INSTRUMENTAL AND SENSORY TEXTURAL PROPERTIES OF FURA

ABSTRACT

A uniaxial compression test and sensory textural analysis was conducted of fura samples made from millet flour. Significant differences (p<0.05) existed among the samples for hardness (the force to compress the sample between molar teeth), cohesiveness (extent to which sample falls apart during chewing) and gumminess (denseness and cohesion persisting during mastication). Correlations between sensory and instrumental tests revealed that a significant relationship exists between modulus of deformability and cohesiveness (r=−0.93, p<0.05); gradient and springiness (r=−0.90, p<0.05); deformation at failure and chewiness (r=0.98, p<0.05); energy per unit mass and gumminess (r=−0.95, p<0.05). A fura quality scale was established based on the peak force; soft and poor quality fura have a peak force of <19 kN; acceptable fura has a peak force of 19–24 kN; a firm and good quality fura, 24–25 kN; very hard and very poor quality fura have a peak force of >25 kN.

INTRODUCTION

In tropical Africa, cereal grains are milled and used to produce thick porridges which are known by various names in different parts of the continent. In West Africa particularly in Nigeria, Ghana and Burkina Faso, one such thick porridge is called ‘fura’—a semi-solid dumpling cereal meal.[1] Fura is produced mainly from moist pearl millet flour, blended with spices, compressed into balls and boiled for 30 min. While still hot, the cooked doughs is worked in the mortar with the pestle (with addition of hot water) until a smooth, slightly elastic, cohesive lump (fura) was formed. The fura dough is rolled into 25–30 g balls by hand and dusted with some flour. The fura is made into porridge by crumbling the fura balls into fermented whole milk (kindrimo) or fermented skim milk (nono).1-2 Sugar may be added to increase taste. The mixture is called ‘fura da nono’ in Nigeria. It is a popular mid-day meal.

Since fura is eaten by chewing between the molars, the textural parameters are particularly important. Texture can be regarded as a manifestation of the rheological properties of a food. It is an important attribute since it affects processing and handling, influences food habits and affects shelf-life and consumer acceptance of foods.3-4 Texture is a sensory property and makes sense only when viewed as a sensory property or “how a food feels in the mouth”.[4] Mohamed et al.[5] studied the effect of flour-to-water ratio and time of testing on sorghum porridge firmness as determined by a uniaxial compression test. The test gave texture results in basic universal unit. Reporting fura texture in basic universal units can facilitate exchange of information between different programs evaluating new millet cultivars for fura quality, and it could lead to the development of standard quality indices. Quality optimisation requires looking at various parameters affecting a food product, particularly a ready-to-eat food like fura.

The objective of this study was to evaluate the instrumental textural properties of fura using uniaxial compression test and correlate the result with sensory properties.

MATERIALS AND METHODS

Source of Sample

Fura samples from four different processors were purchased from a local market in Bauchi, Bauchi State, Nigeria. The samples were sealed in clean low-density polyethylene bags. The samples analysed for proximate composition, sensory and instrumental rheological characteristics related to texture.

Proximate Analysis

The fura samples (50 g) from each retailer were dried and analysed for moisture, crude protein, crude fat, crude fibre and ash using AOAC methods.[6] The carbohydrate content of the samples were estimated by difference. Food energy (kJ/100 g) was calculated according to the method of Marero et al.[7] using the factor

where %protein=protein content of the fura; %carbohydrate=carbohydrate content of the fura; %fat=fat content of fura; 4, 4, and 9 are kilocalories from protein, carbohydrates and fat respectively; 4.2 is a factor for converting from calories to joules.

All determinations were carried out in triplicate and the results expressed on dry weight basis.

Sensory Evaluation

The texture of fura was evaluated by a 10-member trained sensory panel. The panel consisted of students and staff from Abubakar Tafawa Balewa University, Bauchi, Nigeria. Panel training consisted of two 1-h sessions. In the first, session the purpose of the project was explained and the panelists were familiarised with texture terminology such as hardness, cohesiveness, chewiness, gumminess, springiness, adhesiveness, viscosity and fracturability. During the second session, panelists were allowed to discuss the texture terms and develop a ballot for fura. The final ballot consisted of hardness, cohesiveness, chewiness, gumminess, and springiness. Each characteristic was evaluated on a 10 cm unstructured line scale anchored 1 cm from each end with terms representing extremes of the characteristic. Characteristics evaluated, guidelines for evaluating each characteristic and anchor terms for each are presented in Table 1.

Table 1. Sensory Texture Profile Procedure for Fura (Panel Techniques and Definition of Terms)

Characteristics, Scoring Criteria and Guidelines for Evaluation			Scale Anchors^a
Stage 1:	Place a piece of fura in the mouth, chew the fura once between molars. Evaluate for:
	Hardness–	Force required to compress the sample between molar teeth	Soft to hard
	Springiness–	The rate at which fura returns to its original condition after you release pressure	Stays compressed to quickly returns
Stage 2:	Place one fura ball in mouth and chew until ready to swallow. Evaluate for:
	Cohesiveness–	Extent to which sample falls apart during chewing	Not cohesive to very cohesive
	Chewiness–	Number of chews to reach final swallow point	Low to high
	Gumminess–	Denseness and cohesion persisting during mastication	Not gummy to very gummy
	Starchiness–	Feeling of free starch	Not starchy to very starchy

^aFirst term anchors left end of scale; second term anchors right.

Fura Firmness Measurement

Compression tests were performed on cylindrical samples (75 mm height, 38.9 mm diameter) on the ELE International Tritest 10 (107 kN load frame) at unconfined testing mode. The samples were uniaxially compressed until complete breakdown under a 38.9 mm diameter stainless steel plate at a constant deformation rate of 1.5 mm/min. The load ring reading was noted for each strain applied. The method used by Head[8] was used to calculate the strain and stress. A plot of stress against strain was obtained. From the stress-strain curve we calculated the strength of fura in terms of breaking stress (maximum stress at failure, force per unit area); deformation at failure (an indication of the brittleness of the sample) showing how far a sample can be deformed before fracture; the gradient of the slope to the point of failure (indicates sample toughness; the higher the gradient, the tougher the sample i.e., ease of chewing); Modulus of deformability (a measure of stiffness i.e., materials resistance to deformation). From the force-strain curve we calculated the firmness of fura in terms of hardness (maximum force of the force-deformation curve); energy per unit sample mass (EPM) required to shear compress fura was calculated by integrating the area under the force-deformation curve up to the maximum force of the second peak and dividing by the mass of fura; and firmness expressed as maximum force divided by the mass of fura.[5], 9-11 Each sample was tested in triplicate.

RESULTS AND DISCUSSION

Chemical Composition of Fura

The chemical properties of four samples of market fura are detailed in Table 2. Significant differences exist among the samples in all the chemical attributes. Differences in the chemical properties of fura may be attributed to variations in the amount of ingredients used by different processors. There is no standard measure for the proportion of flour to spices used. These ingredients are added according to the processor's taste.

Table 2. Chemical Composition of Fura¹

	Proximate Composition (%)^2
Sample	Dry Matter^3	Ash	Protein (N2×6.25)	Fat	Crude Fibre	Carbohydrate	Energy (kJ/100 g)	Total Sugars^4 (%)	Amylose^4 (%)
QS1	45.0^a	1.25±0.01^a	10.1±0.1^a	2.9±0.3^a	1.14±0.2^a	84.7±0.1^a	1702±0.4^a	6.82±0.08^a	4.52±0.12^a
QS2	40.0^b	1.48±0.02^b	8.4±0.1^b	2.4±0.2^a,b	1.21±0.01^a	86.5±0.1^b	1685±1.7^a	6.26±0.21^b	5.80±0.28^b
QS3	40.0^b	2.25±0.01^c	7.6±0.1^c	2.2±0.2^b	1.82±0.1^b	86.1±0.1^c	1657±0.4^b	1.82±0.31^c	1.90±0.42^c
QS4	45.0^a	2.30±0.01^d	7.3±0.1^d	2.1±0.01^b	1.85±0.07^b	86.3±0.1^b	1652±0.4^b	0.41±0.01^d	2.70±0.42^c

¹Any two means in a column followed by different letters differ significantly (α=0.05) by Duncan's multiple range test.

²Dry basis.

³% of total sample.

⁴% of total solids.

Textural Characteristics of Fura

A typical force-deformation curve of fura is as shown in Fig. 1. The force-deformation curve of fura exhibited a first peak corresponding to the point at which the sample initially yielded and a second peak at which the shear-compression force reached the maximum. The same trend was reported for akara balls.[12]

Figure 1. A typical force-deformation curve of fura.

Table 3 details the instrumental textural characteristics of fura. Fura firmness ranged from 0.202 to 0.241 kN/g corresponding to a 95% confidence interval of 0.200 to 0.238 kN/g. The Energy per unit mass of fura (EPM) ranged from 1.14 to 1.32 J/g, with a 95% confidence interval of 1.16 to 1.32 J/g. The peak force of the fura samples ranged from 19.4 to 25.4 kN, with a 95% confidence interval of 19.0 to 24.2 kN. There was no significant difference (p>0.05) in firmness, EPM and hardness among the fura samples.

Table 3. Strength and Firmness of Fura Samples

	Strength				Firmness
Sample Code	Breaking Stress (N/m)^2	Deformation at Failure (%)^a	Modulus of Deformability (N/m^2)^b	Gradient^c	Firmness (kN/g)	EPM^1 (J/g)^d	Peak Force (Hardness) (kN)
QS1	20.8±1.1	14.0±1.0	88.8±4.6	107.9±3.2	0.241±0.010	1.23±0.17	25.4±1.5
QS2	17.1±1.3	12.7±0.9	73.8±3.2	97.2±4.6	0.202±0.016	1.14±0.3	19.4±1.5
QS3	17.6±1.6	14.7±1.9	85.0±8.5	89.8±2.5	0.204±0.019	1.26±0.13	20.0±1.3
QS4	20.7±1.6	14.0±1.0	92.2±6.2	108.7±2.2	0.229±0.051	1.32±0.18	21.6±2.5

¹Energy per unit sample mass.

^aCohesiveness.

^bSpringiness.

^cChewiness.

^dGumminess.

The breaking stress of the fura samples ranged from 17.1 to 20.8 N/m². The 95% confidence interval for the breaking stress lies between 17.1 to 21.1 N/m². Deformation at failure ranged from 12.7 to 14.7%, with a 95% confidence interval of 13.1 to 14.7%. The resistance of fura to deformation (stiffness) characterised by modulus of deformability ranged from 73.8 to 92.2 N/m² with a 95% confidence interval of 77.2 to 92.8 N/m². The toughness (ease of chewing) of fura measured as the gradient of the stress-strain curve ranged from 89.8 to 108.7 with 95% confidence interval of 92.0 to 109.8. There was no significant difference (p>0.05) in breaking stress, deformation at failure, modulus of deformability and toughness of among the samples.

The ease with which fura yields under an increasing compression load is quite low (13.1 to 14.7%). According to Szczesniak[13] the smaller the deformation under a given load, the lower the cohesiveness and the greater the ‘snappability’ of the product. Szczesniak[13] defined brittleness as the force required to deform a substance. Both crispness and brittleness are measures of the force required to deform a substance. Materials exhibiting greater slope and lower resistance to deform are considered to be more brittle (i.e., less cohesive).[12] Based on the calculated gradient, sample QS4 is more cohesive than the other samples, however not significantly.

Comparison of the various instrumental parameters showed that only three relationship was significant. Firmness showed a strong positive association with peak force (r=0.93, p<0.05), and breaking stress (r=0.97, p<0.05). Modulus of deformability is positively correlated with energy per unit mass (EPM) (r=0.94, p<0.05). A fura quality scale was established based on the peak force; soft and poor quality fura will have a peak force of <19 kN; acceptable fura has a peak force of 19–24 kN; a firm and good quality fura, 24–25 kN; very hard and very poor quality fura will have a peak force of >25 kN.

Sensory Textural Properties

The panel responses for the sensory texture of each sample was normalised by multiplying each response by the inverse of the variance.[14] The mean normalised responses for the sensory attributes of fura are presented in Table 4. The panelists could not detect any difference in the fura samples for springiness (the rate at which fura returns to its original condition after deformation), chewiness (the length of time required to prepare the sample for swallowing)[15] and starchiness (feeling of free starch grains). However, significant differences (p<0.05) existed among the samples for hardness (the force to compress the sample between molar teeth, Brady and Hunecke 1985),[15] cohesiveness (extent to which sample falls apart during chewing) and gumminess (denseness and cohesion persisting during mastication).[16]

Table 4. Mean Normalised¹ Sensory Scores of Texture Attributes of Fura²

	Texture Attributes^3
Sample code	Hardness	Springiness	Cohesiveness	Chewiness	Gumminess	Starchiness
FTP1	2.3±1.1^a	0.4±0.4^a	0.5±0.3^a	0.8±0.4^a	0.4±0.3^a	1.0±0.5^a
	(1.0−4.2)	(0.0−1.2)	(0.1−0.9)	(0.3−1.5)	(0.1−0.9)	(0.2−1.8)
FTP2	0.9±0.5^b	0.4±0.4^a	1.0±0.6^b	0.8±0.4^a	1.2±0.7^b	1.0±0.5^a
	(0.1−1.9)	(0.1−1.3)	(0.2−1.9)	(0.2−1.6)	(0.3−2.2)	(0.4−1.7)
FTP3	0.6±0.6^b	0.6±0.5^a	0.7±0.4^a,b	1.0±0.4^a	0.6±0.3^a	1.0±0.5^a
	(0.1−1.8)	(0.1−1.7)	(0.2−1.4)	(0.5−1.9)	(0.0−1.1)	(0.5−1.9)
FTP4	0.4±0.3^b	0.4±0.4^a	0.6±0.4^a	1.1±0.4^a	0.6±0.4^a	0.8±0.4^a
	(0.0−1.0)	(0.1−0.7)	(0.1−1.4)	(0.4−1.7)	(0.1−1.5)	(0.1−1.4)

¹Normalised sensory figures obtained by weighing each panel score by the inverse of the associated variance (Davis and Goldsmith, 1988).

²The maximum full scale of score is 10 cm (0 the weak and 10 the strong). Values are mean (n=10), standard deviation and range in parenthesis.

³Different lower case in the same column designate statistical differences among samples according to Duncan's multiple range test (α=0.05).

Examination of the correlations among the six sensory parameters revealed that only one of those relationship was significant. Cohesiveness showed a strong positive correlation with gumminess (r=0.98, p<0.05). However, chewiness showed a strong negative relationship (not significant) with hardness (r=−0.74, p>0.05). Since samples compressed in the mouth with low levels of force were not judged to be chewy, this negative relationship was expected. Similar findings were reported by Brady et al.[17] for beef and beef-soy loaves.

Relationship Between Instrumental and Sensory Texture of Fura

Correlations between sensory and instrumental tests revealed that a significant relationship exists between modulus of deformability and cohesiveness (r=−0.93, p<0.05); gradient and springiness (r=−0.90, p<0.05); deformation at failure and chewiness (r=0.98, p<0.05); energy per unit mass and gumminess (r=−0.95, p<0.05). The strong correlations between these sensory and instrumental characteristics would indicate that these parameters were measuring either the same element of texture or ones that were strongly related and would be reliable in predicting sensory evaluations of these characteristics.[16]

CONCLUSION

In summary, some relationships existed between sensory texture profile parameters and instrumental rheological profile values related to texture for fura. The relationships identified in this study provide an understanding of the textural elements being evaluated and provide a basis for relating measurements made by sensory panels and instrumental rheological procedures. However, more work needs to be done in order to totally define the relationships between sensory and instrumental tests of fura texture.

Acknowledgments