Various extraction techniques have been used for extraction, such as solvent,

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supercritical fluid, mechanical pressing, enzymatic, ultrasonic, and heat reflux

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extraction methods (Deng, Li, Li, Zaaboul, Jiang, Li, et al., 2018; Ghafoor, Choi, Jeon,

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& Jo, 2009; Wang & Weller, 2006). Among these techniques, solvent and

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supercritical fluid extractions, and mechanical pressing are used most commonly.

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Extraction using organic solvents is rapid and effective, but most organic solvents are

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highly toxic, and their use can have undesirable effects on oil quality (Jiao, et al.,

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2014). Furthermore, organic solvent extraction requires energy-intensive evaporation

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for removal of the solvent, and the solvent can lead to environmental pollution (Lai,

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De Francesco, Aguinaga, Parameswaran, & Rittmann, 2016; Mubarak, Shaija, &

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Suchithra, 2015). In contrast to organic solvent methods, supercritical fluid extraction

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has lower viscosities and higher diffusivities (Leunissen, Davidson, & Kakuda, 1996;

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Palmer & Ting, 1995). However, despite its potential advantages, use of supercritical

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fluid extraction is limited due to high installation and operating costs (Lam & Lee,

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2012). Mechanical pressing is commonly used to extract oils from nuts and seeds.

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However, the oil produced with high temperature pressing technique has a poor

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sensory quality and a significant loss of vitamin E, sterol, wheat germ phenol,

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phospholipid, and other nutritional factors, and moreover, the stability of the oil is

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poor. On the other hand, only less than 10% of peanut oil can be extracted by using

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cold pressing technique. Thus, a cheap, safe, and environmentally friendly method for

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industrial-scale extraction of edible oils.

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In this study, a green extraction system, citric acid/citric acid sodium, was

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developed. Peanut and pumpkin seeds were used to evaluate extraction efficiency.

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Edible oil extraction was achieved without the use of toxic chemicals or any

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energy-consuming processes, such as cell disruption or solvent evaporation.

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2. Materials and methods

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2.1. Materials

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Nile red, a standard mixture of 37 fatty acids methyl esters (FAMEs), and

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nonadecanoic acid methyl ester were purchased from Sigma-Aldrich (St. Louis, MO,

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USA); citric acid, sodium citrate, chloroform, and methanol were purchased from

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Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China); peanut and pumpkin seeds

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were purchased from a local market (Qingdao, China).

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2.2. Oil extraction

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The peanut and pumpkin seeds were dried in an oven at 60 °C and then ground.

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The volume ratio of citric acid (0.1 mol/L) and citric acid sodium (0.1 mol/L) was

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varied to adjust the pH to 2.2, 3.0, 3.2, 3.4, and 3.8. The mass ratio of ground peanut

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or pumpkin seeds to solvent was fixed at 1:2. The mixtures were maintained at 4 ºC

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for 48 h before the oils were collected from the top of the solution using a pipet. Each

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sample was extracted one time and all the experiments were replicated three times.

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2.3. Total oil extraction

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Total lipids were extracted from the peanut and pumpkin seeds using the Folch

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method (Folch, Lees, & Sloane Stanley, 1957). Briefly, 10 g of freeze-dried peanut

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and pumpkin seeds were grounded and then added to chloroform-methanol (2:1);

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lipids were obtained by evaporating the solvents after extraction. Experiments were

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replicated three times.

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2.4. Conversion of oils

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To determine the oil contents, isolated oils were converted to fatty acid methyl

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esters (FAMEs) via transesterification. Briefly, samples of the crude oils (100 mg)

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were dissolved in 3 mL chloroform/methanol (1:1) and sulfuric acid was added to

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each as a catalyst, and 100 μL of nonadecanoic acid methyl ester as an internal

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standard was added to the solution. The mixtures were heated at 85 ºC for 1 h in a

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water bath.

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FAME contents were analyzed by gas chromatography–mass spectrometry

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(Trace 1310 GC, Thermo Fisher Scientific, Waltham, MA, USA). The injection port

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temperature was 290 °C. Analytes were separated by a TG-5MS column, with helium

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as carrier gas. Mass spectra were recorded under electron ionization (70 eV) with the

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m/z range of 30 to 400 amu. The temperature of the column was increased from 80 °C

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to 200 °C at 10 °C min-1, from 200 °C to 250 °C at 5 °C min-1, and from 250 °C to

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270 °C at 2 °C min-1.

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The oil extraction efficiency (EE) was determined using:

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where Oils

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extraction system, and Oils chloroform/methanol is the oils extracted by Folch method.

citric acid/citric acid sodium

is the oils extracted by citric acid/citric acid sodium

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2.5. Microscopic observation

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To observe any changes in the oil drops before and after treatment, peanut and

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pumpkin seed cells were stained with Nile red. The stain was pre-prepared as a stock

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solution (1 mg/ml) in ethanol and samples stained for 10 min prior to observation

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using confocal laser microscopy (FV1000, Olympus, Tokyo, Japan).

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Structural changes in the samples before and after citric acid/citric acid sodium

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treatment were observed using scanning electron microscopy (JSM-840 SEM, JEOL

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Ltd., Tokyo, Japan). Dried samples were placed on a metal substrate using carbon

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tape and sputter-coated with gold and the results recorded at an acceleration voltage

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of 10 kV.

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3. Results and discussion

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3.1. Oil extraction of citric acid/citric acid sodium extraction system

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The extraction efficiency of the citric acid/citric acid sodium system was

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investigated under various conditions. As shown in Fig. 1, extraction efficiency was

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best when the pH of the citric acid/citric acid sodium mixture was less than 3.

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However, the greatest oil extraction efficiency was at pH 3; above pH 3, the

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extraction efficiency decreased gradually. At pH 3, more than 70% and 57% of oils

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were extracted from peanut seeds and pumpkin seeds, respectively. Although oil

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could not be extracted from peanut seeds at pH 2.2 and 3.8, 25% and 20% of oil

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contents were extracted from pumpkin seeds at pH of 2.2 and 3.8, respectively. The

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citric acid/citric acid sodium extraction system, as a green extractant possesses many

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properties, such as being nonvolatile, biodegradable, nontoxic, recyclable, and stable.

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Furthermore, in comparison with other extraction methods, such as solvent extraction

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and supercritical fluid extraction, the citric acid/citric acid sodium extraction system

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does not require energy-consuming processes or high operating cost. On the other

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hand, compared with cold pressing method, which is considered having a promising

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market prospects, the citric acid/citric acid sodium extraction system resulted in much

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higher oil yield. The oil extraction mechanism of the citric acid/citric acid sodium

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system is not yet well understood and requires further research.

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3.2. Microscopic analysis

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The peanut and pumpkin oils, before and after extraction, were imaged using

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laser scanning confocal microscopy. As shown in Fig. 2, before extraction, the peanut

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and pumpkin oil drops were small and regular with a diameter of about 1 µm. The

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size was in accordance with a previously reported oil drop size distribution in peanut

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cotyledons (Carlton, Halse, Maphossa, & Mallett, 2001), indicating that oils had not

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been released before citric acid/citric acid sodium treatment. After extraction, some

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large coalesced oil droplets were observed. Since surface tension always allows small

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droplets to coalesce and maintain a nearly spherical shape permitting ready drainage

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of surrounding fluid (Neitzel & Dell'Aversana, 2002), this result suggested that oils

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previously bound to the cell structure were separated by citric acid/citric acid sodium

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extraction system and released into the solution after citric acid/citric acid sodium

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treatment.

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The morphology of peanut and pumpkin seed cells, before and after extraction,

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were examined using scanning electron microscopy (SEM). As shown in Fig. 3a and

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3c, SEM images of peanut and pumpkin seeds without treatment were blur. It was

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because before citric acid/citric acid sodium treatment, there were oils on the surface

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of the ground peanut and pumpkin seeds. Oils can affect the current of SEM, resulting

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in blurred images. After treatment, SEM images became clear (Fig. 3b and 3c). The

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results of microscopic analysis indicated that acid/citric acid sodium extraction system

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can effectively extract oils from peanut and pumpkin seeds.

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4. Conclusions

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In this study, citric acid/citric acid sodium extraction system suggest its potential

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for edible oil extraction. Using this system, significant amounts of oils were isolated

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from peanut and pumpkin seeds, which floated on the surface allowing easy

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separation from the aqueous phase without solvent evaporation, avoiding the use of

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volatile solvents and reducing energy consumed in their removal. The proposed

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extraction system could be applied for the extraction and recovery of plant oils with

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the products being used by pharmaceutical, cosmetics, and food industries. Further

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research will be required to determine the citric acid/citric acid sodium extraction

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system mechanisms of action and implications for edible oil production at industrial

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scales.

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Acknowledgments This work was supported by the Key Research and Development Project of Shandong Province (No. 2016YYSP015).

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Fig. 1. Oil extraction efficiency of citric acid/citric acid sodium extraction system at

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various pH levels.

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Fig. 2. Confocal laser micrographs of peanut seeds (a) without treatment and (b)

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treated with citric acid/citric acid sodium; pumpkin seeds (c) without treatment and

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(d) treated with citric acid/citric acid sodium.

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Fig. 3. SEM images of peanut seeds (a) without treatment and (b) treated with citric

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acid/citric acid sodium; pumpkin seeds (c) without treatment and (d) treated with

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citric acid/citric acid sodium.

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Highlights

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1. A citric acid/citric acid sodium extraction system was developed for vegetable oil extraction. 2. More than 70% and 57% oils were extracted from peanut seeds and pumpkin seeds, respectively. 3. Extracted oils floated on the solution surface and can be separated easily without evaporation.