Emmanuel Ohene Afoakwa

Flavor Formation and Character in Cocoa and Chocolate: A Critical Review

Chocolate characters not only originate in flavor precursors present in cocoa beans, but are generated during post-harvest treatments and transformed into desirable odor notes in the manufacturing processes. Complex biochemical modifications of bean constituents are further altered by thermal reactions in roasting and conching and in alkalization. However, the extent to which the inherent bean constituents from the cocoa genotype, environmental factors, post-harvest treatment, and processing technologies influence chocolate flavor formation and relationships with final flavor quality, has not been clear. With increasing speciality niche products in chocolate confectionery, greater understanding of factors contributing to the variations in flavor character would have significant commercial implications.

Response Surface Methodology for Studying the Effects of Feed Moisture and Ingredient Variations on the Chemical Composition and Appearance of Extruded Sorghum-Groundnut-Cowpea Blends

Studies were conducted to evaluate the effects of feed moisture and ingredient variations on the chemical composition and appearance of extruded sorghum-groundnut-cowpea blends using response surface methodology. Central composite rotatable design for k = 3 was used to study the combined effect of feed moisture (12 to 44 percent), groundnut concentration (zero to 10 percent) and cowpea level (zero to 20 percent) on the chemical composition and appearance of extruded ready-to-eat sorghum-legume blends in a single screw extruder. Product moisture, crude protein, crude fat, ash (minerals), crude fibre, carbohydrate and colour (L, a and b-values) were determined on the products using standard analytical methods. The results showed that irrespective of the feed moisture level, the extrusion process could reduce the product moisture to levels between six to 10 percent and low enough to extend the shelf life of the derived products. Increasing additions of cowpea and groundnut resulted in significant increases in protein, fat and ash (minerals - calcium, iron, and phosphorus) contents of the sorghum-legume extrudates. Increasing addition of cowpea decreased the redness of the sorghum extrudates, while no changes were noted with groundnut additions. Contrary, increasing groundnut addition generally increased the yellowness at all cowpea concentrations, while only minimal changes were noted with cowpea additions. The lightness of the sorghum extrudates decreased with increasing cowpea and groundnut concentrations, and this was more pronounced at higher cowpea concentrations and with increasing groundnut levels. The level of feed moisture showed no effect. Models developed for the indices gave R2-values ranging from 67.04 percent (for a-value) to 87.83 percent (for fat content), with insignificant lack of fits indicating the adequacy of the model in explaining the data. The optimal conditions likely to produce ready-to-eat puffed snack with enhanced nutritive value and appearance from sorghum-groundnut-cowpea blend extrudates was at low feed moisture (10 to 14 percent), cowpea level (18 to 20 percent) and groundnut level (six to eight percent).

Chemical composition and quality changes occurring in Dioscorea dumetorum pax tubers after harvest

Studies were conducted on the chemical composition, as well as the biochemical and textural changes, occurring during storage of two cultivars of Dioscorea dumetorum yam tubers. The tubers were harvested and stored under tropical ambient (28C) and cold room conditions (4C) for 24, 48 and 72 h. The samples were evaluated for moisture, starch, sugars, fibres and textural properties. Chemical analysis showed no significant differences between the means obtained for moisture, ash, protein, fibre, fat, carbohydrate, calcium, phosphorus, iron, vitamin A and vitamin C contents of the cultivars (white and yellow) studied. During storage, moisture contents decreased by approximately 6–10% after 72 h of harvest due to dehydration. Starch levels declined from 70.5–66.5 g/100 g, while sugars and fibre contents increased slightly in the samples. Textural properties (peak force and curve areas) also increased considerably during storage. Peak force (hardness) increased from 1143.0 to 5711.5 g in the white cultivars whilst the yellow cultivars increased from 2130.5 to 4194.1 g. Curve areas (adhesiveness) also increased from 12.2254 to 29.5646 g in the white cultivars and from 17.1316 to 24.3108 g in the yellow cultivars. There were very high correlation co-efficients (r=0.9503–0.9913, P40.05) between peak force (hardness) and all the biochemical constituents of the white cultivars, and (r=0.9876–0.9380, P40.05) for the yellow cultivars investigated. However, the rate of hardening varies with cultivars and temperature of storage. # 2001 Elsevier Science Ltd. All rights reserved.

Application of response surface methodology for studying the product characteristics of extruded rice-cowpea-groundnut blends

Response surface methodology (with central composite rotatable design for k=3) was used to investigate the product properties of extruded rice–cowpea–groundnut blends in a single screw extruder. The combined effect of cowpea (0–20%), groundnut (0–10%), and feed moisture (14–48%) levels were used for formulation of the products. The product moisture, expansion ratio, bulk density and total colour change were studied using standard analytical methods. Well-expanded rice–legume blend extrudates of less bulk density and lower moisture content were produced at low feed moisture. Increasing legume addition affected the various shades of colour in the product. Models developed for the indices gave R2 values ranging from 52.8% (for the b-value) to 86.5% (for bulk density). The models developed suggested that the optimal process variables for the production of a puffed snack with an enhanced nutrition and spongy structure from a rice–cowpea–groundnut blend are low feed moisture of 14–20% and maximum additions of 20% cowpea and 10% groundnut. A lack-of-fit test showed no significance, indicating that the models adequately fitted the data.

Viscoelastic properties and changes in pasting characteristics of trifoliate yam (Dioscorea dumetorum) starch after harvest

Studies were conducted on the viscoelastic properties, as well as the changes in pasting characteristics, of trifoliate yam, Dioscorea dumetorum, starch during post-harvest hardening of the tubers. A 34 factorial experiment with blanching treatment time (0, 15 and 30 s) and storage time (0, 12, 24 and 36 h) as the respective variables was performed. The samples were evaluated for pasting temperature, viscosity at 95 C, viscosity at 95 C-Hold, viscosity at 50 C and viscosity at 50 C-Hold, as well as their changes during storage of the tubers, to determine the starch behaviour of the D. dumetorum tubers during the hardening phenomenon and how blanching affects the process during storage. No significant differences (P40.05) in the viscoelastic properties were observed between the means obtained for pasting temperature, viscosity at 95 C, viscosity at 95 C-Hold, viscosity at 50 C and viscosity at 50 C-Hold in the white and yellow cultivars studied. Storage caused decreases in all the Brabender Viscoamylograph indices measured. Pasting temperature decreased from 80.4 to 66.6 C whilst peak viscosity decreased from 30 to 18 BU during the storage period. Viscosities at 95 C and 95 C-Hold also declined, respectively, from 30 to 20 BU and 450 to 330 BU within 36 h after harvest. Similarly, viscosities at 50 C and 50 C-Hold decreased, respectively, from 450 to 265 BU and 480 to 300 BU during storage of the tubers. Blanching, however, increased the pasting characteristics of the stored tubers. The viscoelastic properties of the two cultivars of D. dumetorum tubers studied were similar. However, the hardening phenomenon causes slight decreases in the pasting characteristics of the tubers after harvest. # 2002 Published by Elsevier Science Ltd.

The microflora of fermented nixtamalized corn

Nixtamalization is a traditional process that improves the nutritional quality of corn. To provide a means of utilizing the nutritional benefits of nixtamalized corn and improve product acceptability, lactic acid fermentation was applied. The objective of the study was to study the microbial profile and establish the important lactobacilli of fermenting nixtamalized corn dough. Two batches of cleaned whole corn were subjected to the process of nixtamalization, using two concentrations of lime (0.5 or 1.0%), milled, made into a dough (50% moisture) and fermented spontaneously for 72 h. A control sample was prepared without alkaline treatment. pH and titratable acidity of the dough were measured. Aerobic mesophiles, lactic acid bacteria, yeasts and molds were enumerated on Plate Count Agar (PCA), deMan, Rogossa and Sharpe (MRS) Agar and Malt Extract Agar (MEA), respectively. The identity of lactobacilli present was established at the species level using API 50 CHL. The pH of all the fermenting systems decreased with fermentation time with concomitant increase in titratable acidity. Lactic acid bacteria in numbers of 1.6 x 10(9), 2.3 x 10(9) and 1.8 x 10(9) cfu/g, respectively yeasts and molds, and numbers of 8.0 x 10(7), 5.0 x 10(5) and 1.7 x 10(5) cfu/g, respectively were observed in the control and the two nixtamalized (0.5% and 1.0% lime) samples after 48 h of fermentation. Lactobacilli identified in the fermenting nixtamalized corn dough were Lactobacillus plantarum, Lactobacillus fermentum and Lactobacillus cellobiosus as well as Pediococcus spp. The study demonstrates that nixtamalized corn though alkaline in nature can be subjected to spontaneous fermentation to produce a sour product.

Application of response surface methodology for studying the quality characteristics of cowpea-fortified nixtamalized maize

Studies were conducted on the chemical and functional properties of fermented cowpea-fortified nixtamalized maize using response surface methodology. The central composite rotatable design for Ks3 was used to study the combined effect of lime concentration (0–1%), moisture content (55–65%) and cowpea level (0–30%) on pH, titratable acidity, water absorption, protein and viscosity of fermented nixtamalized maize. Regression models were developed to predict the variables. The presence of lime and cowpea influenced the pH, titratable acidity, water absorption, protein content and the cooked paste viscosity of the fermented cowpea-fortified maize. The presence of lime during fermentation generally decreased titratable acidity, water absorption and cooked paste viscosity while the addition of cowpea increased most of the studied indices. Cowpea fortification can therefore be used to increase the nutritive value of nixtamalized maize with acceptable product quality characteristics. 2002 Elsevier Science Ltd. All rights reserved.

Effect of fermentation on the quality characteristics of nixtamalized corn

Spontaneous lactic acid fermentation is an important process in cereal processing. It is applied to develop and enhance taste and flavour, modify texture and improve the microbial safety of foods. When applied to nixtamalized corn mixed with traditional steeped corn it is expected to further improve the functionality, improve nutritional quality and provide an alternative corn-based ingredient. The objective of this study was to investigate the chemical and functional properties of fermented blends of steeped and nixtamalized corn. A 3� 5 factorial experimental design with fermentation time (0, 24, 48 h) and blends composition (0:100, 25:75, 50:50, 75:25, 100:0 steeped:nixtamalized corn) was performed. The blends were fermented for the specific times and analysed for pH, titratable acidity, water absorption, texture and viscosity. The pH of all the blends decreased with a corresponding increase in titratable acidity as fermentation time was increased. Decreases in water absorption capacity, texture, colour and cooked paste viscosity were measured in all the blends with increasing fermentation time. The cooled paste viscosity of the blends containing 50:50, 25:75 and 0:100 of steeped:nixtamalized corn showed decreases from 1840 to 780, 650 to 240 and 2240 to 1790 BU, respectively, after 48 h of fermentation. The sample derived from 100% steeped corn, however, showed increases in water absorption capacity, texture and cooked paste viscosity with increasing fermentation time. Nixtamalized corn can be subjected to spontaneous fermentation to produce thin, energy dense gruels of acceptable qualities to solve the low energy density problem of weaning foods prepared from fermented corn. # 2002 Elsevier Science Ltd. All rights reserved.

Biochemical and textural changes in trifoliate yam Dioscorea dumetorum tubers after harvest

Biochemical and textural changes were investigated in trifoliate yam Dioscorea dumetorum tubers, after harvest, in an attempt to study the chemical and physical changes associated with the raw and cooked tubers and how these relate to the hardening phenomenon of the tubers after harvest. A 2234 factorial experiment, with cultivar, storage condition, treatment and storage time as their respective variables, was done. Samples were analysed for moisture, starch, reducing sugars, alcohol-soluble sugars, acid and neutral detergent fibre, cellulose, hemicellulose and lignin. The Warner–Bratzler test cell was used in a TA.XT2 Texture Analyser to measure the hardness of cooked tubers. The moisture and starch contents of the tubers decreased from 77.8 to 70.4% and 70.5 to 66.5 g/100g, respectively in a period of 72 h of storage, suggesting rapid dehydration after harvest and breakdown of starch. All the other chemical indices increased with storage time. These changes were influenced by the storage temperature and the treatment given to the tubers prior to storage. In general, samples stored at 4 C showed smaller changes in the chemical indices than those at 28 C, suggesting a temperature-dependence of the changes. Both cultivars of yam showed increase in hardness with storage time. A high correlation (r=0.9503–0.9913) was noted between the texture and the chemical indices of the white cultivars. Storage of the trifoliate yam, Dioscorea dumetorum, immediately after harvest, leads to reduction in moisture and starch contents and increase in sugars and structural polysaccharides. Low temperature storage may reduce the hardening phenomenon. A mechanism for the hardening phenomenon has been proposed. # 2002 Elsevier Science Ltd. All rights reserved.

Changes in rheological properties and amylase activities of trifoliate yam, Dioscorea dumetorum, starch after harvest

Changes in rheological properties and amylase activities occurring in trifoliate yam, Dioscorea dumetorum, starch after harvest were investigated. Trifoliate yam tubers were harvested and stored under tropical ambient (28 C) and cold room conditions (4 C) for 12, 24 and 36 h. The D. dumetorum starches were extracted from the tubers under study and samples were evaluated for changes in their rheological properties (paste characteristics) during storage and to study the action of amylases on D. dumetorum starch after harvesting. The post-harvest activities of a and b-amylases were also studied to evaluate their rate of action on D. dumetorum starch. Storage caused decreases in the rheological properties (paste characteristics) of the tubers within 36 h of harvest. Similarly, a and b-amylase activities in the tubers more than doubled within 24 h after harvesting. Blanching, however, effectively decreased the action of amylases on D. dumetorum starch during storage, with subsequential increase in the paste viscosities of the stored tubers. Low temperature storage of the tubers also slowed down the rates of decrease in both the rheological properties and amylase activities during the 36-h storage period. # 2002 Published by Elsevier Science Ltd.