Plant Proteins - American Chemical Society

19 Protein of the Sweet Potato 1

2

W. M . Walter, Jr. , and A . E . Purcell 1

Agricultural Research Service, U.S. Department of Agriculture, and North Carolina Agricultural Research Service, Department of Food Science, North Carolina State University, Raleigh, NC 27695

2

Downloaded by CORNELL UNIV on May 11, 2017 | http://pubs.acs.org Publication Date: June 18, 1986 | doi: 10.1021/bk-1986-0312.ch019

Department of Food Science and Nutrition, Brigham Young University, Provo, UT 84602

The sweet potato ranks sixth in average yearly production among the world's major food crops. The crude protein content ranges from 1.3% to > 10% (dry weight basis). Significant potential exists for increasing the protein content by breeding/selection and optimization of production practices. From 60-85% of the nitrogenous material is protein, and the remainder i s mostly amino and amide nitrogen. Humans have been maintained in nitrogen balance using sweet potato as the major source of nitrogen. The protein efficiency ratio (PER) for isolated sweet potato protein i s equal to that of casein. Heat processing lowers lysine bioavailability, dependent upon the severity of the heat treatment and the amount of reducing sugar present during heating.

The sweet p o t a t o (Ipomoea b a t a t a s L.) i s an i m p o r t a n t c o n t r i b u t o r t o human n u t r i t i o n i n many p a r t s o f the w o r l d . Sweet p o t a t o ranks s i x t h i n annual w o r l d p r o d u c t i o n a t 137 m i l l i o n m e t r i c tons (1975-1977) (1) b e h i n d wheat, r i c e , maize, p o t a t o , and b a r l e y . Although starchy roots are g e n e r a l l y considered t o provide only c a l o r i e s t o the d i e t , t h e sweet p o t a t o p r o v i d e s 73% o f t h e r e q u i r e d p r o t e i n p e r c a l o r i e (2, 3) f o r an a d u l t male. The average y i e l d f o r sweet p o t a t o e s f o r 1975-1977 (1) was 9,621 kg/ha, making i t second o n l y t o w h i t e p o t a t o e s among t h e t e n l e a d i n g c r o p s produced worldwide. There i s s i g n i f i c a n t p o t e n t i a l f o r i n c r e a s e d y i e l d s , p r o v i d e d p r o d u c t i o n p r a c t i c e s a r e o p t i m i z e d and h i g h y i e l d i n g c u l t i v a r s a r e grown. I n the U n i t e d S t a t e s , f o r example, the mean y i e l d i n 1980 was 13,108 kg/ha ( 4 ) . H i g h y i e l d s and a 110-130 day growing season make t h e sweet p o t a t o an a t t r a c t i v e s o u r c e o f c a l o r i e s and o t h e r n u t r i e n t s f o r t r o p i c a l r e g i o n s o f the w o r l d . I t i s noteworthy t h a t the m a j o r i t y o f t h e c o u n t r i e s w i t h an annual income o f l e s s than $500 (US) p e r c a p i t a a r e l o c a t e d i n the t r o p i c s . Thus, t h e sweet p o t a t o i s p o t e n t i a l l y 0097-6156/ 86/0312-0234$06.00/0 © 1986 American Chemical Society

Ory; Plant Proteins: Applications, Biological Effects, and Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

19.

WALTER AND PURCELL

Protein of the Sweet Potato

235


an o u t s t a n d i n g c a n d i d a t e f o r i n c r e a s e d p r o d u c t i o n in t h i s a r e a . A l t h o u g h not an i m p o r t a n t s o u r c e o f p r o t e i n in the U n i t e d S t a t e s , the sweet p o t a t o is consumed e x t e n s i v e l y in New Guinea, and in p a r t s o f t h a t c o u n t r y , p r o v i d e s up t o 40% o f the crude p r o t e i n in t h e d i e t (5^) . Data a r e n o t a v a i l a b l e f o r p r o t e i n p r o d u c t i o n worldwide. However, an e s t i m a t e o f the p r o t e i n c o n t r i b u t i o n p r o v i d e d by sweet p o t a t o e s can be made i f we assume a mean d r y m a t t e r c o n t e n t o f 28% and a mean p r o t e i n c o n t e n t o f 5%. Based on t h e s e assumptions, t h e sweet p o t a t o p r o v i d e s 1.92 m i l l i o n m e t r i c t o n s o f p r o t e i n worldwide. The y i e l d o f p r o t e i n would be 134 kg/ha u s i n g worldwide y i e l d v a l u e s o r 184 kg/ha u s i n g US p r o d u c t i o n values. Sweet P o t a t o P r o t e i n The d i e t must p r o v i d e t h o s e amino a c i d s which the body cannot s y n t h e s i z e ( e s s e n t i a l amino a c i d s , EAA) and n i t r o g e n in the form o f n o n e s s e n t i a l amino a c i d s (ΝΕΑ). Both EAA and ΝΕΑ a r e r e q u i r e d f o r b i o s y n t h e s i s o f p r o t e i n s and o t h e r n i t r o g e n - c o n t a i n i n g compounds n e c e s s a r y f o r h o m e o s t a s i s o r growth. Thus, the t o t a l n i t r o g e n c o n t e n t o f a s p e c i f i c f o o d must be c o n s i d e r e d t o be nutritionally significant. F o r t h o s e sweet p o t a t o c u l t i v a r s s t u d i e d , the crude p r o t e i n (Ν χ 6.25) c o n t a i n s both p r o t e i n and n o n p r o t e i n n i t r o g e n (NPN). The NPN c o n t e n t has been demonstrated t o range from 15 t o 37% a t h a r v e s t (6, 7 ) . The o n l y p u b l i s h e d r e p o r t o f the c o m p o s i t i o n showed the NPN f r a c t i o n t o be n u t r i t i o n a l l y unbalanced, c o n t a i n i n g m o s t l y amino a c i d s and amides ( 6 ) . The major components were a s p a r a g i n e , 61%; a s p a r t i c a c i d , 11%; g l u t a m i c a c i d , 4%; s e r i n e , 4%; and t h r e o n i n e , 3%. E i g h t y - e i g h t p e r c e n t o f the NPN f r a c t i o n was accounted f o r by amino a c i d s and amides. D u r i n g t h e e a r l y p a r t o f s t o r a g e , the NPN f r a c t i o n d e c r e a s e d , then i n c r e a s e d ( 8 ) . The n o n l i n e a r n a t u r e o f t h e change in NPN, c o u p l e d w i t h the f a c t that n i t r o g e n content decreased during storage, i n d i c a t e d that t h i s f r a c t i o n is p a r t o f a m e t a b o l i c a l l y a c t i v e n i t r o g e n p o o l (9) and t h a t the a p p r e c i a b l e amount o f n i t r o g e n s t o r e d as a s p a r a g i n e is a v a i l a b l e f o r m e t a b o l i c demands o f the r o o t . A l t h o u g h t h e NPN f r a c t i o n o f sweet p o t a t o is a v a i l a b l e t o s a t i s f y n i t r o g e n r e q u i r e m e n t s , o n l y s m a l l amounts o f EAA a r e p r e s e n t in t h i s f r a c t i o n . The i n i t i a l r e p o r t on the n a t u r e o f sweet p o t a t o p r o t e i n i n d i c a t e d t h a t most o f the p r o t e i n was a g l o b u l i n "ipomoein" (10) . The a u t h o r s a l s o s t a t e d t h a t upon s t o r a g e of t h e r o o t , ipomoein was p a r t i a l l y c o n v e r t e d i n t o a p o l y p e p t i d e which was c o n s i d e r a b l y d i f f e r e n t from t h e p a r e n t m a t e r i a l b o t h in i t s c h e m i c a l c o m p o s i t i o n and i t s p h y s i c a l p r o p e r t i e s . L a t e r workers u s i n g modern t e c h n i q u e s r e p o r t e d t h e major s o l u b l e p r o t e i n was a 25 k Da m o l e c u l e (11) . Only s m a l l amounts o f t h i s p r o t e i n were found in r o o t s s t o r e d f o r 1 y e a r , s u g g e s t i n g t h a t t h i s p r o t e i n is r e a d i l y m e t a b o l i z e d and is p r o b a b l y t h e s t o r a g e p r o t e i n . In a d d i t i o n , a second major p r o t e i n i d e n t i f i e d as b e t a amylase was a l s o shown t o be m i n i m a l l y p r e s e n t in r o o t s s t o r e d f o r 1 y e a r . Sweet p o t a t o p r o t e i n is u n e q u a l l y d i s t r i b u t e d w i t h i n t h e



236

PLANT PROTEINS

r o o t . The crude p r o t e i n c o n t e n t is s l i g h t l y g r e a t e r in the stem end than the r o o t end. The o n l y r e g i o n which has been shown t o c o n t a i n much h i g h e r p r o t e i n l e v e l s is the o u t e r l a y e r a d j a c e n t t o the s k i n c o r r e s p o n d i n g t o p r e c a m b i a l t i s s u e (12, 13). Scraping the r o o t s removed ^2.5% o f the f r e s h weight (FW) and d e c r e a s e d the r o o t p r o t e i n c o n t e n t by 4.4%, w h i l e a more d r a s t i c p e e l i n g which removed ^.8.5% o f the FW lowered the p r o t e i n c o n t e n t by 12% (13). The t i s s u e removed w i t h the s c r a p i n g s c o n s t i t u t e d 2.5% o f the t o t a l weight and c o n t a i n e d 87% more p r o t e i n per u n i t weight t h a n d i d the r e m a i n i n g t i s s u e . The t i s s u e removed by the deep p e e l i n g t r e a t m e n t c o n t a i n e d 47% more p r o t e i n per u n i t weight than d i d the t i s s u e r e m a i n i n g a f t e r p e e l i n g . The above d a t a i n d i c a t e t h a t a l t h o u g h the s u r f a c e l a y e r s o f t i s s u e a r e s i g n i f i c a n t l y h i g h e r in p r o t e i n c o n t e n t than the u n d e r l y i n g t i s s u e , the a b s o l u t e amount o f p r o t e i n - r i c h m a t e r i a l is s m a l l . C o n s e q u e n t l y , i t is not f e a s i b l e t o i n c r e a s e the p r o t e i n c o n t e n t by s e l e c t i v e removal o f t i s s u e . A p r o t e i n c o n c e n t r a t e can be o b t a i n e d from sweet p o t a t o r o o t s (_14) . The l a b o r a t o r y method i n v o l v e d g r i n d i n g w i t h t h r e e p a r t s of water, s c r e e n i n g t o remove c o a r s e f i b r o u s m a t e r i a l , s e t t l i n g the s t a r c h , c o a g u l a t i n g the c h r o m o p l a s t s , and p r e c i p i t a t i n g the p r o t e i n . Sweet p o t a t o e s have been used as a commercial s o u r c e o f s t a r c h and are s t i l l b e i n g used as such in Japan (15). Commercial p r o d u c t i o n of s t a r c h i n v o l v e s the f i r s t t h r e e s t e p s , i . e . , g r i n d i n g , s c r e e n i n g and s e t t l i n g the s t a r c h . I t would appear t h a t commercial q u a n t i t i e s of sweet p o t a t o p r o t e i n might be r e a d i l y a v a i l a b l e as a b y - p r o d u c t o f the s t a r c h i n d u s t r y . The l a b o r a t o r y c o n c e n t r a t e s were b l a n d , l i g h t - c o l o r e d powders c o n t a i n i n g 80-88% p r o t e i n . Crude P r o t e i n

Variability

The sweet p o t a t o is a p e r e n n i a l , p r o p a g a t e d v e g e t a t i v e l y as an a n n u a l f o r a g r i c u l t u r a l p u r p o s e s . The p l a n t is h e t e r o z y g o u s and is a h e x a p l o i d w i t h a somatic chromosome number o f 90. As would be e x p e c t e d , g e n e t i c p o t e n t i a l f o r v a r i a t i o n in p r o t e i n c o n t e n t is g r e a t . V a r i o u s workers have r e p o r t e d a p r o t e i n range of from 1.3% t o >10% (dry weight b a s i s ) (16, 17, 18), depending upon the c u l t i v a r . There appears t o be p o t e n t i a l f o r i n c r e a s i n g the p r o t e i n c o n t e n t by b r e e d i n g , s i n c e the sweet p o t a t o has responded q u i t e w e l l t o s e l e c t i o n f o r o t h e r t r a i t s when g e n e t i c v a r i a b i l i t y is p r e s e n t . I n c r e a s e in p r o t e i n c o n t e n t by s e l e c t i o n is e s p e c i a l l y i m p o r t a n t because many p a r t s of the t r o p i c s , which a r e in need o f a d d i t i o n a l p r o t e i n s o u r c e s , c o n s i s t e n t l y produce sweet p o t a t o e s w i t h low (< 4%) p r o t e i n c o n t e n t (dry b a s i s ) . L i (19) demonstrated t h a t a mass s e l e c t i o n t e c h n i q u e was e f f e c t i v e in i n c r e a s i n g crude p r o t e i n c o n t e n t and m a i n t a i n i n g a h i g h y i e l d . A l a t e r study (7) showed t h a t NPN p e r c e n t and t r y p s i n i n h i b i t o r a c t i v i t y d i d not i n c r e a s e as the sweet p o t a t o p r o t e i n c o n t e n t i n c r e a s e d . There appeared t o be some d e t e r i o r a t i o n in the p r o t e i n n u t r i t i o n a l q u a l i t y w i t h an apparent d e c l i n e in r e l a t i v e amounts of v a l i n e , a r o m a t i c amino a c i d s and s u l f u r - c o n t a i n i n g amino a c i d s . I t s h o u l d be n o t e d , however, t h a t sample t o sample v a r i a b i l i t y among amino a c i d s is v e r y g r e a t , and t h u s , more r e s e a r c h is needed in t h i s a r e a b e f o r e a d e f i n i t e r e l a t i o n s h i p can be d e t e r m i n e d .



19.

WALTER AND PURCELL


237

W i t h i n c u l t i v a r v a r i a t i o n o f sweet p o t a t o crude p r o t e i n is h i g h . P u r c e l l e t a l . (20) r e p o r t e d a 13% c o e f f i c i e n t o f v a r i a b i l i t y between r o o t s from a s i n g l e h i l l and a 13% c o e f f i c i e n t o f v a r i a b i l i t y between h i l l s in a s i n g l e f i e l d . F i e l d t o f i e l d v a r i a b i l i t y was v e r y g r e a t w i t h Jewel c u l t i v a r , r a n g i n g from 3.99 t o 8.81% p r o t e i n (dry b a s i s ) , depending upon t h e f i e l d l o c a t i o n . In a c a r e f u l l y c o n t r o l l e d study, C o l l i n s and W a l t e r (21) r e p o r t e d t h a t f o r s i x sweet p o t a t o genotypes grown a t s i x l o c a t i o n s f o r 3 y e a r s (18 e n v i r o n m e n t s ) , p r o t e i n c o n t e n t v a r i e d in a s t a t i s t i c a l l y s i g n i f i c a n t manner (V 4. O.01) by genotype, environment and the environment-genotype i n t e r a c t i o n . A n o t h e r study (22) o f genotype-environment i n t e r a c t i o n f o r sweet p o t a t o e s grown in the s o u t h e r n h i g h l a n d s p r o v i n c e o f Papua, New Guinea, r e i n f o r c e d t h e f i n d i n g o f C o l l i n s and W a l t e r (21) w i t h r e g a r d t o the v a r i a b i l i t y in crude p r o t e i n c o n t e n t . The d a t a o f Bradbury e t a l . (_22) f o r 10 c u l t i v a r s from 5 environments showed a mean crude p r o t e i n c o n t e n t o f 1.51% ( f r e s h weight) w i t h a s t a n d a r d d e v i a t i o n o f O.54%, a c o e f f i c i e n t o f v a r i a b i l i t y o f 35.8% (Table I ) . The g r a d i e n t r e f e r r e d t o in T a b l e I was o b t a i n e d by p l o t t i n g the mean crude p r o t e i n c o n t e n t f o r the 5 environments (bottom row, T a b l e I) a g a i n s t the crude p r o t e i n c o n t e n t f o r each c u l t i v a r in each environment. The g r a d i e n t o r s l o p e of the r e s u l t i n g l i n e p r o v i d e d a measure o f the response o f a g i v e n c u l t i v a r t o v a r y i n g environments. The g r e a t e r t h e g r a d i e n t o r s l o p e , the more the c u l t i v a r is a f f e c t e d by environment. From T a b l e I, i t is apparent t h a t t h e c u l t i v a r 'Simbul Sowar' is l e a s t r e s p o n s i v e t o environment and s t i l l is h i g h in crude p r o t e i n c o n t e n t . On the o t h e r hand, c u l t i v a r 'Takion' has t h e h i g h e s t mean crude p r o t e i n c o n t e n t but much more e n v i r o n m e n t a l i n s t a b i l i t y . T h i s type a n a l y s i s is a v a l u a b l e t o o l f o r improvement o f the crude p r o t e i n c o n t e n t t h r o u g h c u l t i v a r s e l e c t i o n . C u l t u r a l p r a c t i c e s a l s o can a f f e c t sweet p o t a t o p r o t e i n c o n t e n t . P u r c e l l e t a l . (23^) r e p o r t e d t h a t i n c r e a s i n g amounts o f n i t r o g e n f e r t i l i z a t i o n up t o 112 kg/ha caused an i n c r e a s e in p r o t e i n c o n t e n t but no change in the NPN. N e i t h e r s u l f u r nor potassium i n f l u e n c e d the p r o t e i n content. S i m i l a r l y , C o n s t a n t i n e t a l . (24) found t h a t n i t r o g e n f e r t i l i z a t i o n up t o 67.3 kg/ha l i n e a r l y i n c r e a s e d crude p r o t e i n c o n t e n t . Kimber (25>) r e p o r t e d t h a t when a v a i l a b l e n i t r o g e n no l o n g e r a f f e c t s y i e l d s , p r o t e i n c o n t e n t o f t h e r o o t s c o n t i n u e s t o i n c r e a s e . Other workers have demonstrated t h a t crude sweet p o t a t o p r o t e i n c o n t e n t can be i n c r e a s e d t h r o u g h c u l t u r a l management p r a c t i c e s (_26, 27^) . L e n g t h o f the growing season a l s o has an e f f e c t on crude p r o t e i n c o n t e n t . P u r c e l l e t a l . (28) found t h a t t h e p r o t e i n c o n t e n t d e c r e a s e d O.0067% p e r day between 102 and 165 days. C o n c o m i t a n t l y , d r y matter d e c r e a s e d l i n e a r l y a t O.233% p e r day. In a d d i t i o n t o n i t r o g e n f e r t i l i z a t i o n r a t e and l e n g t h o f growing season, h i g h r a t e s of i r r i g a t i o n caused d e c r e a s e s in b o t h d r y m a t t e r and p r o t e i n c o n t e n t (29). The r e s u l t s r e p o r t e d by D i c k e y e t a l . (7) and Bradbury e t a l . (22) r e i n f o r c e the concept t h a t p r o t e i n c o n t e n t is not a r e c i p r o c a l f u n c t i o n o f d r y m a t t e r c o n t e n t . I t appears then t h a t n a t u r a l g e n o t y p i c v a r i a b i l i t y in crude p r o t e i n c o n t e n t p r o v i d e s a p r o m i s i n g avenue t o improve p r o t e i n l e v e l s



1..87 2,.06 2..37 3..00 1..12 1..44 1..81 1..94 1..62 1..37 1..12 1..56 O. .94 .87 O. 1,.69 1,.50 2..37 2,.06 1,.44 1,.69 1 .31 1 .60

1,.48

2..22

1..60

O. .91

1..34

1..50

1..88

1..28

2..69

1..97

b

1..69

1,.94

1,.35

1..63

2..32

2 .10

1..00

1..97

1..56

O..81

2..00

1..91

2 .81

1..06

O..75

1..38

1..31

1,.19

1 .20

1..19

1..50

1..06

O..81

O..88

1..69

1..13

1..25

1..06

1..56

1,.94

1..31

O..94

1..31

O. .88

1..38

1..97

1..00

2..06

1..19

c

From B r a d b u r y

1,.51 0,.45

1..33

1..12

.64 1..71 O. 1,.76 0,.60

O..51

1..10

1..00

O..94

O..31

O..93

1..14

1..61

Gradient (See T e x t )

.23 1..61 O.

1..37 O..60

1..29 O..45

1..42 O..56

1..35 O..46

1..33 O..41

1..81 O..68

1..41 O..74

Upper Mendi, Upper Mendi, 1983 Season^ 1983 Season Gypsum Added Mean SD

Conditions

2..06

O. .88

O..50

Upper Mendi^ 1981 Season

Growth

2..31 2..37 1..87 2..25 2..56 1..37 1..69 1..06 1..94 1..87 2..12 1..81 2..19 2,.44 1,.87 2,.00 2 .31

2..34

Erave, 1982 Season

Roots from two d i f f e r e n t p l a n t s and mean. Two (or three)j r o o t s from same p l a n t and mean. From B r a d b u r y e t a l . ( 2 2 ) . e t a l . ( 1 3 ) . Gypsum added t o s o i l a t a r a t e o f 500 kg/ha.

Mean

Tomun (TO)

Wanmun (WA)

Simbul Sowar (SI)

K a r i k o (KO)

P u l u p u r i (PU)

K a r i a p (KA)

S a p e l (SA)

S o u (SO)

T a k i o n (TA)

Hopomehene (HO)

Name o f C u l t i v a r

Kiburu, 1982 a Season

T a b l e I . Crude P r o t e i n C o n t e n t (% F r e s h Sweet P o t a t o ) o f Ten C u l t i v a r s From Upper Mendi Grown in D i f f e r e n t Environments


C/3

m ζ

Ο Η

70

"0

Η

r > ζ

oo

19.

WALTER AND PURCELL


239

v i a s e l e c t i o n . S e l e c t i o n f o r h i g h p r o t e i n c u l t i v a r s which are r e l a t i v e l y i n s e n s i t i v e t o e n v i r o n m e n t a l d i f f e r e n c e s and o p t i m i z a t i o n of c u l t u r a l p r a c t i c e s a r e a l s o a t t r a c t i v e r e s e a r c h areas f o r i n c r e a s i n g p r o t e i n content.


Nutritional

Value

Feeding Studies. A l t h o u g h sweet p o t a t o e s a r e a s i g n i f i c a n t s o u r c e of c a l o r i e s in many p a r t s o f the w o r l d , v e r y l i t t l e i n f o r m a t i o n is a v a i l a b l e c o n c e r n i n g the n u t r i t i o n a l q u a l i t y o f sweet p o t a t o p r o t e i n as d e t e r m i n e d by c o n t r o l l e d f e e d i n g s t u d i e s in humans. T h i s is in s t r i k i n g c o n t r a s t t o numerous r e p o r t e d s t u d i e s on the f e e d i n g o f w h i t e p o t a t o e s t o humans (30). An e a r l y s t u d y in which the sweet p o t a t o was used as the s o l e s o u r c e of n i t r o g e n in the d i e t o f humans was t h a t o f A d o l p h and L i u (31). They r e p o r t e d t h a t n i t r o g e n b a l a n c e c o u l d be m a i n t a i n e d w i t h sweet p o t a t o n i t r o g e n p r o v i d e d s u f f i c i e n t amounts were consumed. R e s e a r c h by o t h e r workers (32, 33) a l s o suggested t h e sweet p o t a t o p r o t e i n is r e a d i l y u t i l i z e d by humans. Large amounts o f sweet p o t a t o must be e a t e n t o p r o v i d e enough n i t r o g e n . Oomen (34) r e p o r t e d t h a t in New Guinea, where 80-90% o f t h e t o t a l c a l o r i e s were o b t a i n e d from sweet p o t a t o , the s u b j e c t s s t u d i e d were u s u a l l y in s i g n i f i c a n t n e g a t i v e n i t r o g e n b a l a n c e . S i n c e n e g a t i v e n i t r o g e n s t a t u s means c o n t i n u o u s breakdown o f body p r o t e i n l e a d i n g t o s e r i o u s m a l n u t r i t i o n , Oomen (34) was p u z z l e d because the s u b j e c t s seemed t o be in good h e a l t h . As a r e s u l t , he s u g g e s t e d t h a t e a t i n g l a r g e amounts o f sweet p o t a t o might i n d u c e an i n t e s t i n a l m i c r o f l o r a which was a b l e t o f i x gaseous n i t r o g e n so t h a t i t c o u l d be u t i l i z e d t o s y n t h e s i z e amino a c i d s . O b v i o u s l y , i f such were the case, much of the knowledge o f p r o t e i n n u t r i t i o n would be in doubt s i n c e the v a l i d i t y o f n i t r o g e n b a l a n c e s t u d i e s upon which most o f t h i s knowledge is based would be in doubt. A l a t e r study (_35) u s i n g c a r e f u l l y c o n t r o l l e d c o n d i t i o n s i n d i c a t e d t h a t b o t h a d o l e s c e n t and young a d u l t males m a i n t a i n e d in s l i g h t l y n e g a t i v e n i t r o g e n b a l a n c e t h r o u g h use o f sweet p o t a t o as the major n i t r o g e n s o u r c e d e v e l o p e d c l i n i c a l symptoms o f m i l d p r o t e i n m a l n u t r i t i o n . These i n c l u d e d abnormal plasma f r e e amino a c i d p a t t e r n s and a d e c r e a s e in p h y s i c a l s t a m i n a . In a d d i t i o n , no e v i d e n c e o f in v i v o n i t r o g e n f i x a t i o n c o u l d be d e t e c t e d in f e c a l m a t e r i a l , i n d i c a t i n g t h a t the m i c r o f l o r a i n d u c e d by l o n g - t e r m consumption o f sweet p o t a t o e s a r e not c a p a b l e of f i x i n g n i t r o g e n . The r e p o r t t h a t h a b i t u a l sweet p o t a t o e a t e r s are somewhat independent o f d i e t a r y n i t r o g e n appears t o have no b a s i s in f a c t . R e s u l t s r e p o r t e d by Huang e t a l . (35) i n d i c a t e d t h a t w i t h t e e n a g e r s a p o s i t i v e n i t r o g e n b a l a n c e c o u l d be m a i n t a i n e d w i t h an i n t a k e of O.67 t o O.71 g p r o t e i n / k g body weight, where the sweet p o t a t o f u r n i s h e d most o f the p r o t e i n . The energy r e q u i r e m e n t f o r t h i s l e v e l of p r o t e i n consumption was 54 k c a l / k g body weight. The a p p a r e n t p r o t e i n d i g e s t i b i l i t y was found t o be 66%, which was v e r y c l o s e t o a p r e v i o u s l y r e p o r t e d v a l u e of 67% (36). The above r e p o r t s , a l t h o u g h l i m i t e d in number, i n d i c a t e t h a t sweet p o t a t o p r o t e i n is o f good n u t r i t i o n a l q u a l i t y but the q u a n t i t y is low in the c u l t i v a r s used. The c u l t i v a r T a i n o n 57 used by


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240

Huang e t a l . (35) had a crude p r o t e i n c o n t e n t o f from O.8 t o 1.3% ( f r e s h w e i g h t ) . A r e p o r t by B r e s s a n i e t a l . (3_7) , which e v a l u a t e d t h e n u t r i t i o n a l v a l u e o f d i e t s based on s t a r c h y foods and beans, i n d i c a t e d t h a t f o r t h e r a t , sweet p o t a t o p r o t e i n was o f poor n u t r i t i o n a l q u a l i t y . When m e t h i o n i n e was added t o a l l d i e t s t o r a i s e s u l f u r amino a c i d s , sweet p o t a t o s t i l l r e q u i r e d t h e l a r g e s t amount o f s u p p l e m e n t a t i o n w i t h bean f l o u r t o m a i n t a i n animal weight (Table I I ) . Sweet p o t a t o f l o u r c o n t a i n e d 3.8% p r o t e i n , t h e second h i g h e s t amount o f p r o t e i n among s t a r c h y f o o d s , and y e t t h e p r o t e i n appeared t o be t h e p o o r e s t in n u t r i t i o n a l q u a l i t y . However, i t s h o u l d be noted t h a t t h e sweet p o t a t o e s used in t h i s study were d r i e d a t 60 C b u t were n o t cooked. Uncooked sweet p o t a t o s t a r c h is not c o m p l e t e l y d i g e s t a b l e by r o d e n t s . As a consequence, maintenance r e q u i r e m e n t s would i n c r e a s e . T h i s is t h e most l i k e l y e x p l a n a t i o n f o r t h e i n c r e a s e d requirement f o r bean f l o u r , b u t t h e r e a l s o may have been i n t e r f e r e n c e w i t h d i g e s t i o n from p r o t e a s e i n h i b i t o r s p r e s e n t in uncooked sweet p o t a t o e s . W a l t e r e t a l . (38) measured t h e p r o t e i n e f f i c i e n c y r a t i o (PER) o f f l o u r p r e p a r e d from sweet p o t a t o e s which were cooked in a d r y i n g oven. Because t h e PER is determined on t h e b a s i s o f a d i e t c o n t a i n i n g 10% p r o t e i n , t h e 'Jewel' and ' C e n t e n n i a l ' sweet p o t a t o e s used in t h i s s t u d y were s t o r e d u n t i l s u f f i c i e n t s t a r c h had m e t a b o l i z e d t o i n c r e a s e crude p r o t e i n c o n t e n t t o 11.25% (dry b a s i s ) . When t h e f l o u r was f e d t o Sprague-Dawley s t r a i n r a t s , t h e c o r r e c t e d PER v a l u e s were 2.22 and 2.00 f o r ' C e n t e n n i a l ' and 'Jewel' c u l t i v a r s , r e s p e c t i v e l y , compared t o 2.50 f o r c a s e i n . ' C e n t e n n i a l ' had t h e h i g h e s t PER v a l u e o f t h e two c u l t i v a r s because i t s NPN c o n t e n t was lower. The n e t e f f e c t o f i n c r e a s e d NPN c o n t e n t is t o lower t h e amount o f e s s e n t i a l amino a c i d s as a p e r c e n t a g e o f t h e t o t a l n i t r o g e n and thus d e c r e a s e t h e PER value. A n t i - n u t r i t i o n a l Factors I t has been r e c o g n i z e d s i n c e 1954 (39) t h a t sweet p o t a t o c o n t a i n s t r y p s i n i n h i b i t o r s . T r y p s i n i n h i b i t o r s (TI) have an a n t i n u t r i t i o n a l e f f e c t by i n h i b i t i n g p r o t e o l y t i c a c t i o n o f t r y p s i n d u r i n g d i g e s t i o n . S i n c e t h e i n i t i a l r e p o r t , T I a c t i v i t y in sweet p o t a t o e s has been t h e s u b j e c t o f s e v e r a l r e p o r t s . D i c k e y and C o l l i n s (40) r e p o r t e d t h e p r e s e n c e o f 7 T I bands in t h e 4 c u l t i v a r s examined, t h e i n t e n s i t y o f t h e bands b e i n g c u l t i v a r dependent. Heat i n a c t i v a t i o n o f T I a l s o was c u l t i v a r dependent, but h e a t i n g t h e t i s s u e t o 94 C., f o l l o w e d by c o o l i n g t o room temperature d e s t r o y e d 93-97% o f t h e a c t i v i t y in a l l c u l t i v a r s . C o n s e q u e n t l y , c o o k i n g o f sweet p o t a t o e s s h o u l d e l i m i n a t e most of the a n t i - n u t r i t i o n a l e f f e c t . E n t e r i t i s n e c r o t i a n s (EN), a spontaneous form o f e n t e r i c gangrene endemic t o t h e h i g h l a n d s o f Papua, New Guinea, is caused by t o x i n s produced when C l o s t r i d i u m p e r f r i n g e n s o f t h e g u t e n t e r a r a p i d growth phase (41). I t has been p o s t u l a t e d t h a t t h e d i s e a s e o c c u r s in p o p u l a t i o n s which consume a low p r o t e i n d i e t , e.g., sweet p o t a t o as t h e s t a p l e f o o d combined w i t h T I a c t i v i t y which


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T a b l e I I . E f f e c t o f S u p p l e m e n t a t i o n o f Starchy^ Foods With Common Beans on Weight Maintenance

Flours Cassava Plantain Potato Sweet P o t a t o Bean

% Crude Protein 1..4 3,.1 9..5 3..8 22..8

% Bean F l o u r for Nitrogen

Required Balance

14,.5 20..1 14..6 29..3 10..1°

^From B r e s s a n i e t a l . (37) . W i s t a r r a t s were t e s t a n i m a l . D Supplemented w i t h m e t h i o n i n e . C o r n s t a r c h used as s t a r c h y f o o d w i t h bean f l o u r .


242

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e f f e c t i v e l y reduces the p r o t e o l y t i c c a p a c i t y of the d i g e s t i v e system t o such a degree t h a t i t cannot d e s t r o y t h e p r o t e i n a c e o u s t o x i n by h y d r o l y s i s . A r e p o r t by Bradbury e t a l . (13) i n d i c a t e d t h a t t h e r e was no c o r r e l a t i o n between t h e i n c i d e n c e o f EN in a g i v e n r e g i o n and t h e amount o f T I a c t i v i t y in t h e sweet p o t a t o c u l t i v a r s consumed in t h a t r e g i o n . U n l e s s t h e p o p u l a t i o n s i n v o l v e d consume l a r g e amounts o f raw sweet p o t a t o e s , i t is h i g h l y u n l i k e l y t h a t t h e T I is o b t a i n e d from t h i s s o u r c e s i n c e c o o k i n g has been shown t o i n a c t i v a t e t h e i n h i b i t o r (40, 4 2 ) .


Amino A c i d

Composition

In r e c e n t y e a r s , a number o f workers have p u b l i s h e d amino a c i d a n a l y s e s o f t h e sweet p o t a t o (38, 43, 13, 22, 1 8 ) . The o v e r a l l p i c t u r e is t h a t t h e sweet p o t a t o amino a c i d p a t t e r n is o f good n u t r i t i o n a l q u a l i t y but t h a t t h e v a r i a b i l i t y o f i n d i v i d u a l amino a c i d s b o t h w i t h i n t h e same c u l t i v a r and a c r o s s c u l t i v a r s is v e r y h i g h . F o r example, W a l t e r e t a l . (44) r e p o r t e d t h a t w i t h t h e e x c e p t i o n o f a r o m a t i c amino a c i d s , every e s s e n t i a l amino a c i d has a s c o r e o f l e s s than 100 in one o r more c u l t i v a r s . The amino a c i d s c o r e is d e f i n e d as t h e g o f amino a c i d in 100 g o f t e s t p r o t e i n d i v i d e d by t h e number o f g o f t h a t amino a c i d in the FAO/WHO r e f e r e n c e p a t t e r n t i m e s 100. Bradbury e t a l . (22) showed t h a t , f o r t h e same c u l t i v a r , e n v i r o n m e n t a l e f f e c t s on t h e amino a c i d p a t t e r n s is s i g n i f i c a n t . F o r t h r e e c u l t i v a r s , they found a mean p e r c e n t s t a n d a r d d e v i a t i o n f o r a l l amino a c i d s o f 24.2, 23.4 and 20.6 o v e r 5 environments. From t h e i r r e s u l t s , Bradbury (22) c o n c l u d e d t h a t in t h e h i g h l a n d s o f Papua, New G u i n e a , t h e EAA most l i k e l y t o be l i m i t i n g in d e c r e a s i n g o r d e r o f p r o b a b i l i t y were l y s i n e , l e u c i n e and s u l f u r amino a c i d s . These workers s u g g e s t e d t h a t a p a r t o f t h e l a r g e d i f f e r e n c e r e p o r t e d worldwide in the r e l a t i v e amount o f s u l f u r amino a c i d s may be due in p a r t t o d i f f i c u l t i e s in t h e a n a l y s i s o f t h e s e compounds. C o n c e n t r a t e s and I s o l a t e s The l i t e r a t u r e on c o n c e n t r a t e d sweet p o t a t o p r o t e i n is s p a r s e . Amino a c i d p a t t e r n s f o r sweet p o t a t o p r o t e i n i s o l a t e s have been r e p o r t e d by t h r e e groups (16, 45, 4 6 ) . One r e p o r t showed t h a t when compared t o t h e FAO s t a n d a r d (47), no amino a c i d s were l i m i t i n g . The o t h e r r e p o r t s showed t o t a l s u l f u r amino a c i d s and l y s i n e t o be l i m i t i n g (Table I I I ) . The p a t t e r n s i n d i c a t e a n u t r i t i o n a l l y w e l l b a l a n c e d p r o t e i n . The improvement in n u t r i t i o n a l q u a l i t y , when compared t o amino a c i d p a t t e r n s from whole sweet p o t a t o , is due t o t h e f a c t t h a t whole sweet p o t a t o e s c o n t a i n s u b s t a n t i a l amounts o f NPN, which c o n s i s t s m a i n l y o f n o n e s s e n t i a l amino a c i d s . T h i s e f f e c t i v e l y d i l u t e s t h e EAA and lowers t h e amino a c i d s c o r e . F e e d i n g s t u d i e s w i t h t h e r a t as the t e s t a n i m a l v e r i f i e d the h i g h n u t r i t i o n a l q u a l i t y i n d i c a t e d by t h e amino a c i d p a t t e r n (45). U s i n g i s o l a t e s and c o n c e n t r a t e s p r e p a r e d from 'Jewel' and ' C e n t e n n i a l ' c u l t i v a r s , PER v a l u e s were e q u a l t o t h a t o f c a s e i n (milk p r o t e i n ) (Table I V ) . E x a m i n a t i o n o f t h e amino a c i d p a t t e r n s o f sweet p o t a t o p r o t e i n and c a s e i n r e v e a l e d t h a t b o t h c o n t a i n e d


19.

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T a b l e I I I . Amino A c i d C o m p o s i t i o n o f P r o t e i n I s o l a t e s A c i d P e r 100 g o f P r o t e i n )

(g o f Amino

W a l t e r and Purcell Nagase FAO/WHO C a t i g n a n i (45) e t a l . ( 1 6 ) (46) (47) 3


Essential Threonine Valine Methionine Total Sulfur Isoleucine Leucine Tyrosine Phenylalanine Lysine Tryptophan ^ ^ Amino A c i d S c o r e ' Total Sulfur Lysine

6.,4 7.,9 2..0 3..1 5..6 7..4 6..9 8..2 5..2 1..2° 88 95

5.,5 6..8 2..6 3..0 5..3 7..8 5..2 6..7 6..8 1.. i c c

4.6 7.9 2.5 4.1 5.3 8.7 3.6 6.0 6.5 1.8 c

4..0 5..0 3..5 4..0 7..0

5,.5 1,.0

100 100

86 100

Nonessential Aspartic Acid Serine Glutamic A c i d Proline Glycine Alanine Histidine N H

3 Arginine

18,.9 6,.6 9,.6 4,.2 5,.3 5,.4 2,.7 1..6 5,.9

14,.4 5,.1 8,.6 5,.4 4,.3 4,.6 2 •f

13.1 5.5 11.8 4.3 2.6 6.1

6 .0

6.4

-L

"'Jewel' c u l t i v a r . C u l t i v a r unknown. T r y p t o p h a n c o n t e n t measured c o l o r i m e t r i c a l l y on enzyme-hydrolyzed ^material. g o f amino a c i d in 100 g o f t e s t p r o t e i n / g o f amino a c i d in FAO/WHO r e f e r e n c e p a t t e r n χ 100. ^ A l l o t h e r e s s e n t i a l amino a c i d s exceeded FAO/WHO v a l u e s . NH not reported. From W a l t e r e t a l . (44).



Fractions

2.78 + 2.73 + 2.78 +

2.81 + 2.91 + 2.96 +

PER

O.10 O.09 O.10

O.11 O.10 O.07

Wt. Gained, g

2.50 + O.09 109.5 + 7.8 2.47 + O.09 117.6 + 11.3 2.50 + O.10 122.2 + 14.9

394.0 + 25.3 431.1 + 39.5 437.9 + 44.5

477.9 + 37.7 477.1 + 29.0 472.6 + 35.3

Food Consumed, g

b

71.6 + 2.9 71.1 + 2.7 71.3 + 2.7

78.3 + 3.1 78.3 + 3.3 78.4 + 3.2

Initial Group wt., g

f o r P r o t e i n F r a c t i o n s From Sweet P o t a t o e s

2.50 + O.09 134.3 + 11.7 2.64 + O.09 138.9 + 11.7 2.63 + O.07 140.3 + 12.4

Corrected PER

(PER)

Mean and s t a n d a r d d e v i a t i o n c a l c u l a t e d from d a t a from 10 r a t s p e r d i e t group. C o r r e c t e d by a d j u s t i n g t e s t d i e t s t o 2.50 f o r c a s e i n (AOAC). From W a l t e r and C a t i g n a n i ( 4 5 ) .

Casein 'Jewel' 'Centennial'

Chromoplast

Casein 'Jewel' 'Centennial'

White

Protein

T a b l e IV. P r o t e i n E f f i c i e n c y R a t i o


m in

73 Ο Η

-α

> Ζ Η

19. WALTER AND PURCELL


245


l e s s s u l f u r amino a c i d s than r e q u i r e d f o r r a t growth. In a d d i t i o n , sweet p o t a t o c o n t a i n e d l e s s l y s i n e , w h i l e c a s e i n c o n t a i n e d l e s s t h r e o n i n e t h a n is r e q u i r e d f o r r a t growth. A p p a r e n t l y t h e o v e r a l l d e f i c i e n c i e s l i m i t e d r a t growth about the same amount. The end r e s u l t was t h a t r a t s f e d e i t h e r p r o t e i n grew a t about the same rate. Horigome e t a l . (15) r e p o r t e d a PER o f 1.9 f o r p r o t e i n r e c o v e r e d from an i n d u s t r i a l sweet p o t a t o s t a r c h f a c i l i t y . They were a b l e t o i n c r e a s e t h e PER t o 2.5 by supplementing the d i e t s w i t h l y s i n e and m e t h i o n i n e . A p o r t i o n o f t h e s e amino a c i d s were e i t h e r d e s t r o y e d o r made b i o l o g i c a l l y n o n a v a i l a b l e by t h e p r o c e s s i n g o p e r a t i o n . The p o s s i b i l i t y a l s o e x i s t s t h a t t h e s e amino a c i d s were l i m i t i n g in t h e c u l t i v a r s s t u d i e d . E f f e c t o f P r o c e s s i n g on N u t r i t i o n a l

Quality

Heat p r o c e s s i n g o f sweet p o t a t o e s can have d e l e t e r i o u s e f f e c t s on p r o t e i n n u t r i t i o n a l q u a l i t y . P u r c e l l and W a l t e r (48) found t h a t the i n t e n s i t y o f t h e h e a t p r o c e s s i n g c o n d i t i o n s had a d i r e c t b e a r i n g on n u t r i t i o n a l q u a l i t y o f the p r o t e i n . In t h i s study l y s i n e was d e s t r o y e d , presumably v i a i r r e v e r s i b l e r e a c t i o n w i t h r e d u c i n g s u g a r s (40). Both s u c r o s e syrup-canned sweet p o t a t o e s and drum-dried sweet p o t a t o f l a k e s c o n t a i n e d 26% l e s s l y s i n e than d i d baked sweet p o t a t o e s . In a d d i t i o n , syrup-canned sweet p o t a t o e s c o n t a i n e d 25% l e s s t o t a l n i t r o g e n than d i d e i t h e r baked o r drum-dried sweet p o t a t o e s . T h i s l o s s o f n i t r o g e n was a p p a r e n t l y due t o s o l u t i o n o f the NPN f r a c t i o n in t h e s y r u p . Other r e p o r t s on canned sweet p o t a t o e s r e v e a l s i m i l a r changes. Canned sweet p o t a t o e s from v a r i o u s l o c a t i o n s were found t o c o n t a i n 3.8 t o 4.2% (dry b a s i s ) crude p r o t e i n (_50) , r a t h e r t h a n the e x p e c t e d 4.5-7.0%. A l t h o u g h no mention was made of the l o w e r - t h a n - e x p e c t e d crude p r o t e i n v a l u e s , t h e s e were p r o b a b l y due t o d i s s o l u t i o n o f p a r t o f t h e NPN f r a c t i o n in the s y r u p . S i m i l a r l y , M e r e d i t h and D u l l (43) r e p o r t e d t h a t c a n n e d - i n - s y r u p sweet p o t a t o e s c o n t a i n e d c a . 45% l e s s amino a c i d s t h a n d i d t h e r o o t s b e f o r e p r o c e s s i n g . S i n c e s y r u p is d i s c a r d e d b e f o r e t h e canned r o o t s a r e e a t e n , t h i s r e s u l t s in a s e r i o u s l o s s o f n i t r o g e n . The s e v e r i t y o f h e a t t r e a t m e n t d u r i n g d e h y d r a t i o n has a s i g n i f i c a n t e f f e c t on p r o t e i n n u t r i t i o n a l q u a l i t y . Cooked sweet p o t a t o e s d e h y d r a t e d in a f o r c e d - d r a f t oven a t 60 C had a PER o f 2.2, w h i l e a second l o t o f cooked sweet p o t a t o e s d e h y d r a t e d on a steam-heated drum d r y e r had a PER o f 1.3 (38). The l y s i n e c o n t e n t measured by a c i d h y d r o l y s i s - i o n exchange chromatography was somewhat lower in t h e drum d e h y d r a t e d f l o u r but not s u f f i c i e n t l y low t o a c c o u n t f o r the d i f f e r e n c e in PER v a l u e s . F u r t h e r s t u d y u s i n g an a s s a y f o r a v a i l a b l e l y s i n e (SI) showed t h a t a l a r g e p a r t o f t h e l y s i n e was n o t a v a i l a b l e . Thus, a c i d h y d r o l y s i s can l i b e r a t e b i o l o g i c a l l y n o n a v a i l a b l e l y s i n e which is s u b s e q u e n t l y q u a n t i f i e d a l o n g w i t h a v a i l a b l e l y s i n e , c a u s i n g an o v e r e s t i m a t i o n o f t h e n u t r i t i o n a l q u a l i t y o f t h e f o o d . T h i s is most l i k e l y t o happen when h i g h l e v e l s o f r e d u c i n g sugars a r e p r e s e n t in t h e f o o d and l y s i n e is l i m i t i n g , as is the case w i t h sweet p o t a t o e s .


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Summary and

Conclusions

The sweet p o t a t o ranks s i x t h in average p r o d u c t i o n among the major f o o d c r o p s o f the w o r l d . There is s i g n i f i c a n t p o t e n t i a l f o r i n c r e a s i n g t h e p r o t e i n c o n t e n t o f t h i s c r o p by a c o m b i n a t i o n o f b r e e d i n g / s e l e c t i o n and o p t i m i z a t i o n o f p r o d u c t i o n p r a c t i c e s . A c c o r d i n g t o p r e s e n t knowledge, most o f t h e n i t r o g e n of the sweet p o t a t o is in a form s u i t a b l e t o s a t i s f y human n i t r o g e n r e q u i r e m e n t s . The p r o t e i n component comprises from 60-85% o f the n i t r o g e n w i t h the remainder c o n s i s t i n g o f amino o r amide n i t r o g e n . The amino a c i d p a t t e r n o f the sweet p o t a t o is h i g h l y v a r i a b l e . I s o l a t e d sweet p o t a t o p r o t e i n is o f s u f f i c i e n t n u t r i t i o n a l q u a l i t y t o s u p p o r t growth o f l a b o r a t o r y r a t s t o the same e x t e n t as c a s e i n . Humans have been m a i n t a i n e d in n i t r o g e n b a l a n c e u s i n g sweet p o t a t o as t h e major s o u r c e o f p r o t e i n . P r o c e s s i n g o f sweet p o t a t o e s can have adverse e f f e c t s on the p r o t e i n n u t r i t i o n a l v a l u e . Canning sweet p o t a t o e s in a l i q u i d medium causes l e a c h i n g o f s o l u b l e n i t r o g e n o u s compounds i n t o t h e l i q u i d , t h e r e b y l o w e r i n g t h e n i t r o g e n c o n t e n t . Heat p r o c e s s i n g o f the sweet p o t a t o causes a d e c r e a s e in the b i o l o g i c a l a v a i l a b i l i t y o f l y s i n e . The e x t e n t o f the d e c r e a s e in l y s i n e a v a i l a b i l i t y is dependent upon the s e v e r i t y o f the heat t r e a t m e n t and t h e amount o f r e d u c i n g s u g a r s p r e s e n t d u r i n g h e a t i n g . Acknowledgments Paper no. 10141 o f t h e J o u r n a l S e r i e s o f the N o r t h C a r o l i n a A g r i c u l t u r a l Research S e r v i c e , R a l e i g h , NC 27695-7601. M e n t i o n o f a trademark o r p r o p r i e t a r y p r o d u c t does not c o n s t i t u t e a g u a r a n t e e o r warranty o f the p r o d u c t by the U. S. Department o f A g r i c u l t u r e o r N o r t h C a r o l i n a A g r i c u l t u r a l Research S e r v i c e , nor does i t imply a p p r o v a l t o the e x c l u s i o n o f o t h e r p r o d u c t s t h a t may be s u i t a b l e .

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Plant Proteins - American Chemical Society

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