Can extrusion cooking improve and/or stabilize

the flavour of pearl millet porridges?

Isiguzoro Onyeoziri a, Pablo Torres-Aguilar b, Bruce R. Hamaker b, John R.N. Taylor a,

Henriëtte L. de Kock a

3rd September, 2019

a Institute for Food, Nutrition and Well-being and Department of Consumer and Food

Sciencesb Whistler Centre for Carbohydrate Research and Department of Food Science, Purdue

University,745 Agricultural Mall Drive, West Lafayette, IN 47907, USA.DO NOT C

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Problems facing us…

❑ Concerning cereal foods, there is evidence that urban families like the

traditional foods they were brought up on, and will pay for convenience

combined with tradition (Taylor, et.al., 2010).

➢ Demand for the commercial manufacture of value-added cereal food

products by entrepreneurs.

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Pearl millet panicle in a field https://www.harvestplus.org/knowledge-market/pearl millet

Pearl millet grainshttps://www.ebay.com/itm/100gm-Pearl-millet-Grains-Bajra-Grains-Nutritive-Digestive-/282432821840

❑ More than 90 million people depend

on pearl millet for food and income

(ICRISAT, 2019).

➢ Flour is highly subject to

deterioration due to lipid

decomposition and oxidative

rancidity (Taylor, 2016).

➢ Control of free fatty acid

formation is thus critical with

respect to flavour stability (Heiniö

et al., 2016).

Style

Pericarp

Seed coat

Aleurone layer

Corneous

endosperm

Peripheral

endosperm

Floury

endosperm

Scutellar

Epithelium

Scutellum

Hilum

Diagrammatic longitudinal section through a millet grainReproduced from Taylor, (2016).

Pearl millet(Pennisetum glaucum (L.) R. Br.)

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What we knew from other researchers…

• Decortication alone does not inhibit hydrolytic deterioration of

stored pearl millet flour (Tiwari, et. al., 2014).

• Wet and dry thermal treatments of pearl millet flours can inhibit

hydrolytic rancidity by inactivation of lipase enzymes (Nantanga

et. al., 2008).

• The thermal treatments gave porridges prepared from the

flours sensory notes that were generally regarded by

consumers as being pleasant.

• Extrusion cooking is a thermal processing technology that pre-

cooks (instantizes) cereal flours, so that they can be made into

ready-to-eat porridges by simply mixing with boiling water

(Devi et. al., 2013).

• The CONSUMER acceptability of stored ready-to-eat CRISP-

TYPE extrudates produced from decorticated pearl millet for

flavor decreased significantly ONLY after 6 months storage at

ambient conditions and 3 months under accelerated storage

conditions (Sumathi et. al., 2007).DO NOT C

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Extrusion

cooking

conditions

Wholegrain gritsDecorticated grits

12% decortication level

Extruder type

(heat source)

Single-screw (friction

heated)

Single-screw (friction

heated)

Feed moisture

content

30%:

grits equilibrated prior to

extrusion cooking

32%

grits equilibrated prior to

extrusion cooking

Cooking

temperatureAbove 120 °C Above 120 °C

Screw speed 700 rpm 700 rpm

Extruder length

to screw

diameter (L/D)

ratio

40:1 40:1

Die diameter 6 mm 6 mm

Extrudate drying 50 °C for 24 h 50 °C for 24 h

Extrudate

milling

Extrusion cooked

decorticated flour <300

µm particle size

Extrusion cooked

wholegrain flour <300 µm

particle size

What we did…

A Technochem Mini-Extruder©Photo provided by Pablo Torres-Aguilar

Extrusion cooking conditions for the extrusion cooking

of pearl millet

❑ Extrusion cooked wholegrain

and decorticated pearl millet

flours

❑ Native/raw and extrusion

cooked flours were stored at

ambient (25 °C) and elevated (40

°C) temperatures for 6 months

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Some activities in training a sensory panelTop: Information and screening session for recruited potential panelistsBottom left: Panelist carrying out an aroma identification exerciseBottom right: Panelist rating sample attribute intensity on a computer

❑ Recruitment, screening,

and training of the

descriptive sensory panel

followed standard ISO 8586

guidelines (ISO, 2012).

❑ Generic descriptive method

described by Einstein

(1991) was used.

❑ Attribute intensities were

rated on 11-point

continuous linear scales.

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❑ Compusense Cloud® software was used for

capturing and storing evaluation data.

❑ Fat acidity chemical analysis

➢ Total fatty acids in flours were

determined by AACC Method 02-02A (AACC, 1999).

❑ One-way ANOVA and Fisher’s LSD test was

at p≤0,05 probability was used to determine

if differences occurred for each sensory

characteristic as a function of storage time.

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Our results and what they reveal…

Effects of extrusion cooking and grain decortication on the aroma and flavour

attributes of pearl millet porridges

Attributes Native Extrusion cooked

Wholegrain Decorticated Wholegrain Decorticated

Branny

aroma3.52ab 2.06b 4.29a 2.06b

Sweet

aroma1.99b 1.62b 5.30a 5.19a

Starchy

aroma4.13b 4.05b 4.45ab 6.01a

Chemical

aroma0.21ab 0.74a 0.18b 0.18b

Wheaty

aroma3.05ab 2.23b 4.28a 4.50a

Canned

sweet

corn

aroma

3.07bc 2.06c 5.02a 3.79ab

Starchy

flavour4.87ab 3.91b 5.52ab 6.57a

Umami 3.62a 2.75ab 1.47b 1.97b

Values are means of duplicate evaluations; values in rows with differing letter superscripts are significantly different (p≤0.05)

❑ Typical sensory descriptors of

thermally processed non-

wheat cereal products (R.L.

Heiniö et.al., 2016; Kobue-Lekalake

et.al., 2007).

❑ Increased intensities probably

in part due to the formation of

pyrazines which can be

Maillard products of reactions

between proteins and

reducing sugars during

extrusion cooking (Zhou et.al.,

1999)DO NOT C

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0

2

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0 4 8 12 16 20 24

Att

rib

ute

In

ten

sity

Weeks

Sweet aroma

aa

a

a

b

a

aa a a

a b

a

bb

aa

aa

a

aaaaa

aa aa

0

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Att

rib

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In

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Cooked grain aroma

a

b

a

aaa

a

ba

b

aa

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a aa aaa

aa

aaa

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In

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Canned sweet corn aroma

aa

bbb

a

aa

a

aaa

aa

aaa

aa

a

a

a a

aaa

a a

0

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In

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Maize meal aroma

a

aa a

bb

a

b

a aa

aaa

a

aa

aa a

aa

aaa a

a

Effects of extrusion cooking and decortication on the intensities of cereal-like aroma sensory attributes of porridges prepared from stored pearl millet floursValues for same markers on same lines with different letters differ significantly (p≤0.05)

= Wholegrain, native, 25 °C stored flour = Decorticated, native, 25 °C stored flour= Wholegrain, extrusion cooked, 25 °C stored flour= Decorticated, extrusion cooked, 25 °C stored flour= Wholegrain, extrusion cooked, 40 °C stored flour= Decorticated, extrusion cooked, 40 °C stored flour

0

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0 4 8 12 16 20 24

Att

rib

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In

ten

sity

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Wheaty aroma

aa

a

b

a

aa

aaa

a

aa

bb

b

a

aa

a

aa

aa a aa

aDO NOT C

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0

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0 4 8 12 16 20 24

Att

rib

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In

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Painty aroma

aa

c

aa a

aa aa aa

a

b

a

b

a

b

c

aaa

a a0

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4

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0 4 8 12 16 20 24

Att

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In

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Chemical aroma

aba

b

a

a

aa

a

a

a a

b

a

a

b

b a

bbaaaa

aaa

0

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0 4 8 12 16 20 24

Att

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In

ten

sity

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Soapy aroma

aa aa

aa aa

aa

a

aa

b

a

a

bb

aa

a a a aa

0

2

4

6

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10

0 4 8 12 16 20 24

Att

rib

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In

ten

sity

Weeks

Chemical flavour

a

b

aa

a aa

a

a

aa

a

b aa

a

a

b

a

b

c

aa a

aaa

Effects of decortication, extrusion cooking, and storage on the intensities of rancidity-associated aroma and flavour sensory attributes of porridges prepared from stored pearl millet floursValues for same markers on same lines with different letters differ significantly (p≤0.05)

= Wholegrain, native, 25 °C stored flour

= Decorticated, native, 25 °C stored flour

= Wholegrain, extrusion cooked, 25 °C stored flour

= Decorticated, extrusion cooked, 25 °C stored flour

= Wholegrain, extrusion cooked, 40 °C stored flour

= Decorticated, extrusion cooked, 40 °C stored flour

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❑ Classic rancidity-development-indicating sensory attributes include,

chemical, painty, and soapy aromas, as well as chemical flavour (Malcolmson et al., 1996; Chanadang and Chambers, 2019).

❑ Painty and soapy aromas are mainly associated with volatile

secondary products of unsaturated fatty acid oxidation (Broadbent &

Pike, 2003; Chanadang and Chambers, 2019).

❑ Extrusion cooking prevents the development of off-flavours and

aromas by the inactivation of lipase enzymes (Goyal & Chugh, 2017).

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Effects of extrusion cooking and decortication on the fatty acid

content (mg KOH/100 g dry basis) of 25 °C stored pearl millet flours

Storage time

(Weeks)

Native Extrusion cooked

Wholegrain Decorticated Wholegrain Decorticated

Unstored 546aC 76bC NQ NQ

12 2082aA 1206bA NQ NQ

24 1583aB 814bB 91c 11d

Values are means of duplicate determinations; values with different small letter superscripts along a row are significantly (p≤0.05) different; values with different capital letter superscripts down a column are significantly different (p≤0,05)NQ = not quantifiable

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Our conclusions

❑ The application of extrusion cooking is a viable process for both the

production of ready-to-eat porridge-type products from pearl millet, and

modifying and stabilising the aroma and flavour of prepared pearl millet

porridges.

➢ However, extrusion cooking conditions would have to be optimized

for the complete inactivation of lipases.

❑ This will provide convenience, nutritional benefits, and possibly even

increase the consumer acceptance of millet to both traditional and non-

millet urban consumers, and generally improve the pearl millet value

chain.

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Funding: This work was made possible by the support of the American People

provided to the Feed The Future Innovation Lab for Collaborative Research on

Sorghum and Millet through the United States Agency for International Development

(USAID) under Cooperative Agreement No. AID-OAA-A-13-00047. The contents are

the sole responsibility of the authors and do not necessarily reflect the views of

USAID or the United States Government.

Acknowledgements

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Thank you

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