Proceeduuis of the 7 th Internatwnal Working Conference on Stored-product Protection - Volurne 1

The role of microorganisms in deterioration of vegetable oiland measures of controlling

Zhou Jianxm!

Abstract

This paper reported a method for rmcrobiological analysis ofvegetable 011 and the role of microorgamsms dunng 011detenoration. Techmques and methods for 011 safetysupervision and measures for preventmg 011 fromdetenoration were also discussed.

Introduction

Vegetable 011 IS not only an essential nutnent and flavonngfor people, but also an Important raw matenal for chemicaland medical mdustries The quality of 011 has great effect onthe quality of related mdustnal products as well as humanhealth. Under the natural CIrcumstances, vegetable 011 ISunstable and easy to go rancid, or worse detenorated 011rancidity may be caused by biological and non-biologicalfactors There are many reports on 011 detenoration causedby non-biological factors, such as water, light, temperatureetc (Lu Oianyu 1989), but few on biological, especiallyrmcrobiological factors After studied rrucroorgarusms mvegetable Oils and investigated the roles of rmcroorgarusmsin the deterioration process of vegetable Oils, we proposedsome effective techmques and methods for 011 qualitysupervision and controlhng of oil detenoration.

Materials and Methods

Materials

Vegetable tnl , rapeseed 011, soybean 011, peanut 011 andsesame 011 (GB quahty standard, Grade 2)Emuleifier , glycenn monostearate (GMS) (Lm

Guantmg. 1997)Medux , potato dextrose agar (PDA) and czapek agar

(CA) for moulds, malt extract agar (MDA) for yeasts andmeat peptone agar (MPA) for bactena (Xiang Ql. 1992).Apparatus: WH-851 Mixer and other common

mstruments for microbiological analysis and 011 quality

IDepartment of Food Science and Engmeenng , Nanling Institute ofEconomics, POBox 16 Nanjmg , PRof China

analysis.

Methods

EJmuls~fytng efficacy test of emulsifier- The aim of thistest is to choose a SUitableconcentration for the emulsifierand to understand ItS effects on nucroorganisrns growth.Qualtty analys~ items and methods :* Moisture content: GB/f 5528 - 85 ;* ACIdvalue: GB/f 5530 - 85;* Peroxide value: GB/f 5538 - 85 (Wang Zhaoci.

1994)Analytwal method for microbial flora in cnl :* Diluting method: 011 was emulsified With 0.5% GMS

and then diluted as usual.* AnalYSISItems: Total count (see Figure 1 for analysis

procedures); analysis of rrucroflora and dommant species(Xlang QI 1992; Ym Welshen 1990; Institute forMicrobiology of Chma Academy of SCience. 1978).Invest'lf}ation on the effects of microorqarusm« in the

process of oil deterioraium , Two 011 samples (rapeseed 011and peanut 011) were each divided mto SiXportions, eachportion (300 ml) was added to an Erlenmeyer flask The 011m the flask was pretreated as descnbed in Table 1 Eachflask was plugged With a cotton stopper, wrapped Withwaterproof paper, and kept at 30 ± 2°C or 20 ± 2"C in athermostat The rmcroflora of 011 and the 011 quahty wereanalyzed before and after storage.

Results and discussion

Analytical method for microorganisms in oil

FJmuls~fy~ngeffects of emulsifier: Soybean oil, peanut011, rapeseed 011 and sesame 011 were diluted Withwater mto1/2, 1/20, 1/200 respectively, and GMSwas added to makea fmal concentration of 0.5 %, 0.1 %, 0 05%. Then theywere emulsified for 1 - 2 mmutes on a WH-851mixer. Theresults are shown m Table 2.The results m table 2 showed that GMShad very good 011

emulsifying ability in water and achieved the bestemulsifying effect at the concentration of 0.5 % .Effects of emulsifier on, the growth of microorqanisnu: :

Asperqillu» [liurus , Saccharomyces cerev~~ae andMicrococcus sp were cultivated on media CA, MDA andMPA. 0.5 % GMS solution was then used to prepare

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Proceedings of the 7 th Internatwnal Working Conference on Stored-proiuc: Protectwn - Volume 1

microbial suspensions of ten-fold dilutions (.1.0- 1 and 10 - 2)for the test groups, and stenlized water was used for thecontrol groups. Check the total counts of the test andcontrol groups by the pour plate method. The results areshown m table 3.Table 3 shows that 0.5% GMS almost has no mlubiting or

promoting effect on the growth of microorganisms.

The diagram for mtcrobwlogical analysis in emulsifiedand dtluted ou , according to the above test, 0.5 % GMS(sterilized) can make 011 better dispersed in water and haslittle effect on the growth of microorganisms. Therefore,the emulsifier can meet the requirements of analyzingmicroorganisms in OIl.

Oil (50 ml) 0.5% GMS aqious solution

Packed mto Erlenmeye1flasks (50 ml) Packe~ into tubes (18 ml each)I I

\l/Plugged with cotton stoppers, wrapped with waterproof paper

JWell mixed

Emulsified E<------:..:...::..:..::...=:..::..=.::=--------

~ (1 2 microorganism suspensions)

MOIst heat sterilization (121 ·C, 30 mm)

Diluted mto 1. 20 and 1 200 suspensions ~( --J

~

Add suspensions mto petri dishes (l ml each) Medium

If for fungus analysis, streptomycm was addedPour plates < Melt and cooled to 45-50·C

1Incubate at suitable temperature -7 Colony count -7 Keep records of the results (numbers/ml)

1Isolation of cultures -7 Identification -7 Record types & numbers of organisms, and dommant species

Notes Three consecutive dilutrons were used and each dilution was earned out In duplicate or tnphcate

Fig.1. analysis procedures for rrucroorgarusrns in orl.

Table1. Pretreated ways and storage conditions of oil samples.

Pretreated ways before storageall symbol Storage

Rapeseed 011 Peanut oil temperature

Storagepenod

Raw 011 Rr Pr

Moist heat stenlizmg at 115·C for 30 mm

Inoculatmg With dorrunant rrucroorgarusms after stenhzation (1)

Double inoculatmg With dommant microorgarusms After stenhzatrontz)

Addition of water to raw Oil

Rs Ps

Rs+ mW

Rs+m(2)

Ps+ m(l)

Ps+m(2)

40 days

Rw Pw

Raw 011 Rr' Pr' 20 ±2°C 40 days

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Proceedings of the 7th Internatwnal Working Conference on Stored-product Protectum - Volume 1

Table 2. Effects of emulsifier(GMS).

Concentration of GMS 0.05% 0.1% 0.5%all diluting times by water 1/2 1/20 1/200 1/2 1/20 1/200 1/2 1/20 1/200Soybean 011 ttt + + + ttt ++ tH ++ tttPeanut 011 tH + + + Ht ++ ttt Ht tttrapeseed 011 tH + ++ ++ Ht ++ Ht ttt tttSesame 011 + + + ++ ++ ++ ++ ++

Note: '+t+' , '++' , '+' , '- ' mean the best, better, good and bad emulsiiymg effects respectively

Table 3. The effects of GMS on colony numbers.

Asperg~llus [laous Saccharomyces ceremsuie Micrococcaceae sp.Test times 10-1 10-2 10-1 10-2 10-1 10-2

Test Control Test Control Test Control Test Control Test Control Test Control

1 73 75 13 15 120 118 22 25 98 99 13 142 72 70 15 13 125 112 26 20 101 102 12 103 75 78 16 17 130 129 28 19 112 110 11 124 80 72 16 16 115 109 20 28 105 108 12 9

Average (A) 75 74 15 15 123 117 24 23 104 105 12 12At-A: 1 0 6 1 -1 0

Effects of microorganisms in oil deterioration

In order to figure out the role of rmcroorgarusms dunng011 detenora tion , rapeseed 011 and peanut 011 were used msimulatmg storage tests, and the nucroflora patterns of the011 and the 011 quality m different pretreatments wereinvestigated.Changes of microfloro., Changes of microflora before and

after storage are showed m Table 4. The followingconclusions can be drawn from Table 4:All raw rapeseed 011 and peanut 011 contamed a number of

bactena, yeasts and moulds. The average number wasbetween 101 to 102 per ml of OIl. Generally, Aspergillusjla7.YUS was the dominant species.After simulated storage, the number of nucroorgarusms in

OIls increased. For example, the numbers of mould, themore molds there were before storage, the more therewould be after storage (companson between Rr , Rs + mCl)and Rs + m(2); The increase of the mould was positivelyrelated WIth the moisture content of the 011 (companson ofRr and Rw) The lower the storage temperature was, theless the mcrease m mould number would be (comparison ofRr and Rr') A similar result was obtained for rapeseed 011.Otl quahty before and after storage (see table 5):

figures m table 5 showed that 011 qualIty decreased aftersimulated storage, such as the mcreases of aCId value andperoxIde value, especially peroxIde value (ranged from 108to 467%)Discussion on the roles of nncroorgan~ms dunng Otl

deterwration: After storage, the acid value of all the testedoil increased in varying degrees, even that of the stenlized011 increased too. This resulted from non-biological factors.However, the acid value of 011 contammg microorgamsmsmcreased more obviously. Increments of the acid value had apositive correlation With the rrucroorgarusm number,moisture content and the storage temperature. ThIS wasbecause the nucroorgarusms m 011 may decompose fat mtofree fatty acids (FFA) and result m the mcrease of acidvalue, rmcroorgarusm number, moisture content andtemperature That m turn may favor the metabolism ofrrucroorgamsms.The peroxide values of all 011 samples showed ObVIOUS

mcreases, but the mcreased values were different. It wasalso noticed that the peroxide values of stenhzed 011 alsoincreased because of the influence of non-biological factorsBut It was clear that the mcrease rate of peroxide value waspositively correlated WIth rrucroorgarusm numbers, moisturecontent and storage temperature of the tested 011 InGeneral, nucroorgamsms in oil are an Important factor thatcauses 011 detenoration

Conclusions

The applicationof 0 5% GMS as emulsIfIer m the analYSISofmIcrobIal flora m 011 can help to recover microorgamsms mthe 011 and can meet the demands of mIcroorganismanalytlcal methods.

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Proceedmqe of the 7th Iniernatumal Workmg Conference on Stored-product Protectwn - Volume 1

Table 4. Microbial flora illOilbefore and after simulated storage

Bactena Yeast MouldTemperature number number Before storage After storageof storage

BS AS BS AS Number Specres Dommantsf % ) Number Species Dommantsf % )

Rr 40 120 46 79 60 A flavus A ila ous (33) 80 A Favus A flavus(35)

A candzdus A c(IIII!II!II,(30) A nzdulans A n'Ululans (30)

p frequentans A funngatus

asporogenze fung~ R numcans

Rs 0 0 0 0 0 2 A. flavu» A j7,avus(50)

A cermcolor A lX!r8'/color(50)

30 ±2'C Rs+rn(l) 0 0 0 0 100 A tlasus A ,flaVlus(100) 180 A flaou« A flavus (100)

Rs+rn(2) 0 0 0 0 200 A flamus A flavus (100) 320 A fla VlUS A flavus(94)

A [um zgatus

Rw 40 180 46 99 60 A Favu A (lavus (33) 170 A flavus A ,(lavus(44)

A candulus A auuiulus (30) A nululams A nululans (32)

P frequentanas A candulu.

asporogeniC fungi R nujncans

20±2°C Rr' 40 150 46 69 60 A furous A fta mus(33) 68 A Favus A flavas (48)A candulus A euuluius (30) A candulus A candwus(28)P [requenians A nululans

asporogeniC fungI

Pr 20 60 5 12 80 A ,flavus A [la PUS (56 ) 230 A (lavus A Favus (60)

P ciiriun: A fumzgatus

A funuaitus A ternarui

A glaucus

Ps 0 6 0 0 0 0

Ps+rneu 0 0 0 0 160 A flamus A ,flavus(100) 380 A Fatrus A ,flavus (98)30 ±2'C

A furmgatus

Ps+ rn(2) 0 0 0 0 300 A ,flavus A [uious (l00) 790 A flaV1US A flaou» (100)

Pw 20 160 5 42 80 A [laou» A flavus(56) 320 A flatrus A Favus (65)

P citruan P cttruan

A [umuxuus A nuiulan«

A glanc?us A numcans

20±2°C Pr' 20 45 5 8 80 A flavus A flatvus(56) 180 A flavus A flavus(60)P curium P curiumA [umunius A glancusA glancus A fum zgatus

Notes: 'BS' stands for 'before storage' , ' AS' stands for 'after storage', 'A ,stands for Aspergtll us , 'P , stands for Penunllucm , 'R'

stands for Rhizopu»

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Proceed./tngs of the 7th International Working Conference on Stored-product Protection. - Volume 1

Table 5. Oil quality before and after simulated storage.

Storage Moisture Content (%) ACId value (mg KOH/g) Peroxide value

temperature Sample symbol Value Variation ( ± ) Value VanatIon Value Increased ratet % )

30±2°C Rr BS 1.7 2.5 12

AS 1.0 -0.5 2.9 0.4 38 217

Rs BS 1.7 2.5 12

AS 1.1 -0.6 2.7 0.2 25 108

Rs+ mw BS 1.7 2.5 12

AS 1.2 -0.5 3.3 0.8 49 308Rs+ m(2) BS 1.7 2.5 12

AS 1.2 -0.5 3.9 1.4 68 467

Rw BS 2.3 2.5 12

AS 1.8 -0.5 4.5 2.0 52 333

20 ± 2'C Rr' BS 1.5 2.5 12

AS 1.3 -0.2 2.7 0.2 26 117

30±2'C Pr BS 1.1 1.2 15

AS 0.7 -0.4 1.8 0.6 46 207

Ps BS 1.2 1.2 15

AS 0.8 -0.4 1.5 0.3 30 100

Ps+mw BS 1.2 1.2 15

AS 0.9 -0.3 2.9 1.7 58 287

Ps+m(2) BS 1.2 1.2 15

AS 0.9 -0.3 4.5 3.3 78 420

Pw BS 1.8 1.2 15

AS 1.4 -0.4 4.9 3.7 65 333

20 ± 2'C Pr' BS 1.1 1.2 15

AS o 9 -0.2 1.4 0.2 32 113

Notes: 'BS' stands for 'before storage' , 'AS' stands for 'after storage'

From the results of simulated storage of oil by differentpretreated ways, it can be seen that the presence of hpolyticmicroorganisms is an Important factor in oil detenoration(acid value and peroxide value), and the deteriorated degreeis proportional to the number of rrucroorgamsms. Thenumbers of microorgamsms, moisture content m 011 andstorage temperature of the 011 all affect the hpolysis processof rrucroorgarnsms and thus determine the deteriorationdegree of 011 So the effective measures for controllingmicroorgamsm growth and preventing 011 detenoration areto minirmze microorganism pollution of 011, decrease itsmoisture content and store in low temperature.

Acknowledgement

We would hke to thank Associate professor Xiang QI,Nanjing Institute of Economics, for his mstructions durmgthe early stage of this study.

References

Institute for Microbiology of Chma Academy of Science.1978 Common Fungi. Beijing: Science PressLin Guanting. 1997, Manual of Food Additives, 2nd editionBeijing: Press of Chemical Industry, 798 -799.Lu Qlanyu. 1989, Grain and 011 Storage, Beijmg: ChmaFinance and Economics Publishmg House. P254 - 285.Wang Zhaoci. 1994, Quality AnalYSIS of Cereal and 011Foods, Beijing: China LIght Industry Press, 54 - 57, 704-706, 733 -736.Xiang Qi. 1992, Microbiological Test of Cereal Foods.Beijing: China LIght Industry Press.Ym Welshen. 1990. Food Microbiology. Beijing: ChmaFinance and Economics Pubhshmg House. P16 - 21, 34-61.

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