79

Chng 6 - ENZYME LIPASE

1 Gii thiu chung

Lipase (E.C. 3.1.1.3) c ng dng ch yu xc tc thy phn acylglyceride

hoc mt s ester khc nh thiol, polyol hay polyacid ester. Ngai ra lipase cn c th

c s dng lm cht xc tc trong cc phn ng tng hp cc acylglyceride hay ester c

gi tr ng dng cao trong i sng. Lipase c bn cht l mt globulin v c th c sn

xut t tuyn ty v mng d dy ca ng vt. Tuy nhin sinh tng hp lipase t vi sinh

vt mang li thun khit v hiu sut cao hn. Hin ti ngi ta ng dng rng ri

lipase trong cng nghip dc phm, ha m phm, dt da, cht ty ra, thc phm hoc

tng hp cc cht hu c khc nhin liu sinh hc (bio-fuel).

Cng nh a s cc enzyme thy phn khc, lipase khng c nhm ngai m ch c

trung tm hat ng vi cc acid amin c bit nh imidazol ca histidin v nhm

hydroxyl ca mt trong s cc gc serine.

Lipase thy phn lin kt ester ca lipid

nm ti b mt phn gia lipid v nc.

Trong nc, trung tm hat ng cha

serine b kha li bi vng xon helix.

Khi lipase gn vo b mt phn pha gia

c cht lipid v nc th vng xon helix

di chuyn cho php c cht gn vo trung

tm hat ng.

C ch tc dng ca enzyme lipase: do cu trc bc 3 ca enzyme m nhm

hydroxyl ca serine v vng imidazol ca histidin tip xc nhau v to thnh lin kt

hydro. Kt qu l oxy ca nhm hydroxyl thu c tnh cht i nhn. V tm i nhn ny

ca enzyme s tng tc trc tip vi mt trong hai nguyn t tch in dng ca lin kt

b thy phn. V do lin kt ny d b ct t v s thy phn thun li hn. Mt c ch

khc l enzyme to thnh lin kt tng ng vi c cht nhng v tr lin kt gn gi vi

lin kt thy phn. Kt qu l lm tng s khuyt electron vn tn ti trc , v phn

b electron trong phn t c cht thay i theo mt chiu hng cn thit. Nh m vic

ct t lin kt c d dng, hoc lm cho mt trong hai nguyn t ca lin kt tng tc

c vi cc tc nhn i nhn.

Xc tc enzym lipase c chia lm 2 lai nh sau: thy phn v tng hp.

Phn ng thy phn:

RCOOR + H2O RCOOH + ROH

80

Phn ng tng hp: C th c chia thnh 4 nhm nh sau:

a) Ester ha (Esterification):

RCOOH + ROH RCOOR + H2O

b) Ester ha tng h (Interesterification):

RCOOR + RCOOR* RCOOR* + RCOOR

c) Ru phn (Alcoholysis):

RCOOR + ROH RCOOR + ROH

d) Axit phn (Acidolysis)

RCOOR + RCOOR RCOOR + RCOOH

Trong 3 nhm b, c v d thng c gi chung l l transesterification.

2 u th xc tc lipase so vi phng php s dng xc tc ha hc

Hu ht cc ng dng ca lipase ngy nay tp trung thay th cc phn ng xc tc

ha hc ci bin hc to dn xut ca cc lai du m. Hin ti cc triacylglyceride

mch trung bnh c sn xut bng cch ester ha acid bo vi glycerol p sut v nhit

cao (200-250oC) bng cc cht xc tc v c. Cc xc tc v c ny thng khng hiu

qu v i hi phi cung cp ngun nng lng rt ln. Sn phm to thnh cn phi c

tip tc em tinh luyn tip bng cch ra kim, tinh luyn bng hi nc, chng ct phn

t, siu lc, hoc s l bng carbon hat tnh v.v

Mt qui trnh sn xut cng nghip quan trng hin nay l thy phn du thc vt

(du liu, du da, du u nnh) sn xut cc acid bo v glycerol nhm ng dng

rng ri trong cng nghip cht ty ra, ha m phm, dc phm v thc phm. Cc qu

trnh sn xut hin ti c iu kin phn ng cao: p sut 3000-5000 kN/m2 v nhit

250oC. Trong 2h c th thy phn c 96-98% du thu c 12% glycerol. Cc acid

thu c khng th em s dng ngay m cn phi tip tc em i chng ct ty mu

v lai b cc sn phm ph. Qu trnh ny sn xut kiu ny i hi tiu tn nhiu nng

lng v to ra rt nhiu cc phn ng khng mong mun nh l cc phn ng trng hp

ca cc acid bo nhiu ni i v to thnh cc ketone v hydrocarbon.

Chi ph u t cho qu trnh tch bng hi nc rt cao bi v ct chng tch phi

c cu to c bit chu c nhit v p sut cao cng nh kh nng chu n mn acid.

Trong khi , do xc tc enzyme c tin hnh p sut v nhit trung bnh

thp (40 - 60oC) nn c th tit kim c chi ph nng lng. So vi phng php sn

xut truyn thng, chi ph sn xut bng enzyme cng c gim bt nh cc bnh phn

ng khng cn phi chu n mn cao, cng nh cc iu kin mi trng khc nghit khc.

Cc sn phm ca qu trnh xc tc sinh hc cng cho mu sc v mi v tt hn, cng nh

tinh khit cao (c rt t hc khng c phn ng ph). V nh ngi ta cng gim

thiu c chi ph sn xut do khng cn phi tin hnh cc qu trnh tinh luyn tip theo

nh trong phng php xc tc v c. Ngai ra nhit phn ng thp cng gp phn lm

gim thiu s phn hy nhit ca sn phm. V do lipase c s dng cho tt c cc

mc ch trn.

81

3 Cc ng dng c th ca cc loi Lipase

3.1 Thy phn

Thy phn nhm tch du ra thnh cc acid v ru. Sn phm mong mun c th

l mt loi acid c bit hoc l mono, diglycerides, ru... Mt s nghin cu, ng dng

lipase thy phn mt s c cht c trnh by trong Bng 1.

Bng 1. Xc tc thy phn ca Lipase

Lipase C cht Mc ch

Rhizopus delemar +

Pencillium + Rhizopus niveus

Du u nnh Tng tnh hiu qu thy phn bng

cch kt hp nhiu lai lipase

Canida rugosa,

Aspergillus nige

Du c Lm giu cc PUFA

C. rugosa Du u nnh Thy phn kt hp vi h thng

mng lc

C. rugosa M b, m heo Thy phn trong dung mi hu c

nhit thp hn nhit nng chy

ca m

R. niveus Du c Lm giu acid Docosahexaenoic (DHA)

R. delemar Du c Sn xut monoglyceride

3.1.1 Trong phn gii lipid

Nh vo kh nng lipase thy phn du m thnh glycerol v cc acid bo vn c

ng dng quan trng trong cng nghip. Hin ti, cc acid bo c s dng sn xut

x phng. Cc enzym x dng cho mc ch ny c th thu t Candida rugosa v

Pseudomonas fluorescens (quy trnh c cng nghip ha). Cc phng php enzyme

ny hiu qu hn cc phng php ha hc v mt chi ph v sn phm thu c c mu

sc v mi v tt hn. Do cc phn ng enzyme xy ra nhit v p sut thp hoc

trung bnh nn chng c th c s dng sn xut cc lai acid bo t cc lai du d

bin tnh (cha nhiu acid bo khng no hoc c nhiu ni i lin hp). Thng thng

iu ny rt kh t c vi cc phng php sn xut trc y v nhit v p sut

cao d dn n oxt ha du m.

3.1.2 Trong cng ngh thuc da

Trong cng nghip thuc da, mt trong nhng bc quan trng l lai b cc phn

tht v m vn cn dnh trn da v lng. S dng cc quy trnh ha hc nh ty kim

thng khng hiu qu. Ngy nay ngi ta ang dn s dng lipase v protease t

c mc ch trn. Ly v d nh ngi ta s dng lipase t Rhizopus nodosus x l

ty nhn lng cu.

So vi phng php x l bng dung mi truyn thng th cht lng sn phm x

l bng enzyme co cht lng ng nhau, nhng ch r hn v t c hi hn.

82

3.1.3 Trong x l nc thi

Lipase c s dng trong cc bn hat tnh v cc quy trnh x l cht thi hiu

kh m trong lp lipid mng trn b mt bnh hiu kh c lin tc lai b cho php

oxy xm nhp vo ( duy tr iu kin sng cho VSV hiu kh). V d nh Lipase-MY t

Cadida rugosa do cng ty Meito Sangyo, Nht Bn sn xut c s dng trong cc nh

my x l nc thi M.

Trn thc t vic phn hy cc cht thi rn, lm sch v ngn chn lipid lm tc

nghn hay to thnh lp trong cc h thng x l nc thi, cc bnh cha rt quan trng

trong nhiu nh my cng nghip thc phm, thuc gia, git m gia cm.

3.1.4 Trong sn xut cht ty ra gia dng

Cng nhm vi hai nhm ng dng bn trn, lipase c ng dng cng nghip

sn xut cc cht ty ra nh bt git, nc ra chuyn dng Lipase sn xut t

Aspergillus oryzae c dng trong bt git do c kh nng chu kim cao. Hng nm

khang 1000 tn lipaza c cho vo 13 triu tn bt git, vi tng gi tr vo khong 30

triu USD.

3.1.5 Trong sn xut hng v cc sn phm t sa

Cc acid bo t do, cc peptide ha tan v cc amino acid to nn mi v v cu

trc cho cc sn phm b, phmt, margarine. Bi vy cc lipase v protease ng s

dng rt ngn thi gian lm chn ph mt v sn xut cc mi v c trng khc.

Mi v to thnh ph thuc vo lai enzyme s dng. Hin ti cc PGE (pregastric

esterase) c s dng lm ph mt c v tiu cay . Enzyme t Penicillium

roqueforti th c dng to cc lai phmt xanh (blue cheese). Cc sn phm phmt

sn xut nhanh bng enzyme (EMC, Enzyme modified cheeses) ngy cng xut hin rng

ri (v d nh lipase t Mucor meihei, Aspergillus niger, A.oryzae). Bn cnh lipase cn

c s dng to hng cho cc sn phm bt kem c ph, ko b sa cng, scla,

snack, soup....

3.1.6 Trong sn xut cc thc phm khc

y l mt lnh v y tim nng ng dng lipase. Phn gii lipid bng enzyme

c ng dng vo trong sn xut cc lai tht, c khng cha m. Thy phn mt phn

triglyceride lm tng hm lng monoglyceride bng cc cho lipase vo trong bt bnh

m cho php lm chm i qu trnh thai ha tinh bt. S to thnh mono v diglyceride

cng cho php ci thit tnh to xp ca lng trng trng g. Lipase cn c ng dng

trong sn xut sa u nnh, c v tht xng khi, hay ngay c trong sn xut thc n cho

vt nui king nh ch, mo.

3.1.7 Trong iu tr bnh

Bn cnh cc cht hydradcarbon v protein, lipid l thnh phn khng th thiu

trong c th con ngi. V vy, nu xy ra cc sai st trong qu trnh thy gii, hp th hay

tng hp lipid u dn n kh nng gy bnh cho ngi. Do lipase c s dng nh

l mt cht tr gip tiu ha. Hm lng lipase trong serum mu l mt ch nh chn an

tt cho cc bnh tn thng v tuyn ty. Ging nh nhiu lai enzyme khc, lipase cng

c th c c nh trn cc in cc pH hoc oxy ch to cc sensor sinh hc nhm

xc nh hm lng triglyceride v cholesterol trong mu. PGE cng c s dng cho cc

bnh nhn khng th hp thu c lipid.

83

3.1.8 Trong phn tch cu trc triglyceride

Lipase c th c phn lai da trn tnh c hiu ca c cht. C th bao gm:

- Enzym c hiu v tr sn-1,3 (Rhizopus oryzae, Humicola lanuginosa,

Rhizomucor meihei) ;

- mt loi axit bo c trng nh oleic acid (Geotrichum candidum) hoc linolenic

(Brassica napus v Mucor miehei);

- hoc hon ton khng c hiu (Candida antarctica, Candida. rugosa,

Pseudomonas cepacia).

Nh vy s dng tnh c hiu ca Lipase s gip chng ta phn tch c cu trc

ca triglyceride.

3.1.9 Trong sn xut cc acid bo

Sn xut du n giu cc acid bo nhiu ni i (PUFA, polyunsaturated fatty acid)

nh docosahexaenoic acid (DHA) v eicosapentaenoic acid (EPA) l mt trong nhng ng

dng ht sc c quan tm hin nay ca lipase.

V d: Sn xut hn hp giu acid bo omega-3 t du c mi bng lipase (T.

OKADA and M. T. Morrissey - B mn Cng ngh thc phm, i hc Oregon State

USA). Trong vng 30 nm gn y, cc nghin cu cho thy cc acid bo omega-3 c

nh hng tt n sc kho, v rt nhiu loi sn phm cng c tung ra th trng.

Du c mi th trc tin c ty mu bng carbon hot tnh (4%), kh mi bng

cch chn ct chn khng v c n nh vi 200 ppm TBHQ. C th s dng cc loi

lipase thng mi nh t Candida Rugosa, Candida cylindracea, Aspergillus niger hay

Mucor javanicus. m pH bng phosphate t 6.5 7.0 ty thuc vo pH ti u ca mi

loi enzyme.

Nghin cu cho thy i vi c mi th 500U lipase t Candida Rugosa c th thy

phn 82.5% du c trong 9 gi. Phn tch thnh phn acid bo bng phng php sc k

kh cho thy so vi du c mi ban u thnh phn EPA v DHA c ci thin.

Thnh

phn acid

bo

Du c mi

ban u

Thy phn bng 500U/g lipase

t Candida cylindracea trong 6

gi

Thy phn bng 250U/g

lipase t Candida Rugosa

trong 6 gi

EPA 19,44 35,8 29,75

DHA 7,62 19,23 14,85

Nh vy nghin cu cho thy x dng lipase c th dng sn xut cc hn hp

lipid giu cc acid bo omega-3 t du c .

3.1.10 Cc ng dng khc:

Ngoi ra Lipase cn c ng dng ty mc in trn giy, x l t tm, hay trong

sn xut nc sc ming kh mi.

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3.2 Tng hp ester

Kh nng ca lipase nhm xc tc cc phn ng nghch ca qu trnh thy phn

c s dng sn xut cc sn phm mong mun. Hn hp ester ha thng ch cha

cc c cht v enzyme. Nc ch l sn phm ph ca phn ng v c lai b kh d

dng nn y c coi l quy trnh tin tin tng hp cc cht.

Bn cnh qu trnh transesterification thay v to ra nc s to ra sn phm

ru, acid hay ester, do qu trnh ny s sinh li nhiu hn nu nh cc lai sn phm

trn l sn phm chng ta mong mun t c.

Trong ha hc phn ng ester ha ch yu ni n phn ng gia acid vi ru,

trong khi lipase li c th xc tc nhiu lai c cht hn: ester ng, thiol esters,

peptide

3.2.1 Tng hp ester c khi lng phn t thp hoc trung bnh

Cc ester c khi lng phn t thp nh l isoamyl butyrate v benzyl propionate

c s dng lm cht to mi. Hin ti bytyl laurate l thnh phn hng ch yu ca o

v l, v thng c dng trong sn xut hng tri cy. Ethyl butyrate c bit n nh

hng da ca n.

Bytyl oleate v butyl laurate c s dng lm cht lm mm do, cht bi trn.

V do chng c ng dng rng ri trong cng nghip du thm, m phm, x phng,

thc phm

Mt ng dng khc l s dng cc methyl v ethyl ester thay th cho cc lai

nhin liu.

Tt c cc ester trn u c th tng hp mt cch hiu qu bng cch s dng

lipase t M. miehei, R.delemar, P. cyclopium, G. candidum v.v...

3.2.1 Polyester

Lipase c s dng sn xut cc polymer ca dimethyl terphthalate, vinyl

acetate, butyl acrylate c s dng lm cc cht do, sn v cht kt dnh.

85

3.2.2 Thc phm

Lipase dng tng hp monoglyceride nh l monolaurin, ng ester v ester

ca amino v acid bo vn c s dng nh l cc cht nh ha hoc cc cht nn trong

cc mng bao n c.

Hin ti mt sn phm quan trng c sn xut t lipase l cc sn phm thay th

du m. Ngy cng c nhiu nhu cu thay th cc cht du m giu nng lng. Hn th

na cc sn phm thay th li c tnh n nh nhit rt cao.

V d nh sn phm Olestra, hn hp ca hexa-, hepta- v octaester, c hnh

thnh bng cch ester ha c sucrose vi cc acid bo mch di do Proter & Gamble

ang gi bn quyn hay sn phm ester ha ca glycerol vi propoxylate ca ARCO

Chemical. Cc sn phm ester trn c s dng rng ri trong ch bin cc nc st,

mayonnaise, hay kem n.

3.2.3 Dc phm v ha m phm

Mono-, di- v triglyceride ca octanoic v decanoic acid c th c s dng lm

nn cho cc cht mu v mi trong cc sn phm ha m phm v dc phm. Ester ca

ng v sorbitol vi lauric v stearic acid c dng trong thuc chng khi u. Oleyl

monooleate c s dng trong du tm, cc sn phm dng tc, dng da.

3.2.4 Cc sn phm t sa v bnh ko

Mt trong nhng ng dng d thy nht trong lnh vc ny l s dng lipase xc

tc cc lipid tng hp cc triglyceride c tnh cht mong mun. V d nh s dng

lipase t C. viscosum, M. miehei, v P.fluorescens ch to cht thay th b cacao t du

c nhm s dng lm scla v bnh ko, hay nu sn xut t du olive v ht bng vi th

s dng lipase thng mi Lipozyme (sn xut t M. miehei).

86

Ngoi ra ngi ta cng dng lipase ci bin du c thay th cht bo trong

sa m.

Hin ti, Lipases ch chim khong 5-10% sn lng enzyme s dng (protease

chim 60-70%, carbohydrase chim 25%). iu ny c l gii bi vic s dng lipase

cn gp mt s mt hn ch. Trc ht l lipase thng xc tc trong mi trng nc,

trong khi c cht lipid th vn rt t tan trong nc. Bn cnh tnh n nh ca lipase

i vi nhit , pH cha cao. V sau cng l kh thu hi v ti s dng lipase nn dn

n chi ph sn xut cao. Tuy nhin, cng vi s pht trin ca khoa hc k thut, cc c

im hn ch trn ca lipase ang tng bc c gii quyt. Ngy nay cc lipase c th

hat ng trong cc dung mi hu c hoc cc lu cht trng thi siu ti hn. Bng 2

trnh by mt s lipase thng mi ang cung cp cho cc ngnh ngh trn th trng.

Bng 2. Mt s lipase thng mi ang cung cp cho cc ngnh ngh trn th trng.

STT Tn Phm vi ng dng

1 Lipopan Bnh ko

2 Lipozyme Ha du m

3 PalataseTM Sn phm t sa

4 Clear-LensTM LIPO Ty ra

5 Greasex Thuc da

6 NovoCorTM AD Thuc da

7 LipolaseTM Bt git

8 LipoPrime Bt git

9 Novozym 871 Thc n cho vt nuigia sc

87

4 Lipoxygenase

4.1 Gii thiu chung

Lipoxygenase (E.C. 1.13.1.13) xc tc s oxi ha ca cc acid bo khng no nhiu

ni i nh linoleic, linolenic, arachidonic v cc ester ca chng. Lipoxygenase rt quan

trng trong cng ngh thc phm v chng c th nh hng n mu sc, mi v v gi tr

dinh dng ca sn phm.

Lipoxygenase c mt nhiu loi m thc vt v ng vt. c bit l s c mt

ca lipoxygenase trong cc ht mang du (v d nh u nnh) c th c xem l nguyn

nhn chnh hnh thnh cc peroxide trong qu trnh trch ly du thc vt.

a

b

Hnh: Cu trc (a) v trung tm hat ng (b) ca lipoxygenase u nnh

Lipoxygenase thng tn ti nhiu dng isozymes khc nhau v pH hot ng ti

u cng nh l sn phm v tnh c hiu ca c cht. Ly v d nh t u nnh ngi ta

c th tch c 4 loi isozyme khc nhau.

- Isozyme u nnh 1: c pH ti u l 9.0. Loi ny ch tc dng trn cc loi acid

bo t do c nhiu ni i. nhit phng, loi enzyme ny hnh thnh peroxide

v tr Cacbon s 9 v 13 theo t l 1:9.

- Isozyme u ny 2: c pH ti u l 6.8. Loi ny c th xc tc trn triglyceride

cng nh l acid bo t do c nhiu ni i v hnh thnh peroxide v tr 9 v 13

theo t l 1:1 nhit phng.

- Isozyme u nnh 3: tng t nh isozyme 2 nhng hot tnh ca n d b c ch

bi ion Ca2+

- Isozyme u nnh 4: tng t nh loi isozyme 3 nhng c th c tch bi

phng php sc k hay in di

Ngoi ra cc isozymes cn thng c phn thnh loi 1 (pH ti u > 7 v c

hiu vi cc acid bo t do) v loi 2 (pH ti u = 7 v oxi ha c carotenoids). Kh

nng lipoxygenase loi 2 c kh nng ty mu carotenoids ngy nay c ng dng trong

vic sn xut bnh m trng (bng cc thm bt u nnh vo bt m b sung lng

isozymes loi 2).

Lipoxygenase ng vt v thc vt thng c hat ha khi cc m t bo b

tn thng hoc b ph v. Kt qu l cc acid nhiu ni i b oxi ha thnh cc

hydroperoxide. Sau s phn hy thnh nhiu sn phm bay hi khc nhau.

88

Xt v phng din sn phm ca qu trnh xc tc, Lipoxygenase to ra cc sn

phm bay hi tng t nh sn phm trong qu trnh t oxy ha kh, mc d t l ca

thnh phn sn phm khc bit nhau ty vo tnh c hiu ca enzyme cng nh iu kin

phn ng (Bng 3).

Bng 3. So snh thnh phn ca sn phm bay hi ca phn ng oxy ha xc tc

bi lipoxygenase u H Lan (a) vi s t oxi ha kh (b)

C cht: Linoleic acid C cht: Linolenic acid

Sn phm % (a) % (b) Sn phm % (a) % (b)

n-pentanal

n-hexanal

n-hept-2-enal

n-oct-2-enal

n-nona-2,4-

dienal

n-deca-2,4-dieanl

7

41

15

10

5

20

0,7

67

6

18

0,4

5

Acetaldehyde

Propanal

n-but-2-enal

n-penta-2-enal

n-hex-2-enal

n-hex-3-enal

n-hepta-2,4-dieanl

5

27

3

22

2

-

38

-

1

10

1

14

50

4.2 C ch phn ng

Phn t lipoxygenase c cha mt nguyn t st. Nu nguyn t st ny chuyn t

dng Fe2+ sang Fe3+ th lipoxygenase s chuyn t dng th ng sang dng hot ha

xc tc phn ng. Qu trnh xc tc c din t trong s 1. u tin Fe2+ c th b oxi

ha thnh Fe3+

nu c tip xc vi acid bo peroxide hoc l OOH. Do nu c cht

ch n thun l acid bo t do th qu trnh oxy ha din ra kh chm.

Lipoxygenase c kch hot s t t tch nguyn t hydro t nhm methylene

nm gia ca acid bo nhiu ni i. Trong iu kin hiu kh, phc hp enzyme-gc

alkyl s c oxi ha bng phn t oxy tr thnh phc hp enzyme-gc alkyl peroxide.

Sau in t s c truyn t nguyn t st sang nhm peroxide. V nh vy s thm

vo v tch proton t enzyme to ra sn phm hydroperoxide.

Trong iu kin ym kh th gc alkyl s c tch ra khi phc hp enzyme-gc

alkyl, ri t s hnh thnh hn hp cc sn phm nh cc cht nh trng, ketones v

epoxides.

4.3 Hn ch v bt hot lipoxygenase

Vic lipoxygenase gy ra mi i trong cc sn phm cha du m l mt trong

nhng vn ang c quan tm. Hin ti c rt nhiu cc nh nghin cu ang lm vic

ti u ho cc iu kin lm bt hot lipoxygenase trong cc sn phm cha du m.

Cc phng php thnh hnh ang c nghin cu v p dng l b sung cc cht chng

oxi ha, hiu chnh pH v nhit ...

Lipoxygenase c th b hn ch bi rt nhiu loi cht chng oxi ha (v d nh

pyrocatechol, homocatechol, propylgallate, nordihydroguariaretic acid, resorcinol,

phioroglucinol, hydroquinone, butylated hydroxyanisole v nhiu loi flavonoids cng nh

cc hp cht lin quan khc.Ngy nay cc cht chng oxi ha c ngun gc t thc vt

ang c cc khoa hc gia ht quan tm nghin cu v khai thc.

89

S 1: Qu trnh xc tc ca lipoxygenase

X l nhit c th lm nh hng n tnh tan ca protein cng nh lm thay i

cc tnh cht chc nng ca cc sn phm protein theo chiu hng bt li. V d nh

ngi ta hiu chnh u xanh xung m 16,3% vi m pH 9,8 sau un nng trn

90oC trong vng 10s. Bng cc ngi ta c th lm bt hot 99% lipoxygenase, nhng

bn cnh cng ch gi li c 70% tnh ho tan ca protein.

Ngi ta cng thy rng lipoxygenase l nguyn nhn chnh gy nn hin tng tr

mi u H lan xanh. Ngoi ra lipoxygenase cng chu trch nhim cho vic lm bin

i mi v ca chui.

Mc bn nhit ca cc loi lipoxygenase trong cc loi thc phm rt khc

nhau. Ngi ta nhn thy rng lipoxygenase trong u H lan v u xanh chiu nhit km

hn peroxidase 60oC trong 10 pht. Do vy v hot cc loi u trn th cng ch cn

x l nhit mc khng qu cao. Trong khi mt nghin cu khc, ngi ta li nhn

thy lipoxygenase trong mng ty li bn nhit hn so vi peroxidase. Bi vy cho d x

l nhit nhit 70oC trong vng 10 pht c th lm v hot peroxidase nhng li khng

vo hot lipoxygenase

X l nhit bng microwave c th hn ch rt hiu qu hot ng ca

lipoxygenase. Ngi ta nhn thy x l u nnh bng microwave c th va lm tng tnh

ht nc ca ht li va c th lm v hot lipoxygenase v cc cht c ch hp thu

trypsin. Do ngi ta c th x l u nnh (8,7% m ) bng sng siu vi trong 4 pht

trc khi em i sn xut sa u nnh. Nu u c em ngm nc trc th thi gian

x l nhit lm bt hot enzyme s cng ngn hn.

90

4.4 Nhng xu hng pht trin trong tng lai

Hin nay ngi tiu dng ngy cng quan tm n nhng sn phm c cha nhiu

DHA v EPA. V v th cc sn phm ny rt d bi oxi ha bi lipoxygenase.

iu ny i hi cn c nhiu nghin cu hn na v cc cht chng oxi ha, cc

phng php vt l ngn chn s h hng do oxi ha. Cc phng php s ch du n

(c bit l loi du cha nhiu acid bo c nhiu ni i) trc khi a vo ch bin thc

phm c ci tin sao cho lm gim thiu vic to nn cc peroxide.

5 S i ha sinh hc ca lipid

5.1 Gii thiu chung

cc phn trc, chng ta cp n ng dng ca thy gii lipid trong cc

ngnh cng nghip khc nhau. Trong cng ngh thc phm ni ring vic thy gii lipid

cng vi s oxi ha cc acid bo cn ng mt vai tr rt to ln i vi cht lng ca sn

phm trong qu trnh ch bin v bo qun. Trong cc sn phm ng vt v thc vt, hai

nhm lipid chnh l phospholipid v triglyceride. Phospholipid l thnh phn khng th

thiu trong thnh phn mng t bo bi vy chng c gi l lipid cu trc. Cn

triglyceride thng c gi l lipid d tr nng lng, v thng c cha trong cc t

bo m c bao bc bi cc mng phospholipid.

Trong thi gian ch bin v bo qun, lipase v phospholipase s thy gii

triglyceride v phospholipid. Cng vi cc acid bo lin kt, cc acid bo c to ra trong

qu trnh trn (c bit l cc acid bo nhiu ni i) s d dng b oxid ha bi cc tc

nhn ha hc hoc enzyme lipoxygenase.

C ch oxy ha do lipoxygenase:

91

5.2 Phospholipases

Phospholipase A2 (EC 3.1.1.4) l phospholipase u tin c cng nhn v ng

vai tr rt quan trng trong cc ng dng ca phospholipase. Enzyme ny xut hin rt

nhiu trong t nhin v c mt hu nh tt c cc lai t bo. Phospholipase A2 (PLA2)

l mt thnh phn ca thc phm t ngun ng thc vt v v vy lun c a vo c

th ngi thng qua ng n ung. PLA2 ng vai tr tch cc trong vic ci thin cc

tnh nng ca lecithin (u nnh, lng trng), lai gum trong qu trnh tinh luyn du...,

nhng bn cnh n cng l nguyn nhn gy nn mt s h hng trong cc sn phm.

Ngy nay ngi ta khm ph c rng nguyn nhn thy phn phospholipid trong

tht c ch yu l do PLA2. Mt s nghin cu v PLA2 trong c quan trng nh:

- PLA2 c Hi c pH ti u l 7 v khng i hi ion Ca2+

hat ha.

- PLA2 trong c Tuyt khng ph thuc Ca2+

v pHopt >7. PLA2 ny c kh nng

thy phn phospholipid trong c tuyn tng dn trong 8 tun tr lnh u tin -

30oC, sau s gim dn n tun th 12. V chng ta cn lu rng hat ng

ca PLA2 gng lin vi vic c ca c tr nn cng hn do tc ng ca cc acid

bo vi cc c protein.

- PLA2 trong c Plc th i hi s c mt ca Ca2+. Khc vi PLA2 trong c Tuyt,

pHopt cua PLA2 trong c Plc c tnh kim

92

Tuy nhin ngi ta ngi ta vn c th tm thy cc hat ng thy gii

phospholipid acyl khc c. V d nh c ng Euthynnus pelamis, PLA1 li l nguyn

nhn to ra lysophospholipid trong tht c. PLA1 ny khng ph thuc Ca2+

v c pHopt vo

khang 6,5 7,0. Mt nghin cu khc cho thy c thu i Ty Dng cng c loi PLA1

khng ph thuc Ca2+ vi pHopt khong 4,0.

Nh vy c ni chung, nhng tn hi m khi b nh bt hoc do stress s lm

tng hot tnh ca PLA1 v PLA2, v kt qu l nh hng n qu trnh bo qun ca sn

phm.

Phospholipase A2 c xc tc cc phn ng thy phn phosphoatidylcholine

to thnh acylglycerophocholine.

Phospholipase A2 c s dng thy phn lecithin ci bin cc tnh cht nh

ha ca n. Cc lecithin thng mi l mt hn hp phosphatide ca choline,

ethanolamine v inositol vi mt lng nh cc lipid khc v c s dng rng ri trong

nhiu lai thc phm khc nhau. Cng dng ca lecithin nh l mt cht nh ha trong ch

bin thc phm th rt ph bin, tuy nhin cc lai lecithin cha c ci bin th tnh cht

nh ha ca chng rt hn ch trong cc thc phm du m. Phospholipase A2 thy phn

lin kt ester gia glycerol v acid bo ti v tr sn-2 ca lecithin nhm to ra lysolecithin

v mt phn t acid bo. Kt qu l lysolecithin to thnh c kh nng to nh ho cao hn

trong nhiu loi thc phm khc nhau.