Physico-chemical and nutritional evaluation of cookies with different levels of eggplant flour substitution

Abstract

Four types of eggplant – Chinese eggplant (CE), Green Goddess eggplant (GGE), Indian eggplant (IE), and Thai eggplant (TE) – were used to develop cookies (10% and 15% of substitution). Substitution of eggplant flour has significantly increased the nutritional values of cookies (protein, crude fiber, and fat). Eggplant cookies also showed higher total phenolic content (TPC) and free radical-scavenging effect on the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical compared to control. IE cookies showed the highest TPC and DPPH values, followed by TE, GGE, and CE. Cookies with 15% substitution level of all eggplant flour had significantly lower peroxide values showing greater stability against rancidity and oxidation compare to control cookies (IE > TE > GGE > CE > control). Incorporation of eggplant flour had significantly increased the hardness and a* values (color parameter) of cookies. Overall, the nutritional value of cookies was improved by the substitution with eggplant flour and IE cookies showed the greatest in terms of overall properties.

Con el objetivo de producir galletas en las que se sustituyó 10% y 15% de la harina por harina de berenjena, se utilizó harina de cuatro tipos de berenjena: berenjena china (CE), berenjena Green Goddess (GGE), berenjena india (IE) y berenjena tailandesa (TE). La sustitución de la harina por harina de berenjena aumentó significativamente el valor nutricional de las galletas en términos de su proteína, fibra cruda y grasa. En comparación con las galletas de control, las galletas con harina de berenjena también mostraron contenido fenólico total (TPC) más elevado, así como un efecto más pronunciado de eliminación del radical libre 2,2-diphenyl-1-picrylhydrazyl (DPPH). En las galletas IE se registraron los valores de TPC y de DPPH más elevados, siguiéndoles las galletas TE, GGE y CE. Asimismo, en comparación con las galletas de control, las galletas en las que se sustituyó 15% del nivel de harina por los distintos tipos de harina de berenjena, mostraron valores de peróxido más bajos, lo cual apunta a su mayor estabilidad contra la ranciedad y la oxidación (IE > TE > GCE > Control). La incorporación de harina de berenjena incrementó significativamente los valores de dureza y de a* (parámetros de color) de las galletas. En general, la sustitución con harina de berenjena mejoró el valor nutricional de las galletas. Además, las galletas IE mostraron tener niveles más altos en términos de sus propiedades generales.

Keywords:

• eggplant
• cookies
• flour
• antioxidant

Palabras claves:

• berenjena
• galletas
• harina
• antioxidante

Introduction

Foods with high nutritional value are in great demand for proper functioning of body systems and potential health benefits. As a result, value-added foods or functional foods with higher level of dietary fiber and antioxidant have been developed, especially in bakery products such as cookies. Several researches have recommended the use of composite flour in bakery products because the incorporation of composite flour into these traditional wheat-based food products has provided additional nutrients from non-wheat material, and thus improving the nutritional value of the products (De Ruiter, 1978). Composite flour is actually the blend of wheat and non-wheat flours.

Hence, in relation to good health demands, the nutritional value of cookies can be enhanced by supplementation with other nutrient sources such as eggplant flour. Eggplant (Solanum melongena L.) has a great potential for producing improved nutritional value cookies. Eggplant is an important source of fiber (Jenkins et al., 2003). Dietary fiber such as celluloses, hemicelluloses, lignin, pectic substances, gums, mucilages, and certain polysaccharides are carbohydrate polymers that cannot be digested readily by enzymes in human digestive tracts (Slavin, 2005). High dietary fiber foods have been associated with lower blood pressure, lower incidence of obesity, and higher satiety as well as reducing the risks of gastrointestinal disease, hypercholesterolemia, colorectal cancer, and constipation (Brand, Snow, Nobhan, & Truswell, 1990). The varieties of S. melongena L. display a wide range of fruit shapes and colors. They are ranged from oval or egg-shaped to long club-shaped, and from white, yellow, green through degrees of purple pigmentation to almost black. Figure 1 showed the four different types of eggplants.

Figure 1. Different types of eggplants: (a) Chinese eggplant, (b) Green Goddess eggplant, (c) Indian eggplant, and (d) Thai eggplant.

Figura 1. Distintos tipos de berenjena: (a) berenjena china; (b) berenjena Green Goddess; (c) berenjena india, (d) berenjena tailandesa.

Paganga, Miller, and Rice-Evans (1999) and Urquiaga and Leighton (2000) reported that eggplant plants also contain powerful antioxidants such as ascorbic acid and phenolics. The skin of eggplant contains a phytonutrient that helps protect the lipid in brain cell membranes. Anthocyanin (such as nasunin), phenolic compounds (such as caffeic and chlorogenic acid), and flavonoids compounds are examples of phytonutrients in eggplant (Hanson et al., 2006). The extracts from eggplant fruit skin have been demonstrated to possess high capacity in scavenging of superoxide free radicals and inhibition of hydroxyl radical generation by chelating ferrous iron (Nisha, Abdul Nazar, & Jayamurthy 2009). Eggplant has been reported to be having antioxidant and phenolic compounds that can help prevent cancer and cholesterol build up, high fiber contents which can prevent constipation, hemorrhoids, and colitis and low in calories. According to Hanson et al. (2006), polyphenols extracted from eggplant pulp is believed to be able to inhibit digestive enzymes and reduce food digestibility. Inhibitory action of polyphenolics on digestive enzymes such as amylase, glucosidase, pepsin, trypsin, and lipases has been extensively studied. Polyphenolics are believed to be acting as inhibitors of amylolysis enzymes leading to a decrease in post-prandial hyperglycemia (Vinson, Hao, Su, and Zubik (1998), thus producing food which produces fullness for longer time or also known as LGI food.

Apart from their free-radical scavenging activity, eggplant phenolics also have verified antitumoral and anticholesterolemic properties (Whitaker & Stommel, 2003). Several in vivo studies have also demonstrated that phenolic compounds extracted from eggplant fruit have a significant hypolipidemic action; in addition, eggplant extracts have been found to inhibit protein-activated receptor-2 inflammation associated with atherosclerosis.

Apart from that, eggplant is well-furnished with ascorbic acid and phenolic compounds which exert strong antioxidant properties. Phenolic compounds contribute essential positive effect to the diet by providing potential antioxidant benefits for managing oxidation stress-related chronic diseases such as diabetes and cardiovascular disease. The prime function of antioxidants is to delay the oxidation of other molecules by inhibiting the initiation or propagation of oxidizing chain reactions by free radicals, and thus reducing the oxidative damage to human body (Namiki, 1990).

For this research, eggplants were processed into flour before incorporated into cookies formulation, which will produce improved quality and healthy product. By substituting part of wheat flour with eggplant flour in the formulation, the nutritional content of cookies will be further enhanced. To date, the utilization of eggplant flour as substituted flour in bakery products have not been studied extensively. Therefore, the purpose of this paper was designed to evaluate the effect of substitution of wheat flour with different levels and types of eggplant flour on the physico-chemical and nutritional properties of the cookies. This would provide some insights on the attribute and property of end products with regards to the substitution of eggplant flour as a source of dietary fiber and antioxidant in food industry.

Materials and methods

Materials

Four types of eggplants (S. melongena L.) such as Chinese eggplant (CE) (long cylindrical shape and purple in color), Green Goddess eggplant (GGE) (long cylindrical shape and light green in color), Indian eggplant (IE) (round shape and deep-purple to red-purple in color), and Thai eggplant (TE) (round shape and greenish-white in color) were purchased from local market. These eggplants were ensured to have same maturity in terms of ripeness, freshness, color, and size. All the chemicals used for analysis were of analytical grade.

Preparation of eggplant flour

Fresh eggplants were washed with tap water to remove all the soil and unwanted dirt. The eggplants together with their peels were then sliced into thin slices. After that, slices of eggplants were dried in air dryer (AFOS Limited, Kingston Upon Hull, United Kingdom) at 40°C for 72 hours. Next, dried eggplants were ground with grinder (Stainless Steel Vertical Type High Speed Grinding and Pulverizing Machine, Model RT-34, WHL Machinery, Selangor, Malaysia) and further sieved through 500 and 250 µm mesh sieve. The eggplant flour was then kept and sealed in polypropylene plastic bag and subsequently in airtight container. The flour was stored in refrigerator at 4°C prior to use.

Cookies preparation

In this study, four types of eggplant flour were used to prepare different cookie formulations (Table 1). Eggplant flour was incorporated into the formulation to substitute wheat flour by 10–15% based on wheat flour weight as in control formulation. The cookies were prepared according to the method by American Association of Cereal Chemists (AACC), 10–50D (AOAC, 2000) with slight modification.

Table 1. Formulation of cookies.

Tabla 1. Formulación de galletas.

Ingredients (g)	Control	% of eggplant flour
		10%	15%
Wheat flour	250	225	212.5
Eggplant flour	–	25	37.5
Corn flour	60	60	60
Shortening	60	60	60
Margarine	70	70	70
Sugar	120	120	120
Beaten whole egg	80	80	80
Baking powder	6	6	6
Salt	3	3	3

Nutritional analysis

Proximate analysis was conducted on the control cookies and cookies from the four types of eggplant flour at 10% and 15% substitution (triplicate samples) according to AOAC (2000) standard methods. Samples were analyzed for moisture using IR-30 Moisture Analyzer (Denver Instrument, Norfolk, England), which was set at a temperature of 105°C and preheated for 5 minutes. Crude fat analysis was conducted based on AOAC (2000): Soxhlet Extraction Method (AOAC, 2000). Crude fiber analysis was carried out based on AOAC (2000): Neutralization Method (AOAC, 2000). Ash analysis was carried out based on Dry Ashing Method (AOAC, 2000). Crude protein analysis was conducted based on AOAC (2000): Micro-Kjeldahl Method (AOAC, 2000). Carbohydrate content was calculated using the following equation:

The analysis was carried out in triplicates for all samples and expressed in a dry basis. Samples were ground into fine particles before analysis.

Sample extraction

Sample (1 g) was mixed with 100 ml methanol 80% (v/v) in conical flask (250 ml) wrapped with aluminum foil. The mixture was then shaken in an orbital shaker (SI-600 R) for overnight at 160 rpm and 27°C. Next, the mixture was poured into centrifuge tubes (wrapped with aluminum foil) and centrifuged in a centrifuge (Bench Top Centrifuge KUBOTA) at 2500 rpm for 30 minutes to get a clear solution. Light exposure was prevented throughout the extraction process. The extract obtained was used for total phenolic content (TPC) and antioxidant value (2,2-diphenyl-1-picrylhydrazyl, DPPH) analysis.

Total phenolic content

TPC of cookies were determined by using Folin–Ciocalteu’s assay which was described by Singleton and Rossi (1965) with slight modification.

DPPH activity

The antioxidant capacity of the cookies extracts was studied through the evaluation of the free radical-scavenging effect on the DPPH radical. The determination was based on the method proposed by De Ancos, Sgroppo, Plaza, and Cano (2002) with modification. This method was also called as DPPH free radical-scavenging assay.

Peroxide value

Peroxide value was determined based on AOAC (2000). Analysis was conducted after baking on day 1. After that, same analysis procedure was carried out on day 7, day 14, and day 21.

Texture analysis

Texture of cookies was analyzed within 24 hours after baking. Texture analysis on cookies was conducted by using Single Arm Texture Analyzer (TA.XT.Plus Texture Analyzer, Stable Micro Systems, Godalming, Surrey, United Kingdom). Hardness of cookies was measured by using cylindrical probe P/2 and 30 kg load cell.

Color measurement

Color measurement of cookies was carried out within 24 hours after baking. The color of sample cookies was determined by using a colorimeter (Minolta Spectrophotometer CM-3500D, Petaling Jaya, Selangor, Malaysia). The values of lightness (L*), color-opponent dimensions (a* and b*), illuminant (C*), and hue (h) were recorded. Each sample was analyzed in triplicate and mean value was obtained by calculation.

Statistical analysis

The experimental data were analyzed statistically by using SPSS Version 16.0 software program (SPSS Inc., Chicago, IL, USA). Analysis of variance and Duncan’s multiple range tests were used to determine and compare the statistical differences of each data in this study. p-Value of less than 0.05 was regarded as the significance of result.

Results and discussion

Nutritional analysis

The results of nutritional analysis were shown in Table 2. Moisture contents obtained from all samples were complied with the moisture content of regular cookies (2.5–3.0%) as stated by Manley (2000). The incorporation of eggplant flour into cookies formulation had obviously shown the effect of increasing the moisture content of samples. The difference in moisture content between samples might be due to the high fiber content in eggplant (Jenkins et al., 2003). More hydroxyl groups of cellulose in fiber were able to bind with free water molecules through hydrogen bonding (Rosell, Rojas, Benedito De Barber, Nobhan, & Truswell, 2001), and thus resulting in greater water holding capacity. Therefore, the higher the substitution of eggplant flour, the greater the moisture content value. The high ash content in substituted cookies was due to high mineral contents such as calcium, magnesium, phosphorus, sodium, and potassium found in eggplant. Hence, as the level of eggplant flour substituted increased, the ash content of cookies made was also increased.

Table 2. Proximate composition for different formulations of cookies.

Tabla 2. Composición proximal de las distintas formulaciones de galletas.

Types of cookies samples	Composition (%)
	Moisture	Ash	Crude protein	Crude fat	Crude fiber	Carbohydrate
Control	2.51 ± 0.03^a	1.57 ± 0.03^a	8.12 ± 0.02^a	23.85 ± 0.22^a	0.29 ± 0.02^a	63.94 ± 0.23^g
10% CE	2.88 ± 0.02^d	1.74 ± 0.02^b	8.19 ± 0.03^b	24.96 ± 0.07^d	0.76 ± 0.03^c	62.23 ± 0.08^c
15% CE	2.91 ± 0.02^de	1.92 ± 0.02^e	8.21 ± 0.02^bc	25.87 ± 0.03^f	1.65 ± 0.03^d	61.10 ± 0.03^a
10% GGE	2.78 ± 0.02^b	1.82 ± 0.01^d	8.24 ± 0.02^cd	23.98 ± 0.04^a	0.68 ± 0.02^b	63.18 ± 0.05^f
15% GGE	2.81 ± 0.02^bc	1.97 ± 0.02^f	8.34 ± 0.01^e	25.65 ± 0.16^e	1.72 ± 0.03^e	61.22 ± 0.15^a
10% IE	2.81 ± 0.01^bc	1.79 ± 0.01^c	8.25 ± 0.02^d	24.44 ± 0.08^c	0.76 ± 0.03^c	62.71 ± 0.08^d
15% IE	2.83 ± 0.01^c	1.97 ± 0.01^f	8.40 ± 0.02^f	25.13 ± 0.04^d	1.65 ± 0.03^d	61.67 ± 0.02^b
10% TE	2.89 ± 0.02^d	1.76 ± 0.01^b	8.21 ± 0.03^bc	24.19 ± 0.08^b	0.68 ± 0.02^b	62.95 ± 0.05^e
15% TE	2.93 ± 0.02^e	1.93 ± 0.02^e	8.38 ± 0.02^f	25.54 ± 0.04^e	1.63 ± 0.03^d	61.22 ± 0.04^a

Notes: Mean ± standard deviation (n = 3).

Means within the same column bearing the same superscript letter were not significantly different (p > 0.05).

CE = Chinese eggplant flour, GGE = Green Goddess eggplant flour, IE = Indian eggplant flour, TE = Thai eggplant flour.

Notas: La media ± desviación estándar (n = 3).

Medios dentro de la misma columna que lleva el mismo superíndice no fueron significativamente diferentes (p > 0,05).

CE = Chinese harina de berenjena, GGE = Green Goddess harina de berenjena, IE = Indian harina de berenjena, TE = Thai harina de berenjena.

From the results in Table 2, crude protein contents were detected to be significantly increased cookies substituted with each type of eggplant flour compared to control cookies (although the increment is very low). The incorporation of GGE, IE, and TE had greatly increased the crude protein content with 15% substitution and showed higher increment up to 25%. Fiber content in cookies is among the most essential characteristics in this research since high fiber product will bring much advantageous impacts to the human body system.

Crude fiber contents for control cookies were found to be significantly low when compared to all other cookies substituted with eggplant flour. Results showed that larger amount of eggplant flour substituted into formulation will also increase the fiber content in cookies to a much higher value, and this is in accordance with the findings of Jenkins et al. (2003). Addition of eggplant flour has definitely increased the values of the cookie produced as it can significantly increase the fiber content. All 15% addition of eggplant flour showed significant increment up to 400 folds higher when compared to control cookies. Dietary fiber has a significant role in human nutrition. It promotes beneficial physiological effects including laxation, blood cholesterol attenuation, and blood glucose attenuation. Epidemiologic studies have clearly shown that dietary fiber is an important component in the prevention of obesity (Slavin, 2005) and generally associated with food which has low glycemic index.

Analysis of TPC

Table 3 showed the results of TPCs for different formulations of cookies. The trend indicated that as more eggplant flour substituted into formulation, the TPC of cookies became higher. IE has been shown to be a great source of phenolic compounds with highest TPC value (average: 284 mg gallic acid equivalent; GAE) compared to control cookies. This was followed by TE (254 mg GAE), GGE (241 mg GAE), and finally CE (192 mg GAE) (Control < CE < GGE < TE < IE). Higher substitution level (15%) showed significantly higher values of TPC compared to 10% substitution. This result was mainly due to the correlation to the fact that eggplants contain large amount of phytochemicals especially phenolic compounds (Whitaker & Stommel, 2003). Different amounts of phenolic compounds present in different eggplants may be due to high diversity and various proportions of phenolic compounds found in individual eggplant types.

Table 3. Total phenolic contents for different formulations of cookies.

Tabla 3. Contenidos fenólicos totales para las distintas formulaciones de galletas.

Types of cookies samples	Total phenolic content (mg GAE per 100 g of sample)
Control	123.18 ± 2.54^a
10% CE	187.52 ± 1.47^b
15% CE	197.98 ± 1.27^c
10% GGE	207.13 ± 1.27^d
15% GGE	275.08 ± 3.37^g
10% IE	246.72 ± 1.94^f
15% IE	322.40 ± 1.28^i
10% TE	222.50 ± 2.66^e
15% TE	286.17 ± 1.95^h

Notes: Mean ± standard deviation (n = 3).

Means within a column bearing the same superscript letter were not significantly different (p > 0.05).

Notas: La media ± desviación estándar (n = 3).

Medios dentro de la misma columna que lleva el mismo superíndice no fueron significativamente diferentes (p > 0,05).

TPC of eggplant was mainly due to the presence of chlorogenic acid and anthocyanin in eggplant. Chlorogenic acid is among the major phenolic in eggplant (Kahlon, Chapman, & Smith, 2007; Whitaker & Stommel, 2003) and was present in tissues from all zones of the eggplant. Nasunin, an anthocyanin phytonutrient, was isolated from the skin of purple eggplant fruit (Noda, Kneyuki, Igarashi, Mori, & Packer, 2000). Thus, these bioactive compounds would contribute to the presence of total phenolic values in eggplant cookies.

Analysis of antioxidant activity

Results for the percentages of scavenging or inhibition of DPPH free radical activity can be observed in Table 4. Control cookies showed significantly lower DPPH radical-scavenging activity than the other cookies with eggplant flour substitution. The higher incorporation of eggplant flour into cookies had significantly elevated the antioxidant activity for few folds compared to the original control cookies without eggplant flour (Control < CE < GGE < TE < IE).

Table 4. Percentages of scavenging or inhibition of DPPH radical activity.

Tabla 4. Porcentajes de eliminación o inhibición de la actividad del radical DPPH.

Types of cookies samples	% inhibition of DPPH
Control	10.69 ± 0.26^a
10% CE	25.66 ± 0.19^b
15% CE	29.66 ± 0.31^d
10% GGE	29.01 ± 0.33^c
15% GGE	35.78 ± 0.19^f
10% IE	34.43 ± 0.31^e
15% IE	38.68 ± 0.31^g
10% TE	29.27 ± 0.50^cd
15% TE	35.96 ± 0.50^f

Notes: Mean ± standard deviation (n = 3).

The mean values were determined based on dry basis.

Means within a column bearing the same superscript letter were not significantly different (p > 0.05).

Notas: La media ± desviación estándar (n = 3).

Los valores medios se determinaron con base en base seca.

Medios dentro de la misma columna que lleva el mismo superíndice no fueron significativamente diferentes (p > 0,05).

Obviously, IE provided a good scavenging activity toward DPPH radical and this was fully matched with its highest TPC as can be seen from previous section’s result. This result complied with the research done by Nisha et al. (2009), which reported that extract from round, purple color, and small size eggplant fruit illustrated better antioxidant properties and a linear relation was observed between the TPC and antioxidant parameters. A study by Namiki (1990) also showed that the TPCs of S. melongena (eggplant) were positively correlated with the antioxidant activities. High diversity of number of phenolic compounds as well as the proportions of phenolic compounds in individual eggplant types (Whitaker & Stommel, 2003) probably explained the differences in the antioxidant activities obtained in this study.

Peroxide value

From the results in Table 5, it can be seen that the peroxide values of every sample tend to increase gradually as number of storage days increased from day 1 to day 21. As shown, the substitution with eggplant flour has reduced the rancidity level (peroxide value). The effect of incorporation of IE into cookies formulation was illustrated to have highest reduction capacity for the rancidity reaction due to its lowest value for peroxide. The peroxide values of every formulation of cookies were significantly different (p < 0.05) at day 1, day 7, day 14, and day 21, respectively, when compared for different cookies samples at the same day of storage.

Table 5. Peroxide values for different formulations of cookies.

Tabla 5. Valores de peróxido para las distintas formulaciones de galletas.

Types of cookies samples	Storage period	Peroxide values (meq/kg of sample fat)
Control	Day 1	8.98 ± 0.02^aH
	Day 7	9.08 ± 0.19^aG
	Day 14	9.17 ± 0.23^abH
	Day 21	9.43 ± 0.04^bH
		10% substitution	15% substitution
CE	Day 1	7.56 ± 0.24^bcG	7.05 ± 0.21^aF
	Day 7	7.76 ± 0.24^cdF	7.25 ± 0.26^abE
	Day 14	7.93 ± 0.04^dG	7.41 ± 0.04^bcF
	Day 21	8.06 ± 0.18^dG	7.54 ± 0.22^bcF
GGE	Day 1	6.59 ± 0.22^cE	5.46 ± 0.02^aC
	Day 7	6.80 ± 0.28^cdD	5.77 ± 0.26^abC
	Day 14	6.90 ± 0.05^cdE	5.93 ± 0.06^bC
	Day 21	7.08 ± 0.19^dE	6.08 ± 0.22^bC
IE	Day 1	5.93 ± 0.03^cD	4.48 ± 0.01^aA
	Day 7	6.08 ± 0.24^cC	4.59 ± 0.24^aA
	Day 14	6.23 ± 0.26^cdD	4.77 ± 0.26^abA
	Day 21	6.43 ± 0.03^dD	4.94 ± 0.02^bA
TE	Day 1	6.58 ± 0.25^cE	5.12 ± 0.24^aB
	Day 7	6.69 ± 0.24^cD	5.27 ± 0.27^abB
	Day 14	6.89 ± 0.06^cdE	5.44 ± 0.05^abB
	Day 21	7.08 ± 0.25^dE	5.59 ± 0.22^bB

Notes: Mean ± standard deviation (n = 3).

Means bearing the same uppercase letter were not significantly different (p > 0.05) between different samples at the same day of storage.

Means bearing the same lowercase letter were not significantly different (p > 0.05) between same samples at different days of storage.

Notas: La media ± desviación estándar (n = 3).

Medios de soporte de la misma letra mayúscula no fueron significativamente diferentes (p > 0,05) entre las diferentes muestras en el mismo día de almacenamiento.

Medios de soporte de la misma letra minúscula no fueron significativamente diferentes (p > 0.05) entre las mismas muestras en diferentes días de almacenamiento.

On the other hand, for the comparison of same samples at different days of storage, the peroxide value readings for cookies of 15% substitution level with each type of eggplant flour were much significantly lower than the cookies with 10% substitution level. This was in complement to the fact that the higher substitution of eggplant flour which will practically reduce the peroxide value due to improvement of stability toward oxidation, contributed by phenolic compounds in eggplant (Bandak & Oreopoulou, 2011). From this research, it can be concluded that all formulations of cookies made were acceptable in terms of peroxide value attribute.

Texture analysis

The mean values illustrated in Table 6 revealed that there were significant difference (p < 0.05) between the hardness value of control cookies and the other cookies substituted with eggplant flour. Cookies made with GGE were the hardest in texture (25.61 ± 0.17 and 28.27 ± 0.05 for 10% and 15%, respectively). The hardness of cookies is much affected by the composition of flour. A few studies conducted showed that there was positive correlation of fiber and protein contents with the hardness value of cookies made (Piazza & Masi, 1997). The increase of cookies’ hardness was observed as the percentage of fiber compound substitution increased (Arora & Camire, 1994). This was in consistent with the result obtained as eggplant was an important source of fiber (Jenkins et al., 2003). According to Noda et al. (2000), dough prepared from high-absorption flour will have a hard texture. Hemicellulose is a part of fiber components that contributes to the highest water holding capacity. Therefore, high fiber content in eggplant flour was evident to produce cookies with hard texture.

Table 6. Hardness values for different formulations of cookies.

Tabla 6. Valores de dureza para las distintas formulaciones de galletas.

Types of cookies samples	Hardness (N)
Control	12.41 ± 0.12^a
10% CE	22.60 ± 0.03^d
15% CE	24.62 ± 0.21^f
10% GGE	25.61 ± 0.17^g
15% GGE	28.27 ± 0.05^h
10% IE	20.57 ± 0.09^b
15% IE	23.38 ± 0.28^e
10% TE	21.19 ± 0.25^c
15% TE	24.58 ± 0.04^f

Notes: Mean ± standard deviation (n = 3).

The mean values were determined based on dry basis.

Means within a column bearing the same superscript letter were not significantly different (p > 0.05).

Notas: La media ± desviación estándar (n = 3).

Los valores medios se determinaron con base en base seca.

Medios dentro de la misma columna que lleva el mismo superíndice no fueron significativamente diferentes (p > 0,05).

The hardness of the cookies is due to the presence of dietary fiber in eggplant flour. For all types of cookies (with addition of eggplant), 15% addition showed significantly higher values of hardness compared to 10% cookies. Eggplant has been classified as a fruit with high dietary fiber. Dietary fiber can be defined as the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine (Slavin, 2005).

Color measurement

Table 7 illustrated that control cookies had significant difference in terms of L*, a*, b*, C*, and h values compared to all other cookies made by substitution with eggplant flour. It can be observed that control cookies had significantly highest L* value than other eggplant flour substituted cookies (71.51% ± 0.05). This statement was supported by Vratanina and Zabik (1978), who stated that the decrease in lightness of cookies was observed as the substitution level of fiber into formulation was elevated.

Table 7. Color measurement for different formulations of cookies.

Tabla 7. Medición de color para las distintas formulaciones de galletas.

Types of cookies samples	L*	a*	b*	C*	h
Control	71.51 ± 0.05^i	3.35 ± 0.07^a	28.01 ± 0.04^g	28.19 ± 0.02^g	83.18 ± 0.01^h
10% CE	55.43 ± 0.02^h	8.88 ± 0.02^f	25.38 ± 0.03^b	26.89 ± 0.03^c	70.69 ± 0.02^d
15% CE	53.55 ± 0.02^g	10.91 ± 0.04^h	27.70 ± 0.02^f	29.75 ± 0.01^h	68.54 ± 0.02^b
10% GGE	52.45 ± 0.02^e	7.16 ± 0.01^c	26.15 ± 0.01^d	27.14 ± 0.04^d	74.69 ± 0.03^g
15% GGE	51.73 ± 0.01^c	8.26 ± 0.02^e	26.84 ± 0.03^e	28.08 ± 0.03^f	72.91 ± 0.02^f
10% IE	52.40 ± 0.02^d	10.20 ± 0.03^g	28.41 ± 0.03^h	30.17 ± 0.02^i	70.29 ± 0.03^c
15% IE	50.16 ± 0.01^a	11.04 ± 0.04^i	25.69 ± 0.02^c	27.95 ± 0.01^e	66.82 ± 0.04^a
10% TE	53.18 ± 0.02^f	7.04 ± 0.01^b	25.71 ± 0.01^c	26.65 ± 0.02^b	74.71 ± 0.04^g
15% TE	51.04 ± 0.02^b	8.15 ± 0.01^d	24.97 ± 0.01^a	26.24 ± 0.03^a	71.93 ± 0.01^e

Note: Means within a column bearing the same superscript letter were not significantly different (p > 0.05).

Notas: Medios dentro de la misma columna que lleva el mismo superíndice no fueron significativamente diferentes (p > 0,05).

Control cookies had significant difference in b* and C* values compared to other cookies. The differences between those substitutions could be due to uneven exposure of cookies’ surface area to high baking temperature and colored compounds formed from chemical reactions such as caramelization and Maillard reaction which occurred during baking (Purlis & Salvadori, 2007).

Hue (h) refers to a term that describes the pure spectrum color without tint or shade. Hue is one of the main properties of color which can be defined technically as the degree to which a stimulus can be presented as similar to or different from stimuli that are described as red, yellow, blue, and green. As can be seen from Table 7, the increasing substitution of eggplant flour significantly reduced the hue value, with the highest hue value for control cookies. The color changed from light yellow, followed by light yellow-brown, and finally to dark yellow-brown (Control > GGE > TE > CE > IE).

Color tonality of cookies is mainly depending on the amount of sugar and protein in the formulation because Maillard reaction is the prime chemical reaction in the bakery products during baking. Borrelli et al. (2003) reported that the reaction between protein and carbohydrate was responsible for the brown color and organoleptic properties of bakery products. The amount of water or moisture content on the dough surface dramatically decreased when heat was applied, which provided an optimum condition for Maillard reaction to take place in product and resulted in intense brown color.

Conclusion

Nutritional analysis revealed that the increasing substitution level of eggplant flour increased the nutritional content (crude fat, crude protein, and crude fiber) when compared to control cookies. Higher substitution level of eggplant flour (15%) precisely improved the TPC and DPPH free radical-scavenging activity of cookies with the order IE > TE > GGE > CE. A significant correlation observed between TPC and antioxidant activity indicated that phenolic compounds contributed significantly to antioxidant activity of eggplant. Eggplant flour also provided greater stability toward oxidation or rancidity but increased the hardness value of cookies. For the color properties, the substitution of eggplant flour reduced the L* and hue value but increased the a* value. Overall, it can be deduced that the substitution of wheat flour with eggplant flour into the formulation of cookies had enhanced the nutritional value of cookies. Eggplant products could bring much potential health benefits to the consumers. Cookies made by IE provided a good source of fiber as well as greatest TPC and antioxidant activity with better stability against rancidity and oxidation.

Additional information

Funding

This work was funded by Universiti Sains Malaysia Short Term Grant.