Synergism of essential oils with lipid based nanocarriers:emerging trends in preservation of grains and related food products

Satyavani Kaliamurthi,Gurudeean Selvaraj,Lifen Hou,Zhao Li,Yongkai Wei,Keren Gu,Dongqing Wei*

a Center of Interdisciplinary Science-Computational Life Sciences,College of Food Science and Engineering,Henan University of Technology,Zhengzhou 450001,China

b College of Chemistry,Chemical and Environmental Engineering,Henan University of Technology,Zhengzhou 450001,China

c College of Science,Henan University of Technology,Zhengzhou 450001,China

d The State Key Laboratory of Microbial Metabolism,College of Life Sciences and Biotechnology,Shanghai Jiao Tong University,Shanghai 200240,China

Keywords:

Antimicrobial

Essential oil

Food pathogens

Grains

Nanocarriers

ABSTRACT

Grains are one of the major food staples in the world.The cereal grains are easily susceptible to damage by moisture content,flour beetles and food pathogens during storage,harvesting and post harvesting.Food preservative techniques namely drying,freezing,and dehydration,acquire little advantages.However,they cause few undesirable alterations in the organoleptic and nutritional properties of the preserved food items.Therefore,there is a continuous search for new preservation techniques in food industries,to satisfy the customer demands on the addition of natural food preservatives,devoid of pathogenic contaminants and without changes in organoleptic properties.Essential oils(EOs)have been predicted as“natural food additives”in the preservative process.The synergistic potential of EOs with various nanocarriers plays an emerging role in the food industry.Therefore,the present review has focused on the lipid based nanocarriers,and the methods used for the functionalization or encapsulation of EOs and applications in the preservation of food items such as cooked rice,rice flour,grains,sliced breads have also been discussed.The present review ascertains the antimicrobial significance of active EOs loaded lipid nanocarriers in the form of nano emulsions,solid lipid nanoparticles and liposomes for preserving grains and flours.

1.Introduction

Grains are one of the major food staples in the world.Specifically,rice,wheat,and maize offer 60%of the food related energy input around the world[1].According to the recent report by the Food and Agricultural Organization (FAO), the consumption of grains and their products has attained the level of ＞2600 million tons in 2019[2].However,cereal grains are easily accountable to damage by moisture content,flour beetles and food pathogens during storage,harvesting and post harvesting conditions.Food preservation is one of the essential processes during harvesting and transportation.The food preservation process prevents the growth of food spoiling pathogenic microbes(bacteria,fungi or yeast)and decreases the unpleasant odor through decomposition of fat or oil materials(rancidity)in food.The primary food borne pathogens and root infecting pathogens for cereal grains and their products include Aspergillus sp.,Bacillus cereus,Escherichia coli O157,Listeria monocytogenes,Rhizoctonia solani,Salmonella sp.,and Staphylococcus aureus,etc.[3–5].The World Health Organization(WHO)has reported that nearly one in ten people suffers from foodborne illnesses(FBI)via consuming the contaminated food items,which results in about 420,000 deaths per year.Notably,the Africans and the Southeast Asians have an elevated level of health burden due to FBI[6].The preservation techniques such as heating,chilling,canning,fermentation,and dehydration are predominantly used to lower the risk of food poisoning.Even though preservation process endows exceptional advantages it causes undesirable alterations in the organoleptic(flavor,consistency,and appearance)and nutritional properties of the food substances.

Previous studies have reported that weak organic acids,hydrogen peroxide,and chelators are the commonly used chemical preservatives[7].The weak organic acids such as acetic acid (CH₃COOH), lactic acid(C₃H₆O₃)and sorbic acid(C₆H₈O₂),inhibit the growth and germination of bacterial and fungal endospores,which cause adverse effects on the intracellular membrane trafficking pathways of the yeast[8,9].Moreover,hydrogen peroxide acts as a bactericidal agent but its action majorly depends on the concentration,pH,and temperature of the environment[10]. In addition, the chelators are permeabilizing agents (citric acid,EDTA)that lead to the inhibition of the gram negative bacteria.However,the chemical preservatives cause an alteration in the protein and fatty acid contents of the edible seeds[11].Besides,the foodborne pathogens include P.aeruginosa,S.typhimurium,E.coli,and L.monocytogenes showing resistance to the chemical preservatives,is another limitation in food preservation[12].Therefore,there is continuous research in the food industry and its related sectors for finding out new preservation techniques to satisfy the customers'demands on the addition of natural food preservatives but without any organoleptic changes.At present, the distributors from the food industry have recognized the consumer needs,which include cleanly labelled food items(without synthetic preservatives),and less processed,handy,and healthier food items devoid of high fat,salt,and sugar.In recent times,people prefer fast or junk food items.Frequent consumption of such foods with chemicals and artificial preservatives increases health issues such as severe inflammations[13].

EOs are defined as highly hydrophobic liquids that contain“volatile aroma”substances or compounds derived from plant materials.The EOs have the“essence of”the particular material in terms of fragrance.The materials include seeds, flower, leaves, stem, bark, fruits or whole plant[14,15].Therefore,EOs are also known as“volatile oil”“ethereal oil”and“aetherolea” [16]. Traditionally, EOs are widely used in aromatherapy and conventional treatments as a disinfectant and as anti-inflammatory substances.Despite numerous benefits and high phenolic contents of the EOs, they have grabbed the attention of the research community with their antimicrobial,anticarcinogenic,antihyperglycemic properties,and potential for supportive therapy[17–19].Earlier,the EOs have been recommended as“natural food additives”in food preservation.There is an elevating scope for EOs in natural products,not only for the control or prevention of chronic disorders but also to avoid decomposition or deterioration of food constituents.

Usually,the constituents of EOs are of low molecular weight with vast diversity in their antimicrobial actions.Based on the chemical structures,the EOs constituents were divided into(i)Terpenes(p-cyemene),(ii)terpenoids(thymol,carvacrol),(iii)Phenylpropenes(Eugenol,cinnamaldehyde,vanillin)and(iv)others(Allicin,allyl isothiocyanate),respectively[20].Certain factors including lower stability in the presence of oxygen,temperature and light-mediated influences,poor solubility and low bioavailability have limited the use of EOs in food and pharmaceutical industries[20,21].Likewise,some EOs have a unique unpleasant aroma or flavor,which inhibits their direct implementations into food products due to probable sensory changes in the food.Notably,some EOs are highly volatile;therefore,proper consideration is needed while using them for specific applications[22,23].Interestingly,since the past decades,various nanoparticle mediated systems have been playing an important role in therapeutic applications[24,25].In the face of growing interest of the nanoparticles in the food industry,there is an urge to discuss various nanoparticulate systems for the encapsulation of EOs,and their synergistic actions in the preservation of food items against pathogens.Therefore,the present review has specifically focused on the lipid-based nanocarriers and the methods used for the functionalization or encapsulation of EOs.In addition,and the applications of the encapsulated EOs for the inhibition or delay of the growth of the food pathogens during preservation or in postharvest food items such as cooked rice,rice flour,grains,and sliced bread have been discussed.

2.Synergistic impacts of EOs incorporated lipid-based nanocarriers in the preservation of grains and related food products

The nanomaterials,generally used for the transport of molecules(ex.drug, natural phyto-compounds), are known as nanocarriers. Approximately,the size of the nanocarriers varies from 1 to 1000 nm in diameter[26]. Notably, in recent years, the advantages of nanotechnology have attracted numerous researchers due to potential impacts on the antimicrobial delivery system[27,28].Due to their subcellular sizes,the emulsified nanocarriers could enhance the physical stability and distribution of the loaded antimicrobial substances in the food matrices,which gives protection against pathogenic food microbes [29]. There are two types of nanocarriers, which are widely used in food industrial sectors such as lipid and polymer based nanocarriers. Lipid based nanocarriers have unique features such as protection of the loading compounds from premature denaturation without causing changes in the organoleptic properties,extension of the shelf lives of the food products(cereal grains and seeds),and enhancement of antimicrobial and antioxidant activities against the food pathogens[5,30,31].Therefore,the present study has focused on the synergistic impacts of EOs loaded lipid based nanocarriers in food preservation. The synergistic impacts of the lipid-based nanocarriers include nanoemulsion;solid lipid nanoparticles and liposome in preservation of grains and related food products were showed in Fig.1.

2.1.Nanoemulsion

Nanoemulsion is one of the significant colloidal carriers systems within the size range of 10 to 1000 nm.The nanoparticulate colloidal particles usually contain two different immiscible phases,which are aqueous(hydrophilic)and organic(hydrophobic)phases.Moreover,one of the phases is dispersed into the remaining phase as nanosized oil droplets.The dispersed phase is otherwise known as“internal or discontinuous phase,”and the outer phase is known as“external or continuous phase”.The nanocarriers are unstable(thermodynamically)because of the immense surface tension raised in the interface junction of the two phases[32].Upon the addition of a suitable surfactant(polymethyl ester sulfonate)or a stabilizer,the surface tension gets reduced and stabilizes the nanocarrier system[31].In practice,water-in-oil (W/O), oil-in-water (O/W) and bicontinuous types of nanoemulsions can be prepared.In a recent review by Jaiswal et al.[32],the various possible methods for the preparation of nanoemulsions such as high energy and low energy emulsion methods, have been reported.Briefly,the high energy nanoemulsion methods include high energy stirring(HES),high pressure homogenization(HPH),ultrasonic emulsification(UE),microfluidization,and membrane emulsification(ME).On contrary,the low energy emulsion methods include phase inversion temperature(PIT),emulsion inversion point(EIP)and spontaneous emulsification(SE)methods,respectively[33].

Table 1a depicts the examples of synergism of EOs loaded nanoemulsion carriers in the preservation of various food products including rice,sliced bakery bread,sporulating seeds against food pathogens.B.cereus is one of the pathogenic food bacteria,which grows predominantly in cooked rice[34].The frequent incidences of the B.cereus are due to the prolonged storage of cooked rice at room temperature before consumption.Recently, Sharif et al. [31] prepared the black cumin-EOs loaded nanoemulsion by using canola and flaxseed oil with modified waxy maize-starch as a stabilizer through high speed homogenization.The formulated nanoemulsion showed prominent stability,release profile rheological characteristics(shear thinning)and bactericidal activities against the gram positive food pathogens in boiled or cooked rice products.Chia seed(CS) contains high levels of polyunsaturated fatty acids and are highly used in bread products.Due to its poor water solubility,the direct addition of chia seed oil in water based food beverages is very complicated.Teng et al.[35]prepared CS oil loaded nanoemulsion by using SE and microfluidization methods,which showed good transparency with tiny droplets(～47 nm in size),and has facilitated higher chances of its applications in food beverages.

Moreover,Topuz et al.[36]reported the anise oil(Pimpinella anisum)loaded nanoemulsion prepared by using the high speed homogenization(HSH)technique.The anise oil loaded nano encapsulated particles,exhibited stronger solubility and sustained(slower and prolonged drug release)release of the anise oil,which demonstrated significant reductions in the count of foodborne pathogens such as, E. coli O157:H7(2.51 log CFU/mL)and L.monocytogenes(1.64 log CFU/mL),respectively.The particular time duration,whereby a food product is not only harmless to consume but would have retained the taste,flavor,and appearance after being collected from its inhabitant environment,is termed as “shelf life”.Edible films(EF)with active ingredients could be practiced as an option to extend the shelf lives of food products.In the food industry,the significant antimicrobial actions of the EOs have increased more attention but their poor solubility is one of the primary concerns.Acevedo-Fani et al.[37]reported edible films(EFs)with thyme-EOs,or sage EOs(dispersed phase)and sodium alginate (continuous phase) nanoemulsion, prepared by microfluidization technique.They observed that the EF-TH EOs exhibited the most robust bacterial activities against E.coli(4.71 log)up to 12 h.

Fig.1.Lipid-based nanocarriers in the preservation of grains and related food products.Various methods used for the preparation of essential oil loaded nanoemulsion(HES,HPH,USE,MF,ME,PIT,EIP,and SE),solid lipid nanoparticles(HPH and CPH)and liposome(TFH and EH)nanocarriers also depicted.CPH:cold pressure homogenization,EIP:emulsion inversion point,HES:high-energy stirring,HPH:high-pressure homogenization,MF:micro-fluidization,ME:membrane emulsification,PIT:phase inversion temperature,SE:spontaneous emulsification,TFH:thin film hydration,and USE:ultrasonic emulsification.

S.enterica Enteritidis is the crucial factor responsible for causing contamination in a variety of seeds during sprouting(higher humidity).Proper care should be taken to prevent the foodborne microbial pathogen load in the sprouting seeds (high humidity) before germination as the count of pathogenic microbes will be increased during germination (～＞ 108CFU/g). Landry et al. [38] prepared the carvacrol loaded nanoemulsion by using SE method with Tween 80 surfactant.After treatment by the carvacrol nanoemulsion(4000 ppm),significant antimicrobial activities without changes in the germination or total sprout yield were observed in the mung bean and alfalfa seeds.Similarly,growth of yeasts and molds were controlled in sliced bread(during 15 days)while using the edible nanoemulsion films[39].Edible films with antimicrobial actions were successfully prepared by using the ultrasonication method.

2.2.Solid lipid nanoparticles

Solid lipid nanoparticles(SLNps)are one of the well known carriers for pharmaceutical substances since 1990s.SLNps contain the drug,stabilizers or surfactants(which reduce the surface tension)and the solid lipid core(optimal for the inclusion of poorly soluble,lipophilic and hydrophobic molecules), respectively [39]. The spherical shaped SLNps are 50 to 1000 nm in size. The preparation of SLNps is more similar to the nanoemulsion method but the lipid used for the lipid phase preparation is solid(example:palmitic acid)at room temperature,which gives stably dispersed nanoparticles instead of nano oil droplets[25,39,40]with more physical stabilities than the nanoemulsion.Usually,SLNps preparation depends upon the melting point temperature of the solid lipid core; two types of methods are implemented,namely,high pressure homogenization(HPH) or cold pressure homogenization (CPH). Our recent report has reviewed the various lipid core,surfactants,routes of administration,and methods of preparation,advantages,and limitations of the SLNps[39].Several reports are available on the roles of EOs loaded SLNps in therapeutic and topical applications[41].Compared to other nanocarriers,only a minimal number of studies have reported the synergism of EOs loaded SLNps regarding food and agricultural applications.

Table 1b depicts the examples of synergism of EOs loaded SLNps carriers in postharvest fungal infections and the control of pest in agricultural applications.Similarly,Zataria multiflora-EOs loaded SLNps were prepared using high-pressure homogenization(HPH)with the lipid core of glyceryl monostearate, Precirol® ATO5 and Poloxamer 188 as surfactants. The size(100 nm)of the spherical shaped Z.multiflora-EOs loaded SLNps provides higher stability with controlled drug release and protection of EOs against enzymatic degradation by the fungal pathogens [42]. Lai et al.[43]formulated the Artemisia arborescens-EO loaded SLNps for the control of pest in agricultural applications.The SLNps were prepared using the HPH technique with Compritol 888 ATO (lipid core) and surfactants(Poloxamer 188 or Miranol Ultra C32),respectively.A.arborescens EO provide more stability up to 12 months without the evaporation of the incorporated EOs as analyzed by the differential scanning calorimetry(DSC).

2.3.Liposomes

The liposome is a spherical shaped vesicle,structured by the amphiphilic lipid layer,which surrounds an aqueous core or compartment.Liposomes are described in the middle of the 1960s and are usually in the size range of 30 nm to few micrometers.The amphiphilic lipid substances are both water-loving(hydrophilic)and fat-loving(lipophilic)in nature[44].The choice of the lipid bilayer(example:unsaturated phosphatidylcholine and saturated phospholipids with long acyl chains),promisingly decides the rigidity and fluidity of the formulated liposomes.The multilamellar or large unilamellar or small unilamellar liposome vesicles have been prepared by nanoemulsion or thin film hydration(TFH)or ethanol hydration(EH)methods.For EOs encapsulation,TFH was mostly applied to obtainsmaller size liposomes.Liposomes act as potential carrier vehicles in the food sector for various molecules including vitamins,enzymes,EOs,phenolic compounds,and food antimicrobials as reported by Emami et al.[45].The increased drug loading and encapsulation efficiency,stability,flexibility(specific ligand molecules),non immunogenicity,lower side effects,elucidate the advantages of liposomes and their usage in therapeutic applications.However,the lower solubility and half-life,oxidation and hydrolysis of phospholipids,drug molecule leakages,and higher costs for production, have to be paid more attention while considering much more desired applications of liposomes[44,46].

Table 1 Synergism of essential oils with lipid-based nanocarriers in grains and its related food products preservation.

Table 1c illustrates the examples of synergism of EOs loaded liposomes in the antimicrobial actions against the rice flour pathogens.B.cereus is a spore-producing pathogen, which is frequently encountered in various food products, i.e. rice flour. Cui et al. [5] formulated liposomes with curry-EOs(196 nm)by using thin film dispersion,which showed promising bactericidal activity against B.cereus in rice flour.Moreover,Liolios et al.[47] reported the carvacrol- and thymol-EOs of Origanum dictamnus L.loaded liposomes,which were successfully prepared using the TFH technique to act against food pathogens(L.monocytogenes).However,knowledge on the physiochemical properties of the prepared liposomes including lipid composition,size and surface charges,are essential to find out the exact mechanisms of action in the bacterial membranes.

3.Conclusion and perspectives

Owing to an increasing trend in the encapsulation of natural bioactive compounds,this review has presented the synergistic effects of EOs with lipid nanocarriers in the preservation of cereal grains and related products.The antimicrobial actions of EOs are increased by loading or functionalizing with lipid based nanocarriers.Moreover,the possible mechanisms of action of the EOs loaded nanocarriers in the inhibition or control of foodborne pathogens include ribosomal dysfunction,interruption of the functioning of the electron transport chain,inhibition of bacterial toxin release into the environment,changes in the cellular morphology and disruption of cytoplasmic membranes[48,49].Based on literature reports,nanoemulsion and liposome nanocarriers are being actively used for food preservation than SLNps nanocarriers. Besides, most of the published articles have shown the EOs loading capacity,encapsulation efficiency,in vitro drug release profile,particle size,and stability of the EOs loaded nanoparticles.However,the proper quantification of the EOs in the final nano product is lagging.As most of the techniques for the preparation of the nanocarriers involve heating, solvent evaporation or high pressure homogenization,there may be a loss of EOs in the final nano formulations by volatilization,denaturation or degradation.Earlier findings showed that the spectroscopic and chromatographic techniques such as high performance liquid chromatography(HPLC)and gas chromatography and mass spectroscopy(GC–MS)play a crucial role in the quantification and analysis of chemical compounds including EOs[50–54].Therefore,the proper quantification of the EOs in the formulated nanocarrier systems is more critical before moving towards further studies and a wide range of food applications.

Acknowledgment

Authors duly acknowledge the financial support from the Ministry of Science and Technology of China(2016YFA0501703),Henan Natural Science(162300410060)to Dongqing Wei;Henan University of Technology(21450004 and 21450003)and Henan Province Postdoctoral Science grant(001802029 and 001803035)to Satyavani Kaliamurthi and Gurudeeban Selvaraj; China Postdoctoral Science Foundation (2018M632766) to Gurudeeban Selvaraj;The Ministry of Science and Technology of China(2013BAB11B02)to Keren Gu.