EFFECT OF SORGHUM TYPE AND PROCESSING ON THE ANTIOXIDANT PROPERTIES OF ...

144 Pages·2007·2.95 MB·English

EFFECT OF SORGHUM TYPE AND PROCESSING ON THE ANTIOXIDANT
PROPERTIES OF SORGHUM [Sorghum bicolor (L.) Moench] BASED FOODS

A Dissertation
by
NOMUSA RHODA NGWENYA

Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY

May 2007

Major Subject: Food Science and Technology
EFFECT OF SORGHUM TYPE AND PROCESSING ON THE ANTIOXIDANT
PROPERTIES OF SORGHUM [Sorghum bicolor (L.) Moench] BASED FOODS

A Dissertation
by
NOMUSA RHODA NGWENYA

Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY

Approved by:
Chair of Committee, Lloyd W. Rooney
Committee Members, John R.N. Taylor
Ralph D. Waniska
Luis Cisneros-Zevallos
Chair of Food Science
and Technology Faculty, Rhonda Miller

May 2007
Major Subject: Food Science and Technology
iii
ABSTRACT

Effect of Sorghum Type and Processing on the Antioxidant Properties of Sorghum
[sorghum bicolor (L. Moench)] Based Foods. (May 2007)
Nomusa Rhoda Ngwenya, BSc., University of Zimbabwe;
MSc., University of Reading
Chair of Advisory Committee: Dr Lloyd W. Rooney

Antioxidant properties of sorghum are related to sorghum type and method of
processing into foods. Tannin and non-tannin sorghums and their products were
evaluated for total phenols, tannins and antioxidant activity. Total phenols were
determined using the Folin Ciocalteu method, and tannins were determined by the
vanillin-HCl method. Antioxidant activity was evaluated using the ABTS (2,2¢-
azinobis(3-ethyl-benzothiazoline-6-sulphonic acid) and DPPH (2,2¢-diphenyl-I-
picrylhydrazyl) assays. Tannin sorghums and their products had higher total phenols,
tannins and antioxidant activity than non-tannin sorghum grain and products.
Fermentation, extrusion cooking and porridge making reduced measurable phenols,
tannins and in vitro antioxidant activity. Reduction was probably due to phenols
binding to the food components, thus reducing their solubility in the extracting
solvents; 1% HCl in methanol and 70% aqueous acetone.
The procyanidin profile obtained using normal phase HPLC and fluorescent
detection showed that extrusion cooking and porridge making lowered extractability of
polymers (DP>8), while that of oligomers (DP 2-8) and monomers in porridges was
not significantly changed. This indicated increased interactions of procyanidin
polymers with the food matrix, especially with protein. Pepsin treatment of sorghum
extrudates and porridges significantly improved the antioxidant activity and recovery.
The highest antioxidant activity was in the supernatants of pepsin hydrolysates.
Amylase treatment alone did not significantly affect phenol content and antioxidants,
iv
except in bread containing non-tannin white sorghum bran, where there was a slight
increase in phenols. The combination of pepsin followed by amylase treatment of
porridges and extrudates had effects similar to those of pepsin alone.
Improved extractability of antioxidants on pepsin treatment was due to either
the release of phenolic antioxidants or protein hydrolysates high in aromatic amino
acid residues such as tyrosine, also known for their antioxidant activity. In either
situation the improved antioxidant activity could mean that once food is digested it can
potentially protect the gastrointestinal tract against oxidative stress generated from the
diet and that produced by food interactions during digestion.
There is scope to explore other biological methods like use of other proteases to
improve antioxidant recovery. Further work would thus determine contribution of
phenolic compounds to improved antioxidant activity, and also identify the phenolics.
v

DEDICATION

To my children Imani and Mandisa
vi

ACKNOWLEDGEMENTS

I would like to thank my supervisors Dr Lloyd W. Rooney, Dr John R.N.
Taylor and Dr Ralph D. Waniska for making all this work possible and giving me the
opportunity to work with them. I feel honored. I am also indebted to Dr Luis Cisneros-
Zevallos for being on my committee and for providing his valued suggestions.
I would like to extend my gratitude to Fulbright for their financial support while
I was at Texas A&M University, and to The Third World Organization of Women in
Sciences (TWOWS) for financial support while I was at the University of Pretoria,
South Africa.
Many thanks should go to those who assisted me along the way at the
University of Pretoria. I would like to thank Janet Taylor, while in the Cereal Quality
Lab. I extend my thanks to Linda Dykes for assisting me with HPLC analysis, and
general suggestions and comments. Many thanks also go to Cassandra McDonough for
assisting me with materials I needed for the research, the photos and advice. I also
thank Pamela Littlejohn, for the communication channels, and the rest of Cereal
Quality staff and students for being there when I needed them. Thank you for the team
spirit and support.
I am also grateful to my family. I thank my husband, Tula, and my children,
Imani and Mandisa for their support and patience. I thank my mother, Alice Ngwenya,
my sister, Siziwe and my brothers, Themba and Dumisani, for being pillars of strength
and believing in me. I would not be here without your support. Thank you.
vii
TABLE OF CONTENTS

Page

ABSTRACT...................................................................................................................iii(cid:1)
DEDICATION................................................................................................................v(cid:1)
ACKNOWLEDGEMENTS...........................................................................................vi(cid:1)
LIST OF TABLES..........................................................................................................x(cid:1)
LIST OF FIGURES......................................................................................................xii(cid:1)
CHAPTER(cid:1)
I INTRODUCTION...................................................................................................1(cid:1)
II LITERATURE REVIEW.......................................................................................4(cid:1)
Sorghum Phenolic Compounds, Antioxidant Activity and Processing...........4(cid:1)
Structure of sorghum grain........................................................................4(cid:1)
Sorghum phenolic compounds...................................................................7(cid:1)
Determination of phenolic content in sorghum.......................................11(cid:1)
Separation and quantification of phenolic compounds using high
performance liquid chromatography (HPLC)..........................................12(cid:1)
Antioxidant properties of phenolic compounds.......................................13(cid:1)
Determination of antioxidant activity......................................................15(cid:1)
Effect of processing sorghum on phenol content and antioxidant
activity.....................................................................................................15(cid:1)
Bioavailability of dietary phenolic compounds and food
processing................................................................................................16(cid:1)

III EFFECT OF SORGHUM TYPE AND PROCESSING ON ANTIOXIDANT
PROPERTIES OF AFRICAN SORGHUM-BASED FOODS..........................18(cid:1)
Introduction....................................................................................................18(cid:1)
Materials and Methods...................................................................................20(cid:1)
Materials..................................................................................................20(cid:1)
Analyses...................................................................................................21(cid:1)
Sorghum processing.................................................................................24(cid:1)
viii
CHAPTER Page(cid:1)
Sample preparation..................................................................................26(cid:1)
Statistical analyses...................................................................................26(cid:1)
Results............................................................................................................26(cid:1)
Effect of sorghum type, decortication and different processing
methods on total phenols.........................................................................29(cid:1)
Effect of sorghum type, decortication and different processing
methods on tannin content.......................................................................31(cid:1)
Effect of sorghum type, decortication and different processing
methods on antioxidant activity...............................................................33(cid:1)
Discussion......................................................................................................35(cid:1)

IV PROCYANIDINS IN PROCESSED SORGHUM-BASED PRODUCTS.........39(cid:1)
Introduction....................................................................................................39(cid:1)
Materials and Methods...................................................................................41(cid:1)
Sorghum samples.....................................................................................41(cid:1)
Sorghum processing.................................................................................42(cid:1)
Sample preparation..................................................................................43(cid:1)
Analysis...................................................................................................43(cid:1)
Sample extraction and preparation of procyanidin extracts.....................44(cid:1)
HPLC analysis.........................................................................................44(cid:1)
Experimental design and data analysis....................................................45(cid:1)
Results and Discussion..................................................................................45(cid:1)
Phenols and tannin and procyanidin content of grain, porridges
and extrudates..........................................................................................45(cid:1)
Effect of sorghum type and processing on procyanidin profile...............49(cid:1)
Effect of processing on procyanidin polymers........................................54(cid:1)
Comparing the effect of porridge-making and extruder type on
procyanidins.............................................................................................54(cid:1)
Procyanidin content of dough and bread containing 12% tannin
sorghum bran...........................................................................................56(cid:1)
Sorghum procyanidin distribution and its implications on
astringency and bioavailability of procyanidins......................................60(cid:1)

V EFFECT OF ENZYME HYDROLYSIS ON RECOVERY OF PHENOLS
AND ANTIOXIDANT ACTIVITY IN SORGHUM AND PROCESSED
PRODUCTS.......................................................................................................62(cid:1)
Introduction....................................................................................................63(cid:1)
Materials and Methods...................................................................................65(cid:1)
ix
CHAPTER Page(cid:1)
Materials..................................................................................................65(cid:1)
Enzyme hydrolysis of sorghum products ................................................68
Chemical analyses....................................................................................69(cid:1)
Statistical analyses...................................................................................69(cid:1)
Results............................................................................................................70(cid:1)
Enzyme hydrolysis and dry matter loss of sorghum extrudates and
porridges..................................................................................................70(cid:1)
Effect of sorghum type and processing on phenol content of
sorghum products.....................................................................................72(cid:1)
Effect of sorghum type and enzyme treatment of grains on
phenols.....................................................................................................75(cid:1)
Effect of sorghum type, processing and enzyme treatment on
phenols.....................................................................................................79(cid:1)
Effect of sorghum type and processing on antioxidant activity of
sorghum grain..........................................................................................81(cid:1)
Effect of sorghum type and enzyme treatment on antioxidant
activity of sorghum grain.........................................................................84(cid:1)
Effect of enzymes on the antioxidant activity of extrudates and
porridges..................................................................................................87(cid:1)
Effect of pepsin and amylase treatment on tannin content of
extrudates and porridges..........................................................................91(cid:1)
Effect of enzyme treatment on bread containing white or tannin
sorghum bran on phenols and antioxidant activity..................................94(cid:1)
Discussion......................................................................................................98(cid:1)

VI SUMMARY AND CONCLUSIONS...............................................................103(cid:1)
LITERATURE CITED...............................................................................................106(cid:1)
APPENDIX A.............................................................................................................121(cid:1)
APPENDIX B.............................................................................................................125(cid:1)
VITA...........................................................................................................................130(cid:1)
x

LIST OF TABLES

TABLE Page

I Effect of sorghum type on grain quality of Macia, NK 283, Red Swazi, NS
5511 and Framida.............................................................................................27
II Proximate composition (%) (dry basis) of Macia, NK 283, Red Swazi, NS
5511 and Framida.............................................................................................28
III Effect of different processing methods on total phenols of sorghum...............30
IV Effect of different processing methods on tannin content of sorghum.............32
V Effect of different processing methods on antioxidant activity of sorghum.....34
VI Total phenols, tannin content and procyanidin (HPLC) content in sorghum
grain, porridge and extrudates..........................................................................47
VII Pearson’s Correlation Coefficients of total phenols, tannin and total
procyanidin contents of sorghum grain and products.......................................48
VIII Procyanidin content of raw sorghum grain, porridge and extruded grain........50
IX Total phenols and tannin content in Hi Tannin bran, dough and bread with
12% tannin bran................................................................................................58
X Procyanidin content of bread mix containing Hi Tannin bran, dough and
bread containing 12% Hi Tannin bran..............................................................59
XII Dry matter loss (%) in enzyme treated extrudates and porridges.....................71
XIII Effect of pepsin and amylase treatments on total phenols in sorghum grain,
extrudates and porridges...................................................................................74
XIV Effect of pepsin, amylase and pepsin followed by amylase treatment on
antioxidant activity of sorghum extrudates and porridges................................82
XV Effect of pepsin, amylase and pepsin followed by amylase treatment on
tannin content of residues remaining after enzyme hydrolysis of extrudates
and porridges.....................................................................................................92