Partially defatted coconut flour as a functional ingredient in replacement of refined wheat flour for development of fiber rich muffins

Abstract

In the current study, partially defatted coconut flour (PDCF) was prepared using deoiled cake obtained after virgin coconut oil processing. The proximate analysis of PDCF revealed the presence of 5.21% moisture, 14.03% fat, 21.34% protein, 46.39% fiber and 3.27% ash, along with prominent functional properties. Herein, muffins were prepared by incorporation of PDCF as a replacement of refined wheat flour (RWF) at 0, 25, 50, 75 and 100% flour blend. Significant changes in batter rheology were observed after 25 to 100% replacement of RWF with PDCF, which indicated a decrease in peak viscosity and final viscosity by 65.05 to 83.59% and 61.57 to 85.17% respectively, an increase in specific gravity of batter by 0.857–0.929 g/L. The prepared muffins had significant variations in colour of crust and crumb regions as represented by changes in L*a*b*, Hue angle (h°) and Chroma (C) and textural properties such as hardness, springiness, guminess, cohesiveness, chewiness, and resilience. Incorporation of 50% PDCF significantly (P < 0.05) increased the overall acceptability of the muffins (with a maximum score of 8.5), with a fiber content of 5.53 ± 0.23% and protein content of 7.57 ± 0.30%. Storage stability studies performed at 25 ± 2 °C for seven days revealed an increase in microbial count, and reduction in textural properties but both to be in acceptable regime.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05857-2.

Introduction

Dietary fiber is currently a subject of significant interest and discussion, as its importance was not well-established until the twentieth century, unlike many other food components. Prior to this, very few reports have documented the beneficial effects of dietary fiber on human (Dhingra et al. 2012; Pawar et al. 2020; Lin 2022). However, as food processing has progressed, much of the naturally occurring dietary fiber has been removed due to refining processes aimed at improving food quality with improved taste and texture (Ananthanarayan et al. 2019; Ray et al. 2021). Recent epidemiological studies suggest that a decrease in dietary fiber consumption is linked to an increase in certain illnesses, such as gastrointestinal disease, hypercholesterolemia, and colorectal cancer (Patwardhan et al. 2015). Medical observations have demonstrated the benefits of incorporating dietary fiber (DF) into the diet, including a reduction in post-prandial blood glucose and insulin levels, a decrease in serum cholesterol (Yuan et al. 2014; Mao et al. 2021). As consumers become increasingly aware of the importance of dietary fiber in their diets, the demand for healthful, flavorful, high fiber foods has grown.

In response to this, several studies are performed to enhance the fiber content of baked goods by incorporating it from various sources (Indrani et al. 2015; Adsare et al. 2016; Palacio et al. 2018; Lin 2022). Dietary fibers from different sources possess distinct nutritional and functional characteristics due to variations in their structural and chemical composition. Thus, dietary fiber from different sources have a significant impact on rheological properties and quality of bakery products which varies with source (Vijayendra and Sreedhar 2023). One way to enhance the fiber content in baked goods is by using a variety of fiber-rich products to partially substitute for flour, sugar, or fat in the recipe. By incorporating ingredients like mango or potato peels, apple fiber (Struck et al. 2016), fruit such as orange, apple or grape pomace (Grigelmo-Miguel et al. 2001; Rupasinghe et al. 2008) in baked goods can be made to contain more dietary fiber without compromising taste or texture.

Consumers tends towards the high value sweet baked goods, especially muffins, due to their soft texture and distinct taste. The primary components of muffins, including flour, fat, eggs, and sugar, play a crucial role in determining the final product’s structure, appearance, and overall eating experience (Martínez-Cervera et al. 2012). In addition to their soft texture and taste, muffins are recognized for their porous structure and spongy consistency. Various factors impact these attributes, including density, crust-to-crumb ratio, moisture content of both crust and crumb, crumb porosity and density, and textural properties. The operative conditions under which muffins are prepared can significantly affect these characteristics (Ureta et al. 2014). Muffins are a popular baked good that can be fortified with various functional food ingredients to enhance their nutritional profile. Alternative flours made from cereals (Soong et al. 2014; Bhinder et al. 2022), pulses (Shevkani et al. 2015), tubers (Lancetti et al. 2020), oil seeds (Marchetti et al. 2021; Gökşen and Ekiz 2021) and other plant-based ingredients (Caporizzi et al. 2021) can add beneficial nutrients such as fiber, protein, and healthy fats to muffins along with vitamins and minerals. Additionally, functional food ingredients such as probiotics and prebiotics can be added to muffins to promote digestive health. However, it is important to note that the amount and type of fortification ingredients used in muffins should be carefully considered to ensure the desired nutritional benefits without compromising taste or texture.

Currently, desiccated coconut powder is produced by dehydrating either grated or shredded coconut kernel after removing its brown testa, is widely used in confectioneries, puddings, and numerous other food recipe as a substitute for raw grated coconut (Vijayasanthi et al. 2020; Adsare and Annapure 2021). Coconut is naturally low in carbohydrates and high in fiber, making it an ideal fit for low-carbohydrate diets. The conventional refined wheat flour (RWF) has high concentrations of simple carbohydrates which can be rapidly metabolized and cause destructive blood sugar fluctuations in the body. In contrast to RWF, several studies have found that coconut flour has a glycemic-lowering effect (Fernando et al. 2015; Vijayakumar et al. 2018), due to its high fiber content, which makes the flour less disruptive to blood sugar levels. Moreover, it is vegan and gluten free, and best suited ingredient for development of low gluten or gluten free food products.

The focus of this study was to develop partially defatted coconut flour (PDCF) from desiccated coconut cake generated during virgin coconut oil processing. PDCF was investigated for its physicochemical characteristics and as a potential substitute for refined wheat flour in the production of muffin to further enhance its fiber and protein content, resulting in muffins with superior nutritional and functional properties. Throughout the study, the quality attributes of the muffins were assessed during storage, with particular emphasis on their sensory and textural properties. These results could be useful in developing strategies to increase the fiber and protein content of baked goods, while also promoting the use of sustainable ingredients.

Materials and methods

Material

Coconut (Cocus nucifera) was purchased from local super market, Matunga (E), Mumbai, India; Refined wheat flour (Fortune®, Adani Wilmar, India), baking powder (Blue Bird®, India), iodized salt (Tata salt), Powdered sugar (Satyam, India), and Skim milk powder (Everyday®, Nestle, India), Eggs, etc. were procured from local market. Glycerin and other chemicals were procured from Fine Chemicals Inc.; Vanilla flavor from Keva Flavors, Mumbai, India. Specialty bakery fats was gifted by AAK Kamani Oils Ltd., Mumbai, India.

Preparation of partially defatted coconut flour (PDCF)

Preparation of PDCF was prepared using the method given by (Chandrashekar et al. 2019) with slight modification. Fresh and mature coconuts were manually deshelled and pared, with water removal. Pared coconuts were then disintegrated using a slice cutter, and coconut milk was extracted using a manual cold-press method commonly used in virgin coconut oil production. The obtained residual cake from cold press was dried in a hot air dryer at 55 ± 2 °C for 6 h and cooled to 25 ± 2 °C. Further, the dried cake was subjected to grinding using hammer mill to obtain coconut flour coded as partially defatted coconut flour (PDCF), packed in high density polyethylene pouches, and used for further studies.

Proximate and physicochemical characterization of PDCF, RWF and muffins

Both the PDCF, RWF and muffins were analyzed for proximate content using AOAC (2006). Reducing sugar, total dietary fibers, pH (Hanna Instruments, India), water activity (Rotronic Hygrolab C1, USA), functional properties such as pH, peroxide value, water and oil holding capacity, swelling capacity and color and flow properties were also analyzed.

Dietary fiber analysis

The soluble and insoluble dietary fiber present in PDCF, RWF and muffins were analyzed using fiber assay kit (Megazyme K‐TDFR, Wicklow, Ireland) according to the Mes‐Tris AOAC method 991.43 (1995) and AACC method 32‐07 (1995).

Swelling capacity, water and oil holding capacity of flours

Both the flours were analyzed for swelling capacity (SC) as per the method given by (Goswami et al. 2015). The water and oil holding capacity (WHC and OHC) were evaluated by the method given by Pawar et al. (2020).

Color attributes

A spectrocolorimeter (Hunter Lab mini–Scan XE Plus colorimeter) was used to measure the color attributes of PDCF, RWF and muffins in terms of L*, a* and b* values. From the L*, a* and b* values Hue angle, Chroma, and Browning index were calculated as reported by Muley et al. (2022).

Preparation of muffins

The formulation used in the present study was as per the Table 2. The egg yolk with egg white was whipped in a mixer (Kenwood Major Classic, UK) followed by sequential addition of sugar, bakery fat, skim milk powder, glucerin and vanilla flavour. RWF, PDCF, baking powder were added into this mixture and water was added as per requirement to assure uniform mixing of all ingredients of muffin batter. Increase in PDCF results into absorption of bakery fat and egg moisture make it difficult to mixing, so need to add water. The muffins were baked in a conventional oven for 15 min at 180 °C.

Table 2.

Formulations, Proximate and colour analysis of the partially defatted coconut flour (PDCF) based muffins

	RWF replacement with PDCF in muffins
	Control	25%	50%	75%	100%
Formulation of the muffins
Flour blend
RWF (g)	100	75	50	25	0
PDCF (g)	0	25	50	75	100
Other ingredients
Sugar (g)	100	100	100	100	100
Egg (g)	100	100	100	100	100
Fat (g)	100	100	100	100	100
Baking powder (g)	2	2	2	2	2
Skim milk Powder (g)	2	2	2	2	2
Glycerine (ml)	4	4	4	4	4
Vanilla flavour (ml)	1	1	1	1	1
Water (Q.S.) (ml)	0	20	40	60	80
Proximate analysis of muffins
Carbohydrate	43.12 ± 1.78	40.64 ± 1.61	38.20 ± 1.49	36.08 ± 1.43	34.54 ± 1.37
Dietary fiber	0.64 ± 0.03	3.12 ± 0.13	5.53 ± 0.23	8.05 ± 0.31	10.56 ± 0.42
Protein	6.87 ± 0.28	7.24 ± 0.30	7.57 ± 0.30	7.92 ± 0.31	8.36 ± 0.32
Fat	28.01 ± 1.12	27.96 ± 1.12	27.85 ± 1.11	27.87 ± 1.11	28.29 ± 1.13
Ash	1.21 ± 0.05	1.85 ± 0.07	2.50 ± 0.10	3.19 ± 0.13	3.76 ± 0.15
Moisture content	18.90 ± 0.76	20.91 ± 0.84	22.77 ± 0.91	24.66 ± 1.95	27.39 ± 1.98
Water activity	0.781 ± 0.01	0.788 ± 0.02	0.801 ± 0.01	0.817 ± 0.01	0.845 ± 0.01
Colour analysis of muffins
Crumb
L*	77.85 ± 0.26	76.37 ± 0.29	76.04 ± 0.19	73.29 ± 0.21	67.89 ± 1.35
a*	0.21 ± 0.05	0.14 ± 0.06	0.24 ± 0.03	0.27 ± 0.01	0.28 ± 0.01
b*	19.53 ± 0.2	21.53 ± 0.2	20.53 ± 0.06	19.07 ± 0.06	23.68 ± 0.82
Hue angle (H°)	89.37 ± 0.11	89.63 ± 0.09	89.32 ± 0.09	89.24 ± 0.03	89.31 ± 0.01
Chroma (C*)	19.53 ± 0.14	21.53 ± 0.22	19.07 ± 0.05	20.53 ± 0.06	23.66 ± 0.82
Browning index (BI)	28.71 ± 0.19	32.17 ± 0.15	30.70 ± 0.22	30.73 ± 0.20	41.63 ± 0.61
Crust
L*	53.86 ± 0.27	51.27 ± 0.40	50.75 ± 0.53	47.33 ± 0.10	49.79 ± 0.18
a*	13.35 ± 0.13	14.64 ± 0.33	15.61 ± 0.49	16.51 ± 0.09	14.89 ± 0.05
b*	34.32 ± 0.22	34.75 ± 0.26	33.60 ± 0.17	32.88 ± 0.73	34.64 ± 0.22
Hue angle (H°)	68.74 ± 0.53	67.15 ± 0.56	65.10 ± 0.58	63.32 ± 0.58	66.74 ± 0.20
Chroma (C*)	36.83 ± 0.41	37.71 ± 0.38	37.05 ± 0.36	36.79 ± 0.63	37.70 ± 0.18
Browning index (BI)	112.51 ± 3.11	124.55 ± 3.35	122.53 ± 3.31	133.07 ± 3.27	129.83 ± 0.32

Batter properties

Apparent viscosity and temperature dependent rheology of muffin batters

The rheological properties of muffin batter were measured at 25 °C using an Anton Paar dynamic rheometer (MCR-52, Anton Paar, Germany) equipped with a 40 mm diameter plate–plate measuring system, with a plate gap set at 1 mm. An apparent viscosity measurement was conducted by applying a continuous ramp as a function of shear rate over the range of 0.01–100 s⁻¹. For temperature dependent rheology, the batters were subjected to a heating and cooling cycle with a uniform shearing of 160 s⁻¹. Initially the batters were heated to 95 °C from the room temperature of 25 °C at a rate of 6 °C/ min and then kept at that temperature for five minutes. Sequentially, the batters were cooled down to 25 °C at the same rate and then held at that temperature for two minutes. The viscosity of batter during heating and cooling was also recorded.

Specific gravity (SG) of batter

The specific gravity was calculated gravimetrically as the ratio of the mean mass of a standard container filled with batter to that of the same container filled with water.

Muffin characteristics

Determination of the weight loss of muffin, muffin height and muffin volume

Initial and final weights of muffins were measured, and the percent loss in moisture content of baked muffins was determined by comparing the weight of the muffin batter to the weight of the baked muffins (Goswami et al. 2015). The muffin height (mm) was measured by using a vernier caliper (Martínez-Cervera et al. 2012). Muffin volume was measured by seed displacement method (Shevkani et al. 2015).

Sensory analysis of muffins

The sensory characteristics of muffins were evaluated according to Sanz et al. (2009) with slight modifications. The test was performed by a trained panel comprising of 20 people using 9-point hedonic scale. Attributes evaluated were; appearance (height, color, and sponginess), chewiness, coconut flavor, vanilla flavor, taste, and overall acceptability. Data of attributes were analyzed by ANOVA with p < 0.05.

Shelf-life study of muffins

Muffins prepared with various concentration of PDCF were packed in LDPE and stored under ambient temperature (25 ± 1 °C) to assess the effects of PDCF concentration on the longevity of the muffins. The muffin samples were stored and analyzed over a week (7 days) for changes in moisture content, water activity, peroxide value, free fatty acids, textural characteristics, microbial load, and overall acceptability.

Texture profile analysis of muffins during storage

The texture profile analysis (TPA) of the muffins was conducted using a texture analyzer (TA/TX2-plus, Stable Micro Systems, Surrey, UK) equipped with a 5 kg load cell. Compression of the crumbs was performed at a speed of 1 mm/s using a flat 75-mm aluminum probe, with a compression depth of 50%. The texture parameters, hardness, springiness, cohesiveness, chewiness, and resilience were recorded.

Microbiological analysis of muffins during storage

Total plate count (TPC) and yeast and mold count of the muffins were analyzed on initial and 7^th day. Muffin samples were first mashed and mixed in peptone water. Subsequently, 0.1 mL aliquots of diluted samples were inoculated on Potato Dextrose Agar (PDA) and Plate Count Agar (PCA). The PCA inoculated plates and tubes were incubated at 37 °C for 24–48 h whereas PDA plates were incubated at room temperature (30 ± 2 °C) for 3–5 days. Following incubation, the colonies were counted and reported as colony forming units per gram (CFU/g) of the samples.

Statistical analysis

All the parameters were measured in triplicate and means were reported. SPSS software (version 16.0) was used to conduct the statistical analysis of the reported results. The significance was accepted at 5% levels of significance (p ≥ 0.05).

Result and discussion

Evaluation of proximate and functional properties of PDCF and RWF

PDCF and RWF were analyzed for their proximate content (Table 1). The PDCF had 5.21% moisture, 21.31% protein, 14.03% fat, 3.27% ash, and 46.39% total dietary fiber, whereas RWF had 13.6% moisture, 11.98% protein, 0.89% fat, 0.60% ash, and 2.37% total dietary fiber. During the VCO processing, most of the moisture and part of the carbohydrate, protein, fat, and other nutrients were expelled with coconut milk, that results in a de-oiled cake with reduced fat content (14.03%) and improved protein (21.31%) and fiber (46.39%) content. RWF had significantly lower protein and fiber content than PDCF, but gluten makes it an ideal functional ingredient for bakery goods. PDCF and RWF were studied for their essential functional properties. The peroxide value of PDCF was found to be below 0.3 mEq kg⁻¹. Water and oil holding capacity were 4.84 and 3.88 g/g for PDCF and 2.53 and 1.45 g/g for RWF, respectively. PDCF had five times greater swelling capacity than RWF. These variations in the water and oil holding capacities along with swelling capacity could be due to higher content of dietary fiber in PDCF over the RWF. Further, the variations in particle size, porosity, and crystallinity of fiber may also contribute for the same (Dhingra et al. 2012). The flours showed similar L*, a*, and b* color values. Results from Table 1 for flow properties showed that the PDCF flour had lower bulk density (316.32 kg m⁻³), tapped density (370.28 kg m⁻³), compressibility index (14.57), and Hausner ratio (1.17) than RWF.

Table 1.

Proximate and functional properties of PDCF and RWF

Parameters	PDCF	RWF
Proximate analysis
Dry matter (%)	94.79 ± 1.30	86.43 ± 1.30
Ash (g/100 g)	3.27 ± 0.08	0.60 ± 0.01
Fat (g/100 g)	14.03 ± 0.40	0.89 ± 0.01
Total saturated Fat (g/100 g)	0.68 ± 0.01	Nd
Monounsaturated Fat (g/100 g)	0.07 ± 0.01	Nd
Polyunsaturated Fat (g/100 g)	14.75 ± 0.03	Nd
Free fatty acids (% of fat)	0.97 ± 0.01	Nd
Protein (g/100 g)	21.34 ± 0.11	11.98 ± 0.31
Carbohydrates (g/100 g)	54.64 ± 1.20	74.38 ± 1.56
Reducing sugars (g/100 g)	7.88 ± 0.35	3.76 ± 0.22
Total dietary fiber (g/100 g)	46.39 ± 0.23	2.37 ± 0.18
Insoluble fiber (g/100 g)	43.27 ± 0.17	1.41 ± 0.01
Soluble fiber (g/100 g)	2.89 ± 0.11	0.8 ± 0.01
Functional properties
pH	6.69 ± 0.02	6.04 ± 0.07
aw	0.42 ± 0.01	0.62 ± 0.01
Peroxide value (mEq kg−1)	0.28 ± 0.01	Nd
Water holding capacity (g/g)	4.84 ± 0.08	2.53 ± 0.08
Oil holding capacity (g/g)	3.88 ± 0.11	1.45 ± 0.09
Swelling capacity (mL/g)	5.2 ± 0.13	1.02 ± 0.08
Colour properties
L*	91.45 ± 1.04	89.96 ± 0.12
a*	-0.4 ± 0.04	− 0.36 ± 0.03
b*	9.06 ± 0.08	11.05 ± 0.01
Flow properties
Bulk density (kg m−3)	316.32 ± 5.56	335.48 ± 2.50
Tapped density (kg m−3)	370.28 ± 7.00	462.54 ± 5.78
Compressibility index	14.57 ± 0.21	28.26 ± 0.21
Hausner ratio	1.17 ± 0.00	1.39 ± 0.00

Batter properties

Shear rheology of batters

During baking, batter viscosity influences the formation of convection currents, which is dynamic with share rate, time, and temperature. The viscosity of batters is classified as non-Newtonian due to their intricate structural complexity, resulting in a viscosity that varies with the shear rate. Therefore, values for the apparent viscosity versus shear rate for muffin batters prepared with various PDCF concentrations were calculated and displayed in Fig. 1. All the batter formulations demonstrated shear thinning behavior at lower shear rates (up to 40s⁻¹), indicating a decrease in viscosity with increasing shear rate. Conversely, the viscosity remained relatively constant at higher shear rates, displaying Newtonian behavior. At 25% PDCF flour blend in batter, no significant difference was observed in the shear flow behavior compared to the control. However, as the PDCF concentration increased to 50%, 75%, and 100%, the apparent viscosity of the batter decreased significantly.

Fig. 1.

Influence of replacing PDCF flour with wheat flour on flow properties (apparent viscosity) of a muffin batter at 25 °C

Temperature dependent rheology

It was observed that the batter with 100% RWF was showing pasting behavior during the heating and cooling cycle due to the presence of starch. The viscosity of the batter increased when it was heated due to the swelling of starch granules to attain the peak. On continuous heating, the breaking down of the starch granules occurred and thus, a drop in the viscosity was observed. When the batter was cooled down to the normal temperature, the gelatinized starch granules recrystallized to retrograde, increasing the viscosity further. The final viscosity of the control batter was found to be greater than the peak viscosity after gelatinization. However, when the RWF was substituted with the PDCF, the peak viscosity was subsequently reduced with the increase in the PDCF content (Table 2). Also, the pasting behavior was not exhibited in the 100% PDCF flour as there is merely no starchy elements to gelatinize. Nevertheless, after the heating and cooling cycle, the final viscosity of the PDCF substituted batters exhibited a higher final viscosity than their initial viscosity values. This shows the ability of coconut fibers to hydrate and swell in subject to heat energy.

Specific gravity of batter

The specific gravity (SG, g/L) values of the batter serve as an indicator of its capacity to retain air and a lower SG value suggests that more air has been incorporated into the batter. This increased air bubble retention capacity ultimately leads to a larger final volume after baking, resulting in a spongier and fluffier product, an essential requirement for good quality muffins (Cauvain 2016). Table 3 presents the impact of gradually substituting RWF with PDCF on the specific gravity (SG) of the batter. As the level of PDCF in the formulation increased, the SG of the batter also increased. The batter containing the highest proportion of PDCF recorded the highest SG value of 0.929, while the control muffin batter exhibited the lowest SG value of 0.614. These results clearly indicate that the addition of PDCF in the muffin formulation leads to reduced air incorporation and fewer air bubbles formed.

Table 3.

Peak viscosity, final viscosity, specific gravity of muffin batters and physical and microbiological properties of muffin

Batter with PDCF conc.	Peak viscosity of batter (cP)	Final viscosity of batter (cP)	Specific gravity of batter (g/L)	Water activity (aw)	Moisture content (g/100 g)	Weight loss (g/100g)	Height (mm)
Control	1208.6 ± 18.08	1533.7 ± 20.44	0.614 ± 0.07	0.781 ± 0.02	18.90 ± 0.75	6.64 ± 0.93	49.30 ± 0.67
25%	422.4 ± 4.12	589.3 ± 13.65	0.857 ± 0.18	0.788 ± 0.01	20.91 ± 0.98	7.74 ± 1.23	45.86 ± 0.45
50%	474.9 ± 3.97	498.3 ± 9.92	0.874 ± 0.12	0.801 ± 0.02	22.77 ± 0.68	8.72 ± 0.66	37.891 ± 0.33
75%	206.8 ± 3.85	445.6 ± 8.51	0.878 ± 0.15	0.807 ± 0.05	24.66 ± 1.29	10.42 ± 0.72	37.200 ± 0.56
100%	198.3 ± 3.11	228.1 ± 8.64	0.929 ± 0.15	0.865 ± 0.09	30.39 ± 1.32	12.36 ± 1.27	36.245 ± 0.23

	Total plate count (X 102 cfu/g)		Yeast and mold count (X 102 cfu/g)
	0 Day	7 Days	0 Day	7 Days
Control	1.20 ± 0.73	6.62 ± 1.03	< 1	8.58 ± 1.03
25%	1.40 ± 0.98	7.08 ± 1.03	1	6.45 ± 1.15
50%	2.54 ± 1.01	11 ± 1.03	< 1	4.76 ± 1.39
75%	2.59 ± 1.01	10 ± 1.03	< 1	4.53 ± 0.97
100%	3.64 ± 0.62	13 ± 1.03	< 1	5.17 ± 1.09

Height and weight loss of muffins

The progressive substitution of RWF with PDCF led to a significant decrease in the height of the baked muffins, as shown in Table 3. The control muffins, without PDCF, had a height of 49.30 mm. However, as RWF was replaced with PDCF, the height decreased from 45.86 mm (25% PDCF) to 36.24 mm (100% PDCF). The decrease in height of the muffins containing PDCF can be attributed to several factors. Firstly, the increased specific gravity (SG) value and reduced air cell formation in the muffin batter (Cauvain 2016) could contribute to less leavening and subsequently reduced height. Additionally, the incorporation of PDCF, which is gluten-free, may lead to the dilution of gluten, resulting in limited gas cell stability and reduced leavening (Ureta et al. 2014) . Lastly, the lower peak viscosity of the PDCF muffin batter during heating (Baixauli et al. 2008) may further contribute to a reduced leavening effect during the baking process. These combined factors likely resulted in decreased leavening and subsequently a decrease in the height of the muffins containing PDCF.

A significant increase in weight loss of the muffin samples during the baking process was observed with an increase in PDCF. The muffin sample made with a flour blend of 100% PDCF showed the highest weight loss during baking, which was at 12.36 g/100 g. In contrast, the control muffins exhibited the lowest weight loss at 6.64 g/100 g. The higher water binding capacity of the control muffins, attributed to RWF, resulted in minimal moisture evaporation during baking, leading to lower weight loss. In contrast, muffins made with PDCF experienced higher moisture loss during baking, contributing to increased weight loss. The low water binding capacity of muffins was due to the absence of gluten compared to control muffins. The muffins with high PDCF in flour blend exhibited higher moisture content despite higher weight loss during baking, which was attributed to additional water used in batter formulation.

Similar trends were reported by Goswami et al. (2015) in barnyard millet-based muffins. They found that progressive substitution of barnyard millet flour with RWF reduces the muffin height, whereas the specific gravity of the batter increases.

Proximate analysis of muffins

The proximate composition of muffins is summarized in Table 2. Muffins with PDCF in flour blend showed significantly higher dietary fiber content (P < 0.05). Total dietary fiber was increased with increasing PDCF in the flour blend of muffins. The dietary fiber content of muffins prepared with 100% PDCF was more than 10% (w/w), whereas it was limited to less than 1% in control. Bhinder et al. (2022) also found that muffins prepared with buckwheat flour (BF) contain higher fiber content than those with wheat flour. BF is rich in fiber (threefold higher in fiber content than in WF). A slight increase in protein content was observed in PDCF muffins. These increased values in protein content were attributed to the higher protein content of PDCF. Significant variation in moisture content was observed among the muffins. An increase in moisture content of muffins with higher PDCF in flour blend was attributed to the higher water holding capacity of coconut fiber (Raghvendra et al. 2011).

Color attributes of muffins

Table 2 provides the instrumental values for lightness (L*) and color (a*, b* value), as well as the computed Hue angle (h^o) and chroma (C) values of the muffin crumb and crust. The L* values of the muffin crumb with 25–75% PDCF in flour blend exhibited no significant difference compared to the control muffins (77.85). The lightness (L*) values of the crumb of the PDCF muffins decreased with the increase in the proportion of PDCF flour in the formulation because of the loss of the characteristic white color developed due to refined wheat flour during baking. The presence of higher dietary fiber and reduced sugar content in PDCF may have contributed significantly to the development of color characteristics of muffins during the baking process, resulting decrease in crumb lightness, which is similar to findings on the flaxseed meal-based muffins (Shearer and Davies 2005). Muffins with 100% PDCF in flour blend showed significantly lower L* values than others. The b* values for muffin crumb were increased with a progressive increase in PDCF in flour blend from 19.53 (control) to 23.66 (100% PDCF in flour blend).

Sensory characteristics of muffins

A sensory evaluation conducted by a trained descriptive panel is essential to comprehensively assess the impact of the fiber used on various parameters (Lin 2022). The effect of adding different types of fiber on the sensory analysis of a baked product has been studied by several authors. Figure 3 f presents the sensory scores of muffins, showing the influence of PDCF level on different sensory attributes. Despite the significant difference (P < 0.01) in mean sensory scores between the PDCF-based muffins and the control, the overall sensory evaluation results for the PDCF muffins were promising. The muffins with 50% PDCF replacement received the highest score for taste whereas further increase in PDCF in flour blend decreases in the taste score, which can be attributed to the higher fiber content in flour.

Fig. 3.

Effect of storage period on a moisture content, b water activity, c free fatty acids, d peroxide value, e overall acceptability of muffins stored at 30 °C, and f Sensory evaluation of muffins on day 0

Additionally, the chewiness score may be influenced by the texture of the muffins due to the higher retention of moisture by fiber present in the muffins. The sensory panel found that the flavour (both coconut and vanilla) and overall acceptability of muffins made with PDCF:RWF flour blend were more similar and received a good score. Clark and Johnson (2002) carried out a study on the incorporation of lupin kernel fiber in muffins. Their findings showed that the addition of fiber in muffins did not significantly change the consumer rating for its appearance. However, it lowered the consumer ratings for flavor, texture, and overall acceptability. Baixauli et al. (2008) evaluated the muffins enriched with resistant starch for consumer acceptability for appearance, texture, taste, overall acceptance, and consumption intention. They did not find significant differences in scores for sensory parameters taste, overall acceptance, and consumption intention of muffins.

Shelf-life study of muffins

Textural changes during muffin storage

The influence of replacing increasing proportions of refined wheat flour with four different levels of PDCF on the textural properties of freshly baked muffins and the changes that took place during seven days of storage are shown in Fig. 2. Results indicate that the replacement of RWF with PDCF gave the muffins a softer texture which can be seen from the significant decrease in hardness values of the muffins than those of the control muffins Fig. 2a. The decline in hardness was linear with RWF replacement, the lowest hardness value corresponded to the highest concentration of PDCF (100%) in flour blend. The control muffins contain only RWF which is rich in starch but lacks dietary fibers. This starch in control muffins undergoes retrogradation, and causes loss of moisture during storage leading to increased hardness. The lower water holding capacity due to lack of dietary fiber also contributes to hardness. The springiness, cohesiveness and resilience of the muffins decreased as the PDCF rose in flour blend. However, this decrease was only clear and significant from 5 days onwards (Fig. 2). A possible explanation for the lower resilience and springiness when PDCF was added was that the product matrix became denser: at higher PDCF levels, the number and area of the gas cells and the height of the final baked muffins have been found to decrease (Baixauli et al. 2008; Beegum et al. 2017). A gradual decrease in chewiness was found in the muffins after day 3. Higher fiber content absorbs more water, affecting the mouthfeel intensity, which is generally dominated by fat content in muffins. A similar trend was observed by Symons and Brennan (2004), where volume reduction in bread with β –glucan addition has been attributed to gluten dilution resulting in an underdeveloped gluten network, which limits the extent of dough expansion and gas cell stability during proving and leads to reduced loaf volume in tern affects chewiness.

Fig. 2.

Effect of storage period on a hardness, b gumminess, c springiness, d cohesiveness, e chewiness, and f resilience of muffins

Effect of storage on moisture content and water activity of muffins

During the storage period, there was a slight decrease in moisture content observed in control and muffins with different levels of PDCF. Muffins containing PDCF exhibited higher moisture retention compared to control muffins. Higher moisture content can be attributed to the water binding capacity of PDCF flour due to higher fiber content which contributes to retaining moisture in the muffins. The moisture content decreased from 18.90 to 16.39% for control muffins, and 20.91% to 19.62% for 25% PDCF flour blend muffins, whereas moisture loss was not significant and values were very negligible in 50%, 75% and 100% PDCF flour blend containing muffins. Goswami et al. (2015) reported that during the storage of barnyard millet-based muffins, products were softer due to more moisture retention than the control products. The higher moisture retention for PDCF muffins also correlated to lower hardness values presented in Fig. 2. Lower starch content in formulation also restricted the retrogradation phenomenon, preventing moisture loss during storage.

Water activity for control and muffins with PDCF flour blend was increased significantly with storage time. It was increased from 0.78 to 0.83 for control, 0.79 to 0.82 for 25% PDCF, 0.80 to 0.84 for 50% PDCF, 0.80 to 0.86 for 75% PDCF and 0.86 to 0.88 for 100% PDCF flour blend in muffins from day 0 to day 7 of storage. A significant correlation was observed between days of storage and the level of PDCF in muffins. Hussain et al. (2019) reported a similar increase in muffin’s water activity in muffins developed using water chestnut and barley flour.

Effect of storage period on free fatty acid and peroxide value of muffins

Lipid oxidation is recognized as a primary factor contributing to food product quality degradation. It not only introduces rancidity and undesirable flavors to fat-based products but also produces reactive oxygen species, which have been associated with the potential to promote carcinogenesis, inflammation, aging, and cardiovascular disorders (Soong et al. 2014). In addition, lipid oxidation significantly impacts the chemical, sensory, and nutritional characteristics of fat-rich foods, making it a crucial factor in their utilization and shelf life. Figure 3c displays the development of significant variations in the formation of free fatty acids (FFA) in muffins with respect to the PDCF concentration in flour blend and storage period. From Fig. 3c, it can be observed that the rate of FFA formation was increased with an increase in PDCF concentration in flour blend and an increasing storage period. A linear increase in FFA formation was found for muffins with 75% and 100% PDCF flour blend as compared to the control and 25% PDCF flour blend. At the end of 7 days of storage, the FFA of muffins with 50%, 75% and 100% PDCF in flour blend at 25 °C condition reached to 0.431 g/100 g, 0.476 g/100 g and 0.504 g/100 g. However, the content of free fatty acids in the muffins remained within the acceptable limit specified by the Asian and Pacific Coconut Community, which is equal to or below 0.5%.

The primary oxidation products calculated as peroxide value (PV) in all samples were found to be low (0.11–0.13 meq/kg) (Fig. 3d). However, the levels of hydroperoxides varied significantly based on the amount of PDCF in the muffins and the duration of storage. Furthermore, our findings revealed that after a storage period of 7 days, a marginal increase in PV of muffins was found, which was within the acceptable limit for consumption. Among PDCF muffins, 75% and 100% PDCF flour blends showed an accelerated increase in PV due to higher moisture retention, which favors lipid oxidation.

Microbial analysis of muffins during the storage period

The results presented in Table 3, for microbial analysis of muffins performed as a part of storage studies, showed that all the muffins with PDCF were stable for 7 days at ambient room temperature and humidity when packed in LDPE bags, without any preservative such as calcium propionate and acetic acid added into the muffins. On day 0, the TPC count was highest for 100% PDCF muffins (3.64 CFU/100g) and was lower for control muffins (1.20 CFU/100g). The PTC load on muffins was increased after 7 days of storage. The increase in PDCF quantity has significantly increased the PTC load in muffins (Table 3). A slight increase in total yeast and mold count was found for muffins after 7 days of storage period. These results of both PTC and total yeast and mold count of the muffins were within the recommended limits set by FSSAI (2016) till day 7. The muffin prepared with different flour blends was stable after a storage period of 7 days. Hence, it can be concluded that the muffins prepared without preservatives are microbiologically safe for consumption for at least 7 days.

Conclusion

Partially defatted coconut flour in muffin formulations resulted in significant effect on physical, textural and sensory attributes of the muffins. There was a decrease in the baked height, hardness, springiness, chewiness and resilience due to the reduction in gluten. The presence of PDCF provided additional softness and crumby structure in the prepared muffin formulations. Sensory panel liked all the PDCF formulations, except with the muffins containing 100 g PDCF received reduced chewiness and taste score due to high fiber and oil content. The storage stability studies displayed an increase in microbial count and decline in textural attributes, and both to be in acceptable regime. Muffins with 50% PDCF received the highest score for most of the parameters, indicating the potential of developed coconut co-product as a intermediate raw material for foods like muffins. The product has a potential market as a highly nutritious, high-calorie snack food among baked products appreciated by the sensory panel. Currently, the valorization of partially defatted coconut flour from virgin coconut oil cake an underutilized co-product in nutritionally enriched muffins could increase and support the value chain of coconut-based small-scale industries.

Supplementary Information

Below is the link to the electronic supplementary material.

Authors contributions

SA (Conceived and designed the analysis data collection, performed the analysis, contributed data, data analysis, interpretation of data, manuscript writing), UA (critical revision of the article, supervision and final approval for the publication).

Funding

PhD research fellowship from University Grant Commission, Govt. of India.

Availability of data and material

The datasets generated during the current study are available from the corresponding author upon reasonable request.

Code availability

Not applicable.

Declarations

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. There is No conflict of interest among the authors.