The chemistry of phenolic compounds of sorghum

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Statement of the problem

Polyphenols are ubiquitous in plants and are an integral part of both human and animal diets (Bravo, 1998). Polyphenols protect crops from pathogens and predators by acting as phytoalexins and by increasing the astringency of food to make it unpalatable. To reduce bird damage, farmers grow condensed tannin-containing (tannin) sorghums, which are astringent during the immature stages when bird damage is highest (Bullard, Garrison, Kilburn and York, 1980). However, these agronomic advantages of condensed tannins to the farmer are accompanied by nutritional disadvantages (Butler, 1982; Chung, Wong, Wei, Huang and Lin, 1998). Tannins form complexes with proteins, starch and digestive enzymes causing a reduction in the nutritional value of food (Butler, 1982; Chung et al., 1998). Nonetheless, the agronomic advantages of tannin sorghums outweigh such negatives as reduced nutrient availability or astringency (Awika and Rooney, 2004).

Sorghum (Sorghum bicolor [L] Moench)

Sorghum ranks fifth among the most important cereal crops in the world following wheat, rice, maize and barley (FAOSTAT, 2006). In the semi-arid tropics worldwide, sorghum is generally cultivated at a subsistence level and consumed as food by humans (Cothren, Matocha and Clark, 2000). Thus, it contributes significantly to the nutritional livelihood of impoverished populations of the world. Sorghum is eaten as porridge, fermented and unfermented breads, leavened and unleavened bread, snacks, non-alcoholic beverages and sorghum beer and malt (Murty and Kumar, 1995). In Japan, white tan-plant sorghums are processed into flour and other products such as snacks, cookies and ethnic foods (Awika and Rooney, 2004). In the USA, such sorghums are also gaining popularity as a substitute for wheat for people allergic to wheat gluten (Awika and Rooney, 2004).

he chemistry of phenolic compounds of sorghum

Phenolic compounds are one of the most widely distributed groups of substances in the plant kingdom (Ross and Kasum, 2002). There are more than 8000 known phenolic compound structures, the common feature of which is an aromatic ring with at least one hydroxyl group (Ross and Kasum, 2002). There are more than 15 different classes of phenolic compounds in foods, ranging from simple phenolics with molecular weights of less than 500 to polymers of high (3000) molecular weight (Drewnowski and GomezCarneros, 2000). All sorghums contain phenolic compounds (Dykes et al., 2005). Phenolic compounds are located mainly in the pericarp of the sorghum kernel (Awika and Rooney, 2004; Dykes and Rooney, 2006). Phenolic compounds identified in sorghum include phenolic acids, flavonoids and condensed tannins (Hahn et al., 1983; Awika et al., 2003a; Awika and Rooney, 2004). Phenolic acids in sorghum are mainly benzoic and cinnamic acid derivatives (Fig. 1.3) (Hahn et al., 1983; Awika and Rooney, 2004). The benzoic derivatives have a C6-C1 structure and include gallic acid, p-hydroxybenzoic acid, vanillic, syringic and protocatechuic aids (Dykes and Rooney, 2006). Hydroxycinnamic acids have a C6-C3 structure and include coumaric, caffeic, ferulic and sinapic acids (Dykes and Rooney, 2006).

Content of phenolic compounds in sorghum

The amount of phenolic compounds present in any particular sorghum cultivar is influenced by its genotype and the environment in which it is grown (Dykes et al., 2005). These authors determined total phenol, condensed tannins, flavan-4-ols, anthocyanins and antioxidant activity of sorghum grain of clearly identified genotypes. The sorghum grain varied in pericarp colour, mesocarp thickness and the presence and intensity of the pigmented testa layer. Sorghum grains grown from purple/red coloured plants had higher total phenol content than tan plant types. Sorghums with a thick pericarp had higher total phenol content than sorghums with a thin pericarp. Sorghums with a pigmented testa gene B1- B2- and the spreader gene S had increased total phenol content, with B1- B2- S genes having the highest total phenol contents. Sorghums with a red pericarp contained flavan4-ols such as luteoforol and apiforol, produced by flavanones, naringenin and eriodictyol. Consequently tan plant sorghums had the lowest content of flavan-4-ols, followed by purple/red plant sorghums with a thin pericarp. Purple/red plant sorghums with a thick pericarp had the highest content of flavan-4-ols. In tannin-free sorghums with a red pericarp, the total phenols were contributed mostly by the flavan-4-ols. Anthocyanin content followed the same trend as flavan-4-ols. Sorghums with a black pericarp contained the highest levels of anthocyanins. According to these authors, sorghums with a black pericarp are genetically red but turn black during maturation in the presence of sunlight.

Sensory properties of phenolic compounds

Sensory attributes associated with smaller phenolic compounds like phenolic acids include sweet, sour, bitter and astringency (Peleg and Noble, 1995). Peleg and Noble (1995) investigated the sensory properties of phenolic acids (benzoic acid derivatives) commonly found in fruits, vegetables, grains and spices. These included salicylic acid (2-hydroxy benzoic acid), m-hydroxyl benzoic acid (3-hydroxy benzoic acid), gentisic acid (2,5- hydroxyl benzoic acid) protocatechuic acid (3,4-hydroxy benzoic acid) and gallic acid (3,4,5-trihydroxy benzoic acid) in water. Each of these compounds elicited multiple sensations including sweetness, sourness, astringency, bitterness and prickling. Although the compounds were structurally similar their sensory properties differed qualitatively and quantitatively. Gentisic acid was most sour, benzoic acid was highest in prickling sensation, salicylic acid was most astringent, m-hydroxyl benzoic acid was the sweetest and gentisic, benzoic and protocatechuic acids were most bitter.

TABLE OF CONTENTS :

• ABSTRACT
• DEDICATION
• ACKNOWLEDGEMENTS
• LIST OF TABLES
• LIST OF FIGURES
• 1. INTRODUCTION
  • 1.1. Statement of the problem
  • 1.2. Literature review
    • 1.2.1. Sorghum (Sorghum bicolor [L] Moench)
      • 1.2.1.1. Sorghum anatomical structure
      • 1.2.1.2. The chemistry of phenolic compounds of sorghum
      • 1.2.1.3. Content of phenolic compounds in sorghum
    • 1.2.2. Harmful and beneficial effects of phenolic compounds
    • 1.2.3. Sensory properties of phenolic compound
    • 1.2.4. Bitterness
      • 1.2.4.1. Bitter taste transduction and other basic tastes
      • 1.2.4.2. Genetic variation
      • 1.2.4.3. Sensitivity to PROP and bitterness of other compounds
      • 1.2.4.4. Sensitivity to PROP and phenolic compounds
      • 1.2.4.5. PROP sensitivity on acceptability of bitter foods
    • 1.2.5. Astringency
      • 1.2.5.1. Compounds that cause astringent sensations
      • 1.2.5.2. Sensory perception of astringency
      • 1.2.5.3. Acceptability of astringency in food
      • 1.2.6 Time intensity sensory evaluation procedure
      • 1.2.6.1. Single attribute time intensity and dual attribute time
      • intensity sensory methods
      • 1.2.6.2. ‘Panellist’s signature’
• 1.3. Conclusions
• 1.4. Hypotheses
• 1.5. Objectives

• 2. RESEARH
  • 2.1. Effects of phenolics in sorghum grain on its bitterness, astringency
  • and other sensory properties
    • 2.1.1. Abstract
    • 2.1.2. Introduction
    • 2.1.3. Materials and methods
      • 2.1.3.1. Materials
      • 2.1.3.2. Grain characterization
      • 2.1.3.3. Bran isolation
      • 2.1.3.4. Determination of phenolics
      • 2.1.3.5. Descriptive sensory panel selection and training
      • 2.1.3.6. Sample preparation, presentation and assessment
        • 2.1.3.6.1. Sorghum bran infusions
        • 2.1.3.6.2. Sorghum (whole-grain) rice
      • 2.1.3.7. Statistical analysis
    • 2.1.4. Results and discussion
    • 2.1.5. Conclusions
    • 2.1.6. References
  • 2.2. Bitterness and astringency of bran infusions of tannin-free and
  • condensed tannin sorghums determined using a dual attribute time
  • intensity sensory method
    • 2.2.1. Abstract
    • 2.2.2. Introduction
    • 2.2.3. Materials and methods
      • 2.2.3.1. Sorghum grain
      • 2.2.3.2. Sorghum bran infusions
      • 2.2.3.3. Descriptive sensory panel selection and training
      • 2.2.3.4. Sample presentation and assessment
      • 2.2.3.5. HPLC analysis
      • 2.2.3.6. Statistical analysis
    • 2.2.4. Results and discussion
      • 2.2.4.1. Main effects
      • 2.2.4.1.1. Cultivar effect
      • 2.2.4.1.2. Panellist effect
      • 2.2.4.1.3. Session effect
      • 2.2.4.1.4. Replicate effect
      • 2.2.4.1.5. Sample order effect
      • 2.2.4.2. Interaction effects
      • 2.2.4.2.1. Panellist x cultivar
      • 2.2.4.2.2. Panellist x session
      • 2.2.4.2.3. Panellist x replicate
      • 2.2.4.2.4. Panellist x sample order
    • 2.2.4. Conclusions
    • 2.2.6. References
  • 2.3. Consumer acceptability of sorghum rice from condensed tannin and
  • tannin-free sorghums and the influence of PROP taster status
    • 2.3.1. Abstract
    • 2.3.2. Introduction
    • 2.3.3. Materials and methods
      • 2.3.3.1. Sorghum
      • 2.3.3.2. Consumer recruitment
      • 2.3.3.3. Sample preparation, presentation and assessment
      • 2.3.3.4. PROP classification
      • 2.3.3.5. Statistical analysis
    • 2.3.4. Results and discussion
    • 2.3.5. Conclusions
  • 2.3.6. References
• 3. GENERAL DISCUSSION
  • 3.1. Methodologies
  • Effects of total phenol and condensed tannin content on the sensory
  • properties, bitterness and astringency, and acceptability of different
  • sorghums
  • 3.3. Condensed tannin threshold limit
• 4. CONCLUSIONS AND RECOMMENDATIONS
• 5. REFERENCES
• 6. APPENDIX
  • Papers published

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Sensory perception of bitterness and astringency in sorghum