Extruded Taro - American Chemical Society

Chapter 30

Extruded Taro (Colocasia esculenta) Volatiles J . A . Maga and M. B. L i u

Downloaded by UNIV OF ARIZONA on March 16, 2017 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch030

Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, C O 80523

Taro corms were peeled, sliced, dried, ground and extruded under conditions to produce maximum expansion. The volatiles from the resulting extrudate were extracted and identified using G C / M S . A series of common aliphatic hydrocarbons, acids and alcohols was identified along with Strecker degradation and Maillard reaction products. A total of 41 compounds were identified, the most abundant being octane, eicosanoic acid, 2-methylbutanal and pyridine.

Taro is an ancient crop that originated in Asia but is now grown in many parts of the world where it is also known as eddo, dasheen or cocoyam. It is a member of the family Araceae and is one of the most important edible aroids (7). Currently, it is primarily grown as a subsistence crop and eaten as a staple because of its high starch content (2). The cooked corms have a relatively bland flavor similar to that of potatoes or cereals and its volatile composition has been recently characterized (3). Due to its high starch content, taro is postulated to have good expansion properties, and thus perhaps would have application as an expanded snack product that could be extrusion processed. Therefore, the major objectives of this study were to determine the optimum extrusion temperature required to maximize taro expansion and to identify and semi-quantitate the volatiles associated with such a product. Materials and Methods Taro Preparation. Fresh taro corms averaging 1 kg each were directly imported from the Caribbean. Upon receipt, they were manually peeled and mechanically sliced into 0.5 mm slices. The resulting slices were placed in a heated forced-air dehydrator operating at 90°C and held for eight hours. The dehydrated product was ground to pass through a 2 mm screen. The moisture content of the resulting flour was determined using standard gravimetric procedures and was adjusted to 15% using tap water.

0097-6156/94/0543-0365$06.00/0 © 1994 American Chemical Society Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Extrusion Conditions. The moisture-adjusted flour was extruded using a singlescrew Brabender model P L V500 laboratory extruder equipped with a 3:1 compression screw operating at 100 ppm. The unit was equipped with a die having an opening of 3.75 mm. Dough temperatures just before the exit die were maintained at 80°, 100°, 120° or 140°C. The resulting extrudates were permitted to air dry overnight. Expansion ratio was determined by dividing the average extrudate diameter by the die diameter. Volatile Compound Extraction/Identification. Representative samples of extrudate were ground to pass through a 0.5 mm screen. The ground extrudate was extracted for 4 hours in a Likens/Nickerson apparatus using pentane as the solvent. The recovered solvent was concentrated under a stream of nitrogen and injected into a Hewlett Packard Model 5890 gas chromatograph equipped with a 10 m χ 0.32 mm i.d. capillary column coated with Carbowax 20M. Helium was the carrier gas. A n oven temperature of 50°C was maintained for 7 minutes, then the oven temperature was increased 2°/minute until a final temperature of 210°C was obtained. The injection port was operated at 230°C, and the detector was maintained at 250°C. For compound identification, the gas chromatograph was connected to a Hewlett Packard Model 5970 mass selective detector. Results and Discussion Taro Composition. Upon receipt, the proximate composition of the fresh peeled taro was determined using standard analytical procedures. These data are shown in Table I. Table I. Raw Peeled Taro Composition Component Moisture Protein (Ν χ 6.25) Lipid Carbohydrate Fiber Ash

Percent

71.3 1.6 0.3 25.4 0.9 1.4

As can be seen, the major functional component was starch. However, it should be noted that a small amount of protein and lipid material were also present that could serve as precursors for the thermal formation of flavor compounds. Extrudate Expansion. The influence of extrusion temperature on resulting extrudate expansion ratio is summarized in Figure 1. As can be seen, extrudate expansion continued as the temperature increased to 120°C and then expansion decreased at 140°C. With any starch-based food, optimum expansion temperature needs to be determined. Product moisture content, which in this study was held constant at 15%,

Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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20

EXT. TEMR (°c) Figure 1. Influence of extrusion temperature on taro extrudate expansion ratio.

as well as the type and amount of starch are important contributors to overall product expansion. A maximum expansion ratio of slightly over 16 was observed, thus demonstrating that taro has very good expansion properties normally associated with an extruded snack product. Volatiles Identified. The sample extruded at 120°C was used for volatile identification/quantitation. A total of 41 compounds were identified, including the hydrocarbons shown in Table II. Octane was the major volatile identified. Three fairly common acids were also identified which represented 10.9% of the total volatiles. The nine aldehydes shown in Table II were also identified, with 2methyl butanal being the most prevalent at 4.1%. Due to thermal action on various classes of precursors, a series of heterocyclics was identifed with pyridine being the most abundant. Heterocyclics represented 6.70% of the total volatiles. The eight alcohols shown in Table II were identified with hexadecan-l-ol being the most prominent at 1.6%. Several ketones were also present, with the compound decan-2one being the most dominant. Several miscellaneous compounds including two lactones and a phenol were also identified. Formation Pathways. The formation of all of the compounds identified can be explained by commonly accepted pathways. For example, the Maillard reaction could explain the formation of furans, thiophenes, thiazoles, and pyrroles. Sugar degradation can account for ketones and certain furans. Amino acids are likely precursors for the aldehydes identified, and lipids can account for the presence of the hydrocarbons, acids, alcohols, aldehydes and lactones. Conclusion The extrusion of taro at 120°C and 15% moisture produced optimum product expansion and resulted in the identification of 41 typical flavor volatiles.


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Table Π. Volatiles Identified in Extruded Taro (120°C) Classl Compound

Relative %

Hydrocarbons

Octane Tetracosane Pentacosane Total

16.3 1.5 3.7 21.5

Acids


Nonanoic Octadecanoic Eicosanoic Total Aldehydes

2-Methylpropanal 2-Methylbutanal 3-Methylbutanal Hexanal Nonanal Decanal 5-Methyl-2-phenylhex-2-enal Phenylacetaldehyde Benzaldehyde Total Heterocyclics

2-Methylthiophene Thiophene-2-carboxaldehyde Thiophene-3-carboxaldehyde Pyridine N-Methylpyrrole 2-Pentylfuran 2,3-Dihydrobenzofuran 2-Furaldehyde Benzothiazole 2-Acetylthiazole Total Alcohols

2-Methylbutan-2-ol 2-Methylbut-3-en-2-ol 3-Methylbutan-1 -ol Pentan-l-ol Hexan-l-ol (Z)-Hex-3-en-l-ol Oct-l-en-3-ol Hexadecan-l-ol Total

1.4 2.6 6.9 10.9

0.2 4.1 0.4 1.1 0.1 0.1 0.2 0.2 0.4 6.8

0.5 0.05 0.05 3.5 0.2 0.1 0.3 0.5 0.1 1.4 6.7

0.1 1.1 0.2 0.2 0.2 0.1 0.2 1.6 3.7


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Table II. Continued


ClasslCompound Ketones Butanedione Pentane-2,3-dione Nonan-2-one Decan-2-one 3,5,5-Trimethylcyclohex-2-en-1 -one Total Miscellaneous γ-Decalactone δ-Decalactone 2-Methoxy-4-vinylphenol Total

Relative % 0.02 0.2 0.05 0.1 0.05 0.42 0.04 0.06 2.1 2.2

Literature Cited 1. Bradbury, J. H . ; Holloway, W. D . Chemistry of Tropical Root Crops: Significance for Nutrition and Agriculture in the Pacific, Australian Centre for International Agricultural Research, Canberra. 2. Wills, R. B . H . ; Lim, J. S. K ; Greenfield, H . ; Bayliss-Smith, T. J. Sci. Food Agric. 1990, 38, 1137-1142. 3. Macleod, G. Food Chem. 1990, 38, 89-96. RECEIVED August 5, 1993


Extruded Taro - American Chemical Society

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