Flavor Chemistry - American Chemical Society

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Chapter 13

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Role of Free Amino Acids and Peptides in Food Taste Hiromichi Kato, Mee Ra Rhue, and Toshihide Nishimura Department of Agricultural Chemistry, University of Tokyo, Bunkyo-Ku, Tokyo, Japan Free amino acids and/or some peptides have some sweetness, bitterness, sourness, saltiness and umami, and are very important as taste substances in foods. In this paper, we discuss 1)some tastes of free amino acids and some peptides, 2)the role of free amino acids in the characteristic tastes of vegetables and marine foods, 3)the role of the bitter peptides in cheese and the traditional Japanese foods "miso" and "natto", and 4)the contribution of free amino acids and peptides to the improvement of the meat taste during storage of meats (beef, pork and chicken). Free amino a c i d s and p e p t i d e s are v e r y i m p o r t a n t as taste substances. The i m p o r t a n c e o f amino a c i d s t o f o o d t a s t e was f i r s t recognized by Ikeda i n 1908 ( 1_). He d i s c o v e r e d t h a t monosodium glutamate (MSG) was the e s s e n t i a l taste component o f traditional Japanese seasoners, such as sea t a n g l e . MSG i s a t y p i c a l umami s u b s t a n c e . Almost a l l f r e e amino a c i d s , i n c l u d i n g MSG, have some sweetness, b i t t e r n e s s , s o u r n e s s and umami (2,2) and t h e r e f o r e cont r i b u t e to the characteristic taste of foods. The characteristic t a s t e o f many marine foods is e l i c i t e d by f r e e amino a c i d s . The t a s t e o f t r a d i t i o n a l Japanese foods such as s a k e , miso and soy sauce i s thought t o be caused by amino a c i d s r e l e a s e d from p r o t e i n s d u r i n g fermentation. Many s t u d i e s on t h e t a s t e o f amino a c i d s i n foods and t h e i r p r o d u c t i o n by e x t r a c t i o n , fermentation, or chemical syntheses have resulted i n mass p r o d u c t i o n o f v a r i o u s amino a c i d s . Today, amino a c i d s used i n f o o d processing n o t o n l y enhance the nutritive value o f many p r o c e s s e d foods such as c e r e a l s , but a l s o enhance the n a t u r a l c h a r a c t e r i s t i c t a s t e s o f many f o o d s . S t u d i e s on the t a s t e o f p e p t i d e s have been done o n l y r e c e n t l y . The b i t t e r t a s t e produced d u r i n g the s t o r a g e o f cheese and i n t h e fermentation of the t r a d i t i o n a l Japanese f o o d "miso" and "soy sauce" has been shown t o be caused by t h e p e p t i d e s i n the h y d r o l y s a t e of proteins. Since then, a number o f s t u d i e s on b i t t e r p e p t i d e s have

c

0097-6156/89/0388-0158$06.00/0 1989 American Chemical Society

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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13. KATO ET AL·

Free Amino Acids and Peptides in Food Taste

159

been conducted. A l s o , r e s e a r c h has been conducted on sweet and umami peptides r e c e n t l y . T h i s paper d e a l s w i t h t h e t a s t e s o f f r e e amino a c i d s and p e p t i d e s , and t h e i r r o l e s i n t h e t a s t e o f f o o d s . For t h e convenience o f o u r r e a d e r s , t h e amino a c i d and p e p t i d e symbols used a r e : Alanine Leu Leucine Ala Arginine Lys Arg Lysine Asparagine Met Methionine Asn Phe Aspartic acid Phenylalanine Asp Asn + Asp Pro Proline Asx Cysteine Ser Serine Cys Glutamine Thr Gin Threonine Trp Glutamic a c i d Tryptophan Glu Gin + G l u Tyr Glx Tyrosine Glycine Val Valine Gly Histidine Anserine Ans His Car Isoleucine Carnosine He TASTE OF FREE AMINO ACIDS Amino a c i d s a r e n o t o n l y t h e b u i l d i n g b l o c k s o f p r o t e i n s b u t a l s o o c c u r i n t h e f r e e form. Amino a c i d s commonly found i n p r o t e i n s have t h e L - c o n f i g u r a t i o n . Of t h e s e amino a c i d s (Table 1 ) , Asn was f i r s t discovered i n asparagus i n 1806, and T h r , t h e most r e c e n t l y discovered, was i s o l a t e d from t h e h y d r o l y s a t e s o f f i b r i n i n 1935· Most o f them were i s o l a t e d from h y d r o l y s a t e s of various proteins. Glu, f i r s t obtained from wheat g l u t e n h y d r o l y s a t e i n 1886, was found t o be t h e most i m p o r t a n t t a s t e component i n s e a t a n g l e by Ikeda i n 1908. L a t e r , i n d u s t r i a l p r o d u c t i o n o f MSG was u n d e r t a k e n t o u t i l i z e i t as a seasoner. Almost a l l amino a c i d s e l i c i t t a s t e . Most h y d r o p h o b i c L-amino a c i d s have a b i t t e r t a s t e . However, hydrophobic D-amino a c i d s , which are formed s i m u l t a n e o u s l y by t h e s y n t h e s i s o f L-amino a c i d s , b r i n g out a s t r o n g sweet t a s t e . D-Trp, Phe, H i s , T y r and Leu a r e 3 5 , 7, 7, 6 and U times as sweet as s u c r o s e , r e s p e c t i v e l y ( 2 ) . G l y and L - A l a e l i c i t a s t r o n g sweet t a s t e . I t i s thought t h a t t h e s t r o n g sweet t a s t e e l i c i t e d by these amino a c i d s i s due t o t h e a b i l i t y o f t h e s e m o l e c u l e s t o b i n d t o t h e sweet substance r e c e p t o r s . L - G l u and Asp a r e sour s t i m u l i i n d i s s o c i a t e d s t a t e , b u t t h e i r sodium s a l t s d i s s o c i a t e on s o l u t i o n and e l i c i t t h e umami t a s t e . Free L-glutamate i s c o n t a i n e d i n n a t u r a l foods, as shown i n Table 2 and c o n t r i b u t e s t o the savory t a s t e o f foods as i t s sodium s a l t . I b o t e n i c and t r i c h o l o m i c a c i d s ( I A and TA) d i s c o v e r e d i n mushrooms are t h e d e r i v a t i v e s o f o x y g l u t a m i c a c i d and a r e a l s o umami substances (Λ,5). The umami t a s t e i n t e n s i t y o f IA o r TA i s Λ t o 25 t i m e s t h a t o f MSG. As t h e s e compounds a r e n o t amino a c i d s commonly found i n an a n i m a l system, t h e y have n o t been used as s e a s o n e r s . The umami t a s t e o f a MSG-, I A - o r T A - 5 ' - r i b o n u c l e o t i d e m i x t u r e i s much more i n t e n s e than t h a t o f o n l y MSG, IA o r TA. Among 5 ' - r i b o n u c l e o t i d e s , 5 i n o s i n a t e and g u a n y l a t e have s y n e r g i s t i c e f f e c t s i n a m i x t u r e w i t h MSG, IA o r TA. T h i s phenomenon i s c a l l e d t h e s y n e r g i s t i c e f f e c t o f f

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Table 1. T a s t e , T h r e s h o l d Value and D i s c o v e r y o f Amino A c i d s

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Amino acid

Taste

His Met Val Arg He Phe Trp Leu Tyr Ala Gly Ser Thr Lys Pro Asp Glu Asn Gin Cys

Bitter Bitter Bitter Bitter Bitter Bitter Bitter Bitter Bitter Sweet Sweet Sweet Sweet Sweet and b i t t e r Sweet and b i t t e r Sour Sour Sour Flat

G l u Na Asp Na

Umami Umami

Threshold value(mg/dl) 20 30 AO 50 90 90 90 190 ND 60 130 150 260 50 300 3 5 100

Where found

C a s e i n and s t u r i n * ( 1 8 9 6 ) Casein*(1922) Albumin*(1879) Casein*(1895) Syrup(1904) Bean sprouts(1881 ) Casein*(1901) unknown(1819) Casein*(1846)

ND

Fibroin*(1875) Gelatin*(1820) Sericin*(l865) Fibrin*(1935) Casein*(1889) Casein*(1901) Asparagine*(1827) Gluten*(1886) Asparagus(1806) Beet(1883) Cystine(1884)

30 100

Sea t a n g l e ( 1 9 0 8 ) unknown

ND, n o t determined; *, h y d r o l y s a t e .

t a s t e ( 6 ) . When G l y was added t o a M S G - 5 ' - r i b o n u c l e o t i d e m i x t u r e , the umami t a s t e i n t e n s i t y o f t h e m i x t u r e was g r e a t e r t h a n t h a t o f t h e m i x t u r e b e f o r e a d d i t i o n ( 7 ) . A l a , Cys, H i s , Met, P r o and V a l , b e s i d e s G l y , were a l s o r e c o g n i z e d as h a v i n g t h e s y n e r g i s t i c e f f e c t o f t a s t e i n t h e m i x t u r e o f MSG, 5 ' - r i b o n u c l e o t i d e and f r e e amino a c i d s ( 8 ) . Though amino a c i d s can e l i c i t any one o f t h e p r i m a r y t a s t e s , t h e t h r e s h o l d v a l u e o f t a s t e o f each amino a c i d i s h i g h . As t h e l e v e l s of some f r e e amino a c i d s i n n a t u r a l foods a r e l o w e r t h a n their t h r e s h o l d v a l u e s , i t may be thought t h a t t h e y may n o t c o n t r i b u t e d i r e c t l y t o f o o d t a s t e . However, t h e y may have an i m p o r t a n t r o l e i n making t h e f o o d s a v o r y because o f t h e s y n e r g i s t i c e f f e c t .

TASTE OF PEPTIDES Sweet Taste The sweet p e p t i d e , aspartame (L-Asp-L-Phe-OMe) which has a sweet t a s t e 180 t i m e s t h a t o f s u c r o s e , was d i s c o v e r e d by Mazur e t a l . (2)· Aspartame i s s t a b l e a t pH 4 and u n s t a b l e a t pH 1 o r 7-8. I t i s a l s o u n s t a b l e a t h i g h t e m p e r a t u r e s . Under t h e s e u n s t a b l e c o n d i t i o n s , t h e

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

13.

Free Amino Acids and Peptides in Food Taste

KATO ET A L

Table 2.

Free L-glutamate i n N a t u r a l Foods

Food

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161

L-glutamate (mg/100 g)

Kelp Parmesan cheese Green t e a Seaweed Fresh sardine F r e s h tomato j u i c e Champignon Tomato Oyster Potato Chinese cabbage F r e s h s h i i t a k e mushroom Soybean Sweet p o t a t o Dried sardine Prawn Clam Chicken bones Cabbage Carrot Bonito f l a k e s Pork f i l l e t

2240 1200 668 640 280 260 180 140 137 102 100 67 66 60 50 43 41 40 37 33 26 23

e s t e r l i n k a g e o f aspartame may h y d r o l y z e t o produce a s p a r t y l phenylalanine (AP) o r c y c l i z e t o produce the c o r r e s p o n d i n g d i k e t o piperazine (DKP). As none o f these c o n v e r s i o n p r o d u c t s f i t s the sweet t a s t e r e c e p t o r , none o f them i s sweet. Thaumatin (1_0) and m o n e l l i n ( H ) a r e sweet and a r e p r o t e i n s i s o l a t e d from a p l a n t n a t i v e t o Western A f r i c a . Thaumatin and m o n e l l i n a r e r e s p e c t i v e l y 1600 and 3000 times sweeter than s u c r o s e . As both p r o t e i n s a r e b a s i c , t h e y a r e assumed t o b i n d e a s i l y t o the n e g a t i v e l y charged t a s t e c e l l . There have been no r e p o r t s r e g a r d i n g the d e t e c t i o n o f sweet p e p t i d e s i n n a t u r a l l y o c c u r r i n g f o o d s t u f f s o t h e r than thaumatin and monellin.

B i t t e r Taste Almost a l l p e p t i d e s o f hydrophobic L-amino a c i d s e l i c i t a b i t t e r t a s t e , which i n d i c a t e s t h a t the b i t t e r n e s s o f p e p t i d e s i s caused by the hydrophobic p r o p e r t y o f the amino a c i d s i d e c h a i n . Ney (12) has r e p o r t e d t h a t whether a p e p t i d e has a b i t t e r t a s t e o r not depends on i t s hydrophobic v a l u e Q. The v a l u e Q i s o b t a i n e d by a d d i n g the AÎvalues (Table 3) o f each c o n s t i t u e n t amino a c i d r e s i d u e o f a p e p t i d e and d i v i d i n g the sum by the number o f amino a c i d r e s i d u e s ( n ) .

η

If

the

value

Q

of a peptide

is

g r e a t e r than

1400,

the p e p t i d e

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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FLAVOR CHEMISTRY: TRENDS AND DEVELOPMENTS Table 3· A f - v a l u e o f the s i d e c h a i n o f amino a c i d

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amino a c i d Gly Ala Val Leu He Phe Pro Met Tyr Thr Ser Asp Glu Arg Lys Trp

Af

(cal/mol) 0 730 1690 2420 2970 2650 2600 1300 2870 440 40 540 550 730 1500 3000

w i l l e l i c i t b i t t e r t a s t e . This r u l e i s a p p l i c a b l e t o almost a l l peptides. H y d r o l y s i s o f p r o t e i n s w i t h o u t t a s t e by p r o t e a s e s o f t e n produces b i t t e r p e p t i d e s . Hydrophobic amino a c i d r e s i d u e s l o c a t e d i n the i n t e r i o r o f p r o t e i n m o l e c u l e s i n aqueous s o l u t i o n a r e exposed by fragmentation o f the p r o t e i n m o l e c u l e s t r e a t e d w i t h p r o t e a s e s , and the p e p t i d e s c o n t a i n i n g a number o f h y d r o p h o b i c amino a c i d r e s i d u e s occur i n the s o l u t i o n (13)· Many b i t t e r p e p t i d e s a s shown i n Table 4 have been i s o l a t e d from p r o t e i n d i g e s t s w i t h p r o t e i n a s e s (14-22). The comparison o f the amino a c i d sequence o f the above-mentioned b i t t e r p e p t i d e s shows a l a r g e p r o p o r t i o n o f h y d r o p h o b i c amino a c i d s i n each p e p t i d e . And the amino a c i d sequence o f p e p t i d e s a l s o p l a y s an i m p o r t a n t r o l e i n the i n t e n s i t y o f the b i t t e r t a s t e . For example, the b i t t e r n e s s o f Phe-Pro i s more i n t e n s e t h a n t h a t o f Pro-Phe, and the b i t t e r n e s s o f Gly-Phe-Pro i s more i n t e n s e than t h a t o f Phe-Pro01y (23). C-terminal groups o f a l l b i t t e r p e p t i d e s i n pepsin hydrolysates o f the above-mentioned soy p r o t e i n were c h a r a c t e r i z e d by the l o c a t i o n o f the L e u r e s i d u e (14-17)· The r e s e a r c h on the r e l a t i o n s h i p between the s t r u c t u r e and b i t t e r t a s t e i n t e n s i t y o f A r g Gly-Pro-Pro-Phe-Ile-Val (BP-Ia) showed t h a t Pro and Arg l o c a t e d on c e n t e r and the N - t e r m i n a l s i t e , r e s p e c t i v e l y , p l a y e d an i m p o r t a n t r o l e i n the i n c r e m e n t o f b i t t e r t a s t e i n t e n s i t y b e s i d e s the h y d r o phobic amino a c i d s l o c a t e d on C - t e r m i n a l site (24-26). T h i s may i n d i c a t e t h a t the p e p t i d e m o l e c u l a r s t r u c t u r e formed by the a r r a n g e ment o f A r g , Pro and h y d r o p h o b i c amino a c i d r e s i d u e s c o n t r i b u t e s t o the b i t t e r t a s t e i n t e n s i t y c f the p e p t i d e .

Sour Taste As shown i n T a b l e 5, d i p e p t i d e s c o n t a i n i n g Gly-Asp-Ser-Gly, Pro-Gly-Gly-Glu and V a l - V a l - G l u

G l u and/or Asp, i n water elicit

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

13. KATO ET A L

Free Amino Acids and Peptides in Food Taste

T a b l e A. B i t t e r P e p t i d e s I s o l a t e d from P r o t e i n a s e H y d r o l y s a t e s Proteins

of

Protein (proteinase*)

P e p t i d e s i s o l a t e d from

Soy p r o t e i n (pepsin)

G l y - L e u , Leu-Phe, Leu-Lys, Arg-Leu, Arg-Leu-Leu, Ser-Lys-Gly-Leu, PyroGlu-Gly-Ser-Ala-Ile-Phe-ValLeu, Tyr-Phe-Leu, Phe-Leu A l a - I l e - A l a , A l a - A l a - L e u , G l y - A l a - L e u , Leu-Gln-LeuLeu-Glu-Leu, L e u - V a l - L e u , Leu-Pro-Phe-Asn-Gln-Leu, Leu-Pro-Phe-Ser-Gln-Leu Ala-Gln-Thr-Gln-Ser-Leu-Val-Tyr-Pro-Phe-Pro-GlyPro-Ile-Pro-Asn-Ser-Leu-Pro-Gln-Asn-Ile-Pro-ProLeu-Thr-Gln Gly-Pro-Phe-Pro-Val-Ile, Phe-Phe-Val-Ala-Pro-ProGlu-Val-Phe-Gly-Lys, Phe-Ala-Leu-Pro-Glu-Tyr-LeuLys Arg-Gly-Pro-Pro-Phe-Ile-Val, Val-Tyr-Pro-Phe-ProPro-Gly-Ile-Asn-His, cyclo(Leu-Trp-Leu-Trp)

Zein (pepsin)

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163

Casein (papain) Casein (trypsin) Casein (bacterial proteinase) used

hydrolysate

to obtain hydrolysate of p r o t e i n .

sour t a s t e Q ) . T h i s a c i d i c s e n s a t i o n i s assumed t o be produced by the b i n d i n g o f the hydronium i o n , produced by the d i s s o c i a t i o n of a c i d i c amino a c i d , t o the t a s t e c e l l membrane.

T a b l e 5.

Sour P e p t i d e s

Gly-Asp, G l y - G l u ; A l a - A s p , A l a - G l u ; Ser-Asp, S e r - G l u ; V a l - A s p , V a l - G l u ; A s p - A l a , Asp-Asp; G l u - A l a , Glu-Asp, G l u - G l u ; Glu-Phe , Glu-Tyr , y-Glu-Gly , y - G l u - A l a , y-Glu-Asp , y-Glu-Glu ; Phe-Asp, Phe-Glu, Trp-Asp, T r p - G l u ; Gly-Asp-Ser-Gly, Pro-Gly-Gly-Glu, V a l - V a l - G l u a

a

b

b

b

b

A l l amino a c i d s have the L - c o n f i g u r a t i o n . a, Accompanied by b i t t e r n e s s and a s t r i n g e n c y ; b, Accompanied by a s t r i n g e n c y .

Salty

Taste It has been r e p o r t e d t h a t t h e r e a r e s a l t y s t i m u l i i n p e p t i d e s . Tada e t a l . (27) i n a d v e r t e n t l y d i s c o v e r e d the s y n t h e s i z e d salty dipeptides, L-0rn-3-Ala«HCl, L-Orn-Tau · HC1, Lys-Tau*HCl and L-OrnG l y · HC1 h a v i n g the same i n t e n s i t y t a s t e as N a C l . The s a l t y t a s t e o f L-Orn-Tau · HC1 and Lys-Tau*HCl was more i n t e n s e t h a n t h a t o f L - 0 r n - 3 A l a · HC1 and L-Orn-Gly · HC1. The degree of d i s s o c i a t i o n of the c a r b o x y l or s u l f o n y l group i n p e p t i d e s was assumed t o c o n t r i b u t e t o the i n t e n s i t y o f the s a l t y t a s t e . These d i p e p t i d e s may be u s e f u l as new s e a s o n i n g s f o r d i a b e t i c s and h y p e r t e n s i v e s because t h e y c o n t a i n no Na i o n s .

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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R e c e n t l y , Huynh-ba and P h i l i p p o s s i a n (28) have r e p o r t e d t h a t t h e L-Orn-Tau · HC1, L-0rn-3-Ala«HCl and L - O r n - G l y H C l t h e y s y n t h e s i z e d e l i c i t e d no s a l t y t a s t e . The s a l t y t a s t e o f L-Orn-Tau · HC1 s y n t h e ­ s i z e d by Tada e t a l . seemed t o r e s u l t from t h e NaCl p r e s e n t as an a r t i f a c t i n t h e method o f p r e p a r a t i o n . However, t h e p r e p a r a t i o n o f L-Orn-β-Ala-HCl (OBA-HCl) and L - O r n - G l y H C l d i d n o t c o n t a i n N a C l a t all. The r e a s o n f o r t h i s d i s c r e p a n c y i s n o t c l e a r y e t . We heard from Okai group t h a t t h e p u r i f i e d OBA w i t h o u t HC1 d i d n o t e l i c i t s a l t y t a s t e i n w a t e r . When t h e molar r a t i o o f HC1 t o OBA i n OBA-HCl became 0.97 i n t h e s o l u t i o n , t h i s s o l u t i o n e l i c i t e d a s l i g h t l y s a l t y t a s t e . T h i s s a l t y t a s t e e l i c i t e d s t r o n g l y w i t h i n c r e a s i n g HC1, t i l l the molar r a t i o o f HC1 t o OBA became 1.3 ( O k a i , Η., H i r o s h i m a U n i v e r s i t y , p e r s o n a l communication, 1988.). As t h e molar r a t i o o f HC1 i n 0BA*HC1 s y n t h e s i z e d by Huynh-ba and P h i l i p p o s s i a n was 1.0, i t s s a l t y t a s t e might be v e r y weak.

Umami Taste S e v e r a l d i p e p t i d e s h a v i n g L-Glu a t N-terminus e l i c i t t h e umami t a s t e , though i t s umami t a s t e i n t e n s i t y i s much l e s s than t h a t o f MSG. A r a i e t a l . (29) s y n t h e s i z e d L-Glu-X (X= amino a c i d ) and examined t h e i r t a s t e i n aqueous s o l u t i o n c o n t a i n i n g NaCl a t pH 6. Glu-Asp, G l u - T h r , G l u - S e r and G l u - G l u were found t o produce t h e umami t a s t e . Ohyama e t a l . (30) showed t h a t Asp-Leu and Glu-Leu were umami substances. I n s e c t i o n "Sour T a s t e " , t h e p e p t i d e s c o n t a i n i n g Asp or/and G l u were shown t o e l i c i t a sour t a s t e i n w a t e r . However, s e v e r a l o f t h e i r p e p t i d e s b e s i d e s Glu-Asp and G l u - G l u may a l s o be umami s t i m u l i i n aqueous s o l u t i o n s c o n t a i n i n g NaCl a t pH 6. When f i s h p r o t e i n s were t h o r o u g h l y h y d r o l y z e d by pronase, t h e hydrolysate elicited t h e complex t a s t e c o n t a i n i n g b i t t e r n e s s . P e p t i d e s h a v i n g n o t o n l y t h e b i t t e r b u t a l s o t h e umami t a s t e were produced i n t h i s h y d r o l y s a t e . The f r a c t i o n o f compounds w i t h m o l e c u l a r weight under $00 was o b t a i n e d from t h i s h y d r o l y s a t e by ultrafiltration. T h i s f r a c t i o n was d i v i d e d i n t o f o u r f r a c t i o n s , a r o m a t i c , a c i d i c , n e u t r a l and b a s i c f r a c t i o n s . The a c i d i c f r a c t i o n had a v e r y i n t e n s e umami t a s t e . Though G l u was removed from t h e a c i d i c f r a c t i o n by t h e t r e a t m e n t w i t h ion-exchange chromatography, the t r e a t e d a c i d i c f r a c t i o n a l s o e l i c i t e d an umami t a s t e . Therefore, the umami t a s t e o f t h e a c i d i c f r a c t i o n can be a s c r i b e d i n p a r t t o p e p t i d e s , a l t h o u g h MSG was m a i n l y r e s p o n s i b l e f o r t h e umami t a s t e . Umami p e p t i d e s from t h i s f r a c t i o n (Table 6) were c h a r a c t e r i z e d by c o n t a i n i n g G l u r e s i d u e and a number o f h y d r o p h i l i c amino a c i d s except f o r G l u (31,32).

Table 6. Pronase*

Umami P e p t i d e s I s o l a t e d from F i s h P r o t e i n H y d r o l y s a t e s by

Dipeptides: Tripeptides:

G l u - G l u , Glu-Asp, T h r - G l u , G l u - S e r Glu-Gly-Ser, Ser-Glu-Glu, Glu-Gln-Glu, Asp-Glu-Ser

Glu-Asp-Glu,

*, T h r e s h o l d v a l u e s o f t h e s e p e p t i d e s a r e 150-300 mg/dl.

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13.

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An octapeptide, L y s - G l y - A s p - G l u - G l u - S e r - L e u - A l a , which has a d e l i c i o u s t a s t e , was i s o l a t e d from beef t r e a t e d w i t h p a p a i n by t h e use o f g e l f i l t r a t i o n and ion-exchange chromatography, and f i l t e r paper e l e c t r o p h o r e s i s (33). A sensory e v a l u a t i o n showed t h a t t h i s o c t a p e p t i d e produced s y n t h e t i c a l l y a l s o e l i c i t e d a d e l i c i o u s t a s t e (3Λ). The e l i m i n a t i o n o f two amino a c i d r e s i d u e s a t N-terminus, L y s G l y , l e d t o t h e d i s a p p e a r a n c e o f t h e s a v o r y t a s t e and changed i t i n t o a sour t a s t e . T h i s indicated that the residues, Lys-Gly are important t o the savory t a s t e .

Other T a s t e s Kirimura e t a l . (3) have r e p o r t e d that the dipeptides formed by t h e b i n d i n g o f γ-COOH group i n G l u t o NH2 group i n a n o t h e r amino a c i d , such as γ-L-Glu-Gly, γ-L-Glu-L-Ala, γ-L-Glu-L-Asp and γ-L-GluL - G l u , have n o t o n l y sour t a s t e b u t a l s o a s t r i n g e n t t a s t e . I n g e n e r a l , because p e p t i d e s were amphoteric e l e c t r o l y t e s , t h e y have a b u f f e r a c t i o n on t a s t e . 3 - A l a - H i s (Car) and 3 - A l a - 1 - m e t h y l H i s (Ans) w i d e l y d i s t r i b u t e d i n a n i m a l t i s s u e s were found t o have a l a r g e b u f f e r a c t i o n i n t h e pH range above 6.0 (35). Dipeptides, G l y Leu, P r o - G l u and V a l - G l u , were a l s o found t o have a b u f f e r a c t i o n (3). When t h e s e p e p t i d e s were added t o a s y n t h e t i c "sake", a t r a d i t i o n a l Japanese a l c o h o l i c d r i n k , composed o f a l c o h o l , g l u c o s e , s u c c i n i c a c i d , l a c t i c a c i d , p h o s p h o r i c a c i d , N a C l , MSG, G l y and A l a , the b u f f e r a c t i o n s o f sake c o n t a i n i n g G l y - L e u and P r o - G l u were l a r g e r i n t h e pH range above pH 8 t h a n t h a t o f sake w i t h o u t G l y - L e u adn P r o Glu. The b u f f e r a c t i o n o f sake c o n t a i n i n g V a l - G l u was l a r g e r i n t h e pH range 7-9 than t h a t o f sake n o t c o n t a i n i n g V a l - G l u . The b u f f e r a c t i o n seems t o p l a y an i m p o r t a n t r o l e i n t h e improvement o f f o o d t a s t e by enhancing t h e t a s t e o f f o o d and k e e p i n g t h e e l i c i t a t i o n o f i t s taste (36). A glutamic a c i d - r i c h oligopeptides f r a c t i o n was found t o be e f f e c t i v e i n masking b i t t e r t a s t e (37). The a d d i t i o n o f t h e s e p e p t i d e s t o t h e b i t t e r m e d i c i n a l drugs and d r i n k s (summer orange and v e g e t a b l e j u i c e s , and cocoa) seems t o decrease o r mask t h e b i t t e r n e s s of t h e p r o d u c t s .

ROLE OF FREE AMINO ACIDS AND PEPTIDES IN FOOD TASTES V e g e t a b l e Foods Free amino a c i d s p l a y an i m p o r t a n t r o l e i n t h e t a s t e o f vegetables. There a r e l a r g e amounts o f G l u , Asp, S e r , V a l , A l a , P r o and G i n i n v e g e t a b l e s as shown i n T a b l e 7 (38). The d e t a i l e d r e s e a r c h on t h e t a s t e o f green t e a , o n i o n and p o t a t o r e v e a l s t h e presence o f umami s u b s t a n c e s . The most i m p o r t a n t umami substances o f green t e a are G l u and L - t h e a n i n e , which i s an e t h y l a m i d e d e r i v a t i v e o f G l u (3941). I t has been shown t h a t t h e most i m p o r t a n t umami substance o f o n i o n (42) i s Glu. B u r i e t a l . (A3) examined t h e r o l e o f f r e e amino a c i d s i n t h e f l a v o r o f b o i l e d p o t a t o e s . A n a l y t i c a l d a t a has shown t h a t t h e t a s t e o f b o i l e d p o t a t o soup s t o c k was s i m i l a r t o t h a t o f s y n t h e t i c p o t a t o soup composed o f f r e e amino a c i d s and n u c l e o t i d e s . T h i s i n d i c a t e d t h a t f r e e amino a c i d s a r e v e r y i m p o r t a n t i n p o t a t o t a s t e . A l t h o u g h t h e y c o n t a i n l a r g e amounts o f G l u and Asp, t h e r e a r e

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T a b l e 7.

Contents

Amino acid

Tomato

Glu Ser Gly Asn Lys Thr Gin Ala Arg Tyr Val Phe Leu Pro Asp Cys

3.99 10.07 A.83 6.61 1.89 0.11 6.58 0.92 6.81 0.75 9.17 2.37 2.U

o f F r e e Amino A c i d s i n V e g e t a b l e s (α-amino Ν mg/100 g)

Egg plant

-

Cucumber

Carrot

0.65 2.85 0.54 3.33

3.02

0.84 0.59 0.48 2.66

-+ 3.02 0.97 2.05 0.34 2.35

-

0.95 0.54 0.99 0.50

3.03 2.28 0.82 9.77 1.16 0.46 5.09 2.24 1.95 0.45 4.01 2.05 0.74 1.71

-

2.34

0.35 -

-

0.33 1.44 1.33 4.11 0.12 1.23

0.95 0.53 0.65 1.10 0.65 0.95 0.50 0.80

-

0.45 0.55 2.88

-

Pumpkin

--

-

Maize

0.33 0.55 0.44 1.32

-

0.22 4.18 1.32 1.32 0.11 3.52 0.33 0.77 1.76 0.66

+

+, t r a c e ; -, n o t d e t e c t e d .

T a b l e 8.

f

5 --guanylate i n N a t u r a l Foods

Food D r i e d s h i i t a k e mushroom Matsutake E n o k i t a k e mushroom F r e s h s h i i t a k e mushroom T r u f f l e mushroom Pork Beef Chicken

f

5 -guanylate

(mg/100 g)

156.5 64.6 21.8 16-45 5.8 2.5 2.2 1.5

smaller amounts o f n u c l e o t i d e s , IMP o r GMP, i n p o t a t o e s than i n animal foods. I n a n i m a l f o o d s , IMP o r GMP enhances umami and b r o t h y taste e l i c i t e d by MSG. T h i s may account f o r t h e absence o f the brothy taste i n v e g e t a b l e s (44)· However, mushrooms c o n t a i n excep­ t i o n a l l y h i g h l e v e l s o f GMP ( T a b l e 8 ) . GMP i n mushrooms enhances t h e umami t a s t e o f G l u by a s y n e r g i s t i c effect and i m p a r t s a b r o t h y taste. Miso and n a t t o a r e t r a d i t i o n a l Japanese f o o d s made from soybeans by a f e r m e n t a t i o n p r o c e s s . These f o o d s a r e produced by a mixed fermentation process using a characteristic microorganism and ripening f o r a given time. In these processes, the h y d r o l y s i s o f p r o t e i n s by m i c r o b i a l p r o t e a s e s results i n the production of free

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amino a c i d s and p e p t i d e s . F r e e amino a c i d s and p e p t i d e s produced i n each f o o d contribute t o t h e c h a r a c t e r i s t i c t a s t e o f each f o o d . The r a t e o f l i b e r a t i o n o f G l u and Asp as umami s u b s t a n c e s d u r i n g r i p e n i n g o f miso was v e r y slow and amounts l i b e r a t e d from p r o t e i n s were s m a l l (45) . Examination o f t h e change i n p e p t i d e s d u r i n g s t o r a g e o f miso showed that the peptides o f A . P . L . ( t h e average number o f amino a c i d r e s i d u e s i n t h e p e p t i d e ) 3-4 g r a d u a l l y d e c r e a s e d , b u t t h e p e p t i d e s o f A.P.L. 3-4 r a p i d l y i n c r e a s e d d u r i n g t h e i n i t i a l s t a g e s o f storage. The p e p t i d e s o f A.P.L. 13-20 i n c r e a s e d w i t h s t o r a g e time ( F i g . 1) (46) . As AO % o f t h e c o n s t i t u e n t amino a c i d s r e s i d u e i n t h e p e p t i d e s o f A.P.L. 13-20 was G l u , t h e s e p e p t i d e s seemed t o p l a y an i m p o r t a n t r o l e i n t h e umami t a s t e o f miso. The f r e e amino a c i d content i n natto was v e r y small and c o r r e s p o n d e d t o o n l y about 10 % o f t h e t o t a l n i t r o g e n compounds. Most of the other n i t r o g e n compounds were p e p t i d e s . These p e p t i d e s have been shown t o c o n t r i b u t e t o the b i t t e r n e s s o f the c h a r a c t e r i s t i c taste of natto ( 4 7 ) . One of these peptides was i s o l a t e d and i t s amino a c i d sequence was i n v e s t i g a t e d . The amino a c i d c o m p o s i t i o n o f this p e p t i d e was Asp 1, T h r 1, G l u 1, A l a 1, Pro 2, V a l 3, H e 3 and Leu 5. The amino a c i d a t N-terminus o f t h i s p e p t i d e was Leu and the C - t e r m i n a l s t r u c t u r e was - A l a - V a l - I l e - L e u . A c y c l i c dipeptide, Pro-Leu anhydride, having b i t t e r n e s s was i s o l a t e d from a traditional Japanese a l c o h o l i c d r i n k "sake" (48)· T h i s p e p t i d e i n c r e a s e d t h e l o n g e r sake was s t o r e d i n sake p r o d u c t i o n . So t h i s p e p t i d e seems t o c o n t r i b u t e t o the b i t t e r t a s t e o f sake.

A n i m a l Foods T a s t e components o f a number o f s e a f o o d p r o d u c t s have been examined f o r each s e a f o o d p r o d u c t has i t s i n d i v i d u a l c h a r a c t e r i s t i c taste. In s t u d i e s on t h e f r e e amino a c i d s a n a l y s i s ( T a b l e 9 ) , i t was shown t h a t t h e major amino a c i d i s H i s i n r e d meat o f f i s h , G l y and Pro i n c u t t l e f i s h , G l y and A r g i n prawns, and Tau and A r g i n a b a l o n e (49). The major amino a c i d s i n s e a u r c h i n a r e G l y , A l a and L e u (50)» However, t h e components contributing to their i n d i v i d u a l characteri s t i c t a s t e s were not e l u c i d a t e d because t h e r e l a t i o n s h i p between the t a s t e components and t h e t a s t e was n o t t h o r o u g h l y c o r r e l a t e d and i n v e s t i g a t e d i n most o f t h e s e s t u d i e s . Detailed research on t h e r e l a t i o n s h i p between t h e t a s t e components and t h e t a s t e o f s e a u r c h i n , shrimp and c r a b l e d t o the identification o f the c h a r a c t e r i s t i c t a s t e components. The c h a r a c teristic components o f s e a u r c h i n a r e G l y , A l a , V a l , G l u , Met, i n o s i n e 5'-monophosphate (IMP) and guanosine 5 -monophosphate (GMP) (51) . The c o n t r i b u t i o n o f G l y and A l a t o sweetness, V a l t o b i t t e r n e s s , and G l u , IMP and GMP t o umami t a s t e was f o u n d . Met was shown t o be r e s p o n s i b l e f o r t h e c h a r a c t e r i s t i c t a s t e o f s e a u r c h i n . The characteristic taste o f shrimp i s sweet t a s t e which i s attributed t o G l y , the l a r g e s t component o f a l l t h e f r e e amino a c i d s i n shrimp (52) . The 12 components - G l y , A l a , A r g , G l u , CMP, AMP, GMP, N a , K^7 C l ~ , P0^3" and b e t a i n - were shown t o c o n t r i b u t e t o t h e c h a r a c t e r i s t i c t a s t e o f b o i l e d crab e x t r a c t ( 5 3 ) . Free amino a c i d s and p e p t i d e s released by such p r o t e o l y t i c enzymes as chymosin and l a c t i c a c i d b a c t e r i a l p r o t e a s e s i n cheeses contribute t o t h e f o r m a t i o n o f cheese t a s t e . B i e d e and Hammond (54) r e p o r t e d t h a t f r e e amino a c i d s and s m a l l p e p t i d e s p l a y e d an i m p o r t a n t 1

+

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Table 9.

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Amino acid Tau Asp Thr Ser Gin + Asn Glu Pro Gly Ala Cys Val Met He Leu Tyr Phe Trp His Lys Arg

Contents

Plaice

171

+

A

3 1 6 1 5 13

o f Free Amino A c i d s i n Sea Foods (mg/100 g)

Yellowfin tuna 26 1 3 2

3 2 3 7

-1

-7

1 1 1 1 1

3 3 7 2 2

-1

1220

17 3

35 0.6

Cuttle­ fish 160

+ 9 27 _ 3 7Λ9 832 181 3 3 7 6 12 8 2 5 16 15 2Λ6

Abalone

Scallop

Prawn

Snow crab

946 9 82 95 _

176

150

2A3

+

+

38 6 _

13 133

109 83 17Λ 98

99 36 613 82 3 10 12 3 0.3 2

-

37 13 18 2A

57 26 20 23 76 299

-

+

3A

19 327 623 187

203 1222 A3

+

A

17 12 9 13 20 7

7 935

16 52 902

-10

10 U U

+

-30 19 29 30 19 17 lO 8 25 579

+, t r a c e ; -, n o t d e t e c t e d .

r o l e i n p r o d u c i n g t h e sweet and b r o t h y t a s t e s o f Swiss cheese, and t h a t medium s i z e d ( t r i t o hexa) p e p t i d e s p l a y e d an i m p o r t a n t r o l e i n b i t t e r n e s s . A number o f s t u d i e s on b i t t e r p e p t i d e s o f cheese have been c a r r i e d o u t . S e v e r a l b i t t e r p e p t i d e s were i s o l a t e d from d i f f e r e n t cheeses and t h e i r s t r u c t u r e s were determined. L-Leu-Trp0H, a b i t t e r p e p t i d e , was i s o l a t e d from Swiss cheese (5.5), and Pro-Phe-Pro-Gly-Pro-Ile-Pro-Asn-Ser from Butterkâse (56). From Cheddar cheese were i s o l a t e d P r o - P h e - P r o - G l y - I l e - P r o , Pro-Phe-ProG l y - P r o - I l e - A s n - S e r , and G l n - A s p - L y s - I l e - H i s - P r o - P h e - A l a - G l n - T h r - G l n Ser-Leu-Val-Tyr-Pro-Phe-Pro-Gly-Pro-Ile-Pro (57). Recently, the f o r m a t i o n mechanisms o f f r e e amino a c i d s and p e p t i d e s c o n t r i b u t i n g t o cheese t a s t e have been shown (58). As shown i n F i g . 2., t h e p e p t i d e , aSI-CN(f1-23), o b t a i n e d from Gouda-type cheese was produced by t h e a c t i o n o f chymosin w i t h a S I - c a s e i n ( - C N ) . T h i s p e p t i d e was degraded by l a c t i c a c i d b a c t e r i a l p r o t e a s e and s m a l l p e p t i d e s , i n c l u d i n g t h r e e major p e p t i d e s , aSI-CN(f1-9), aSI-CN(f1-13) and a S I - C N ( f 1 - U ) , were formed. These s m a l l p e p t i d e s were f u r t h e r degraded i n t o s m a l l e r p e p t i d e s and f r e e amino a c i d s by aminopeptidase of l a c t i c acid bacteria. I t i s s a i d t h a t f l a v o r o f b e e f , pork and c h i c k e n i s improved by s t o r a g e a t a l o w temperature f o r g i v e n p e r i o d s . We examined t h e e f f e c t o f t h e s t o r a g e a t l o w temperature on t h e t a s t e o f meats (59). A f t e r beef, pork and c h i c k e n were s t o r e d a t 4°C f o r 8, 5 and 2 days, r e s p e c t i v e l y , t h e changes i n i n t e n s i t y and l e v e l s o f b r o t h y t a s t e and

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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KATO ET AL.

Free Amino Acids and Peptides in Food Taste

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F i g . $. R e l a t i v e s t r e n g t h o f each t a s t e among b e e f , pork and c h i c k e n soups. The N a C l c o n c e n t r a t i o n o f each soup was a d j u s t e d t o 0.$08%. 0 - - - 0 » beef;£*---&, p o r k ; D • , chicken. C l o s e d symbols, s i g n i f i c a n t l y d i f f e r e n t from o t h e r s (p ( 0.0$). ( R e p r i n t e d w i t h p e r m i s s i o n from r e f . 60. C o p y r i g h t 1988 M a r c e l Dekker.)

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

13. KATOETAL.

173 Free Amino Acids and Peptides in Food Taste

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October 6, 1988

In Flavor Chemistry; Teranishi, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.