Effect of Apple Puree as a Substitute for Fat and Sugar on the Texture and Physical Properties of Muffins

Abstract

In the context of global public health concerns, reducing the fat and sugar content of baked goods has become a major priority. The excessive consumption of sugar and fat is associated with an increased risk of obesity, type II diabetes and cardiovascular disease. This study realizes the optimization of muffin formula by incorporating apple puree as a substitute for sucrose and fat, with the aim of increasing the nutritional value and reducing the caloric content of the product. A comprehensive analysis was performed to evaluate the impact of this substitution on various textural and physical properties of muffins. The firmness, chewiness, cohesiveness, height, volume and weight loss during baking of the muffins were determined. The result of the study showed that in order to obtain optimal textural and physical properties, the sugar and fat in the muffins can be substituted with applesauce in percentages of 34.04% and 43.78%, respectively. This research highlights the potential of apple puree to reduce the sugar and fat content and to lead to obtaining optimal physical and textural properties.

1. Introduction

In recent decades, the consumption of pastries has increased significantly in many parts of the world. This is partly due to urbanization and lifestyle changes, which have led to a greater demand for fast and convenient foods [1].

Pastries are now available in a wide range of types and flavors, from muffins and croissants to pies and cookies [1]. This diversification responds to diverse consumer preferences and influences consumption habits. Europe is one of the regions with the highest consumption of pastry products. Countries such as France, Italy and Germany have a long tradition in pastry making, with world-renowned products. For example, France is known for its croissants and tarts and Italy for its frolla and biscotti. Muffins are also extremely popular in the United States and the UK, where they are often eaten for breakfast or as a snack [2]. In the US and UK, muffins are available in a wide range of flavors, from traditional ones such as blueberry to more inventive ones with chocolate or nuts. Muffins are commonly found in cafes, bakeries and supermarkets. Large chain coffee shops, such as Starbucks, Dunkin’ Donuts and Panera Bread, offer a variety of muffins [2].

Baked goods such as croissants, pies and muffins are often high in fat, including saturated fat and, in some cases, trans fat. These types of fats can contribute to elevated LDL cholesterol and increase the risk of cardiovascular disease [1,3,4]. These products also contain large amounts of sugar, which add to the total caloric intake and can cause the blood sugar to rise. Excessive sugar consumption is associated with an increased risk of type 2 diabetes and obesity. Due to their high fat and sugar content, pastry products are often dense in calories, and frequent consumption and in large quantities can lead to caloric excess and, implicitly, to weight gain [1,3,4].

Sugar and fat are two fundamental ingredients in baked goods, each playing an essential role in achieving the desired texture, taste and appearance. Sugar adds sweetness, which is essential to the taste of baked goods. Fat also has the ability to intensify flavors, improving the overall taste of the product. Sugar and fat help retain moisture in baked goods, keeping baked goods soft and fresh. They help to aerate the dough, which contributes to a larger volume and fluffier texture of the products. It also inhibits excessive gluten formation, resulting in a softer and more delicate dough. The sugar caramelizes during baking, giving a golden color and characteristic taste and contributes to the formation of the brown crust and complex flavor in baked products [4,5,6,7].

As the demand for low-calorie foods increases, so does the need for low-sugar and low-fat products. Therefore, finding natural substitutes for these ingredients has become a priority for researchers and manufacturers [1,4].

Sugar and fat substitutes in baked goods are ingredients used to reduce the energy value while improving the nutritional profile of baked goods. Fat and sugar substitutes include: inulin, oligofructose, fruit purees (bananas, apples), pectin, apple fiber, berry pomace, etc. [4,6,8,9,10,11,12,13,14].

Apple puree can be used as a substitute for both sugar and fat in baked goods. The high fiber content (2.03 g/100 g) contributes to digestive health and blood sugar regulation [15]. Antioxidants in apple puree, such as vitamin C and polyphenols, help fight free radicals. Cooked apple puree has a total polyphenol content of 911.66 mg GAE/kg and an antioxidant capacity of 1.24 μM TE/g, with a percentage of 93.78%, inhibiting DPPH radical scavenging activity. Apple puree is also rich in essential sugars, vitamins and minerals. The total soluble solids content is 18.56 °Brix, and the mineral content is as follows: Ca (48.67 mg/kg), K (41.67 mg/kg), Na (0.61 mg/kg), Mg (0.17 mg/kg), Mn (0.07 mg/kg), Cu (0.02 mg/kg), Zn (0.02 mg/kg) and Fe (0.01 mg/kg) [16]. In addition, apple puree has a lower caloric intake compared to refined sugar. In addition to the multiple nutrients that apple puree contains, it is rich in pectin, which makes it possible to use it as a substitute for sugar and fat in baked goods [17].

Pectin is a natural polysaccharide found mainly in the cell walls of plants, especially in fruits (citrus, apples, etc.) [18,19,20]. This is a dietary fiber known for its gelling properties, which is why it is widely used in the food industry to stabilize and thicken products such as jams, jellies and other food products; in baked goods, pectin retains moisture and improves texture. Pectin contributes to the increase in dough volume by maintaining gas and stabilizing the dough structure and slows the aging process of baked goods, keeping them fresh for longer [20,21].

Pectin can form hydrogen bonds with the starch molecules in the flour, stabilizing their structure and preventing the rearrangement of the starch molecules in a rigid network. These hydrogen bonds create a more stable and flexible structure, preventing rapid degradation, which leads to an increase in the hardness of the products and to the acceleration of their aging [22,23,24]. Pectin interacts with gluten in bakery products, contributing to the formation and maintenance of a more uniform and stable gluten network structure [20,25,26,27,28]. Also, pectin can facilitate the creation of exudated starch granules, thus contributing to the formation of the structure of bakery and pastry products, and structure is essential for the stability and texture of the products [21]. Essentially, pectin acts as a hydrocolloid, meaning it holds water in the dough, which can prevent excessive drying and keep the product moister for a longer period of time [20,22,23,24,26,29].

Oligosaccharides derived from pectin can act as prebiotics, stimulating the growth of beneficial bacteria in the intestines, improving digestion and providing protection against pathogens. Studies suggest that certain fractions of pectin can induce apoptosis (programmed cell death) in colon cancer cells, thus providing a potential protective effect against colon cancer [20]. Pectin is resistant to the action of digestive enzymes, such as salivary amylase, pepsin and trypsin, which means that it reaches the colon intact, where it can act as a prebiotic [20,30,31].

The aim of this research is to optimize the formula of muffins by using apple puree as a substitute for the simultaneous reduction in sucrose and fat content, with the objective of increasing the nutritional value and reducing the energy value of the product. The study aims to evaluate the impact of this substitute on the texture of the muffins (hardness, chewiness and cohesiveness) and on the physical properties (height, volume and weight loss during baking). The Design Expert v11 program (trial version, Stat-Ease, Minneapolis, MN, USA) was used to optimize the production of muffins with apple puree.

2. Materials and Methods

2.1. Materials

Ingredients for making muffins (wheat flour, sugar, sunflower oil, fresh eggs, milk, baking powder and iodized salt) were purchased from a local supermarket. Cooked apple puree was used to replace sugar and fat (sunflower oil) in muffins. Cooked apple puree was obtained from apples purchased from a local producer in Fălticeni, Suceava county, and was obtained according to the scheme described by Huțu and Amariei in 2024 [16].

2.2. Dosage of Pectin from Apple Puree

A citric acid (Merck KgaA, Darmstadt, Germany) solution pH = 2 was used for pectin extraction from apple puree. The stirring of apple puree and citric acid solution was carried out for 10 min for proper homogenization. The obtained mixture was subjected to ultrasound using an ultrasonic device (Sonopuls HD 2070; Bandelin, Germany) at an amplitude of 60% for 20 min [12,32,33]. To precipitate the extracted pectin, the ultrasonic mixture was subjected to centrifugation at 4000 rpm for 40 min using a Z216-MK refrigerated centrifuge (Hermle Labortechnik, Wehingen, Germany). The obtained supernatant was mixed with ethyl alcohol (Merck KgaA, Germany) and stored at 4–6 °C for 12 h. The precipitated pectin was centrifuged again at 4000 rpm for 40 min and then washed 3 times with ethyl alcohol. The purified pectin was dried at 50 °C using laboratory oven with air circulation ZRD-A5055 (Zhicheng Analysis Instrument, Shanghai, China) until a constant mass was obtained [12,32,33].

2.3. Determination of the Degree of Esterification of Pectin

The degree of esterification of pectin was determined by titrating a mixture (50 mg of dry pectin dissolved in 10 mL of boiled distilled water) with 0.1 N NaOH (V1) in the presence of phenolphthalein until a persistent pink color appeared. After the titration, 20 mL of 0.5 M NaOH (Honeywell, Seelze, Germany) was added to the mixture, and after 30 min, 20 mL of 0.5 M HCl (Sigma-Aldrich, Munich, Germany) was added. The obtained mixture was titrated again with 0.1 N NaOH (V2) [12,33]. The degree of esterification of pectin was calculated with the relation:

$$DE \left(\mathrm{\%}\right)=V2/(V2+V1)\times 100;$$

(1)

where V1 is the volume of 0.1 N NaOH used in the first titration, mL, and V2 is the volume of 0.1 N NaOH used in the second titration, mL.

2.4. Preparation of the Muffin Dough

The doughs were obtained using a KitchenAid—Professional Mixer (KPM5, KitchenAid, St. Joseph, MI, USA). After obtaining the dough, it was kept for one hour at the refrigeration temperature. The samples were marked from 1 to 16 or with the letter R, followed by two numbers (the first number is represented by the percentage of substituted sugar, and the second by the percentage of substituted oil).

2.5. Determination of Texture

The determination of the texture profile of the muffins using the Perten TVT-6700 textrometer (Perten Instruments, Hägersten, Sweden) was carried out by applying a double compression of up to 25% of the initial height of the sample, using a cylinder probe with a diameter of 25 mm, at a speed of 1.0 mm/s and a 5 s interval between compression cycles [34,35]. The texture parameters obtained were hardness, chewiness and cohesiveness. All measurements were performed in triplicate.

2.6. Determination of Muffin Height

One hour after the muffins were removed from the oven and cooled, the height was determined by measuring the distance from the bottom of the muffins to the highest point. Three measurements were made for each sample using a digital caliper (Mitutoyo Deutschland GmbH, Neuss, Germany) [4,36,37]. All measurements were performed in triplicate.

2.7. Determination of Muffin Volume

Volume determination is an easy method for evaluating the impact of sugar and/or oil percentage substitution on the physical properties of muffins. Thus, one hour after cooling the muffins, their volume was determined three times for each sample by the rapeseed displacement according to the AACC (2000) method 10-05 [36,38,39,40,41,42]. All measurements were performed in triplicate.

2.8. Loss of Weight during Baking

To determine the weight loss during baking, the mass of the dough before baking (Wb) and the mass of the muffin (Wm) one hour after baking were determined using an analytical balance Partner AS 220.RS (Radwag, Torunska, Poland). All measurements were performed in triplicate. The calculation formula used for weight loss during baking was [35,37,43,44,45,46]:

$$WL(100\%)=(Wb-Wm)/Wb\times 100$$

(2)

2.9. Experimental Design

The Design Expert v11 program (trial version, Stat-Ease, Minneapolis, MN, USA) generated 17 samples as representative. Response surface methodology (RMS) was used to model the results [47,48]. For the Optimal (Custom) experiment, the effects of the three independent variables (percentage of substituted sugar, percentage of substituted oil and baking temperature) on muffin texture (hardness, chewiness and cohesiveness) on physical properties (muffin height, volume and baking loss) were studied (Figure 1). The three factors varied as follows: the percentage of substituted sugar (SS%) and the percentage of substituted oil (SO%) varied from 25% to 100%, and the baking temperature (T) varied between 180–220 °C. The coded levels of the design variables are presented in

Table 1. Levels of the independent variables in the experimental Optimal (Custom) model regarding the influence of percentage of substituted sugar (SS%), percentage of substituted oil (SO%) and baking temperature (T) on muffin properties.

Variable	−1	−0.33	0	0.33	1
Percentage of substituted sugar, %	25	50	-	75	100
Percentage of substituted oil, %	25	50	-	75	100
Temperature, °C	180	-	200	-	220

.

The Optimal (Custom) process (3 factors and three levels), resulting from the establishment of the experimental matrix, involved 16 experiments (

Table 2. Coded values and actual values of the factors used in programming the experiment regarding the influence of the percentage of substituted sugar (SS%), the percentage of substituted oil (SO%) and the baking temperature (T) on the muffin properties.

Run	Actual Values
	Percentage of Substituted Sugar (%)	Percentage of Substituted Oil (%)	Temperature (°C)
1 (R 75 25)	75	25	200
2 (R 25 75)	25	75	200
3 (R 75 25)	75	25	200
4 (R 75 75)	75	75	220
5 (R 75 75)	75	75	220
6 (R100 25)	100	25	220
7 (R 100 100)	100	100	200
8 (R 50 50)	50	50	180
9 (R 25 25)	25	25	180
10 (R 100 50)	100	50	180
11 (R 25 25)	25	25	200
12 (R 25 25)	25	25	220
13 (R 25 75)	25	75	200
14 (R 25 100)	25	100	220
15 (R 100 50)	100	50	200
16 (R 50 100)	50	100	180

).

2.10. Crumb Microstructure

The microstructures of the core were made using a stereomicroscope (Microscope Optika SZ A1, Italy). For this purpose, the muffin samples were cut into 10 mm slices.

3. Results

3.1. Determination of Pectin and Degree of Esterification of Pectin in Apple Puree

Pectin is classified into two main types based on the degree of methylation: high- methylated pectin (HM) and low-methylated pectin (LM). The pectin with DE > 50% is known as high-methoxy (HM) pectin, and that with DE < 50% is known as low-methoxy (LM) pectin. These types have distinct properties and uses depending on their chemical structure [18,49,50]. The pectin content and the degree of pectin esterification in the analyzed apple purees are presented in

Table 3. Pectin content and degree of pectin esterification in apple purees.

Sample	Pectin, %	DE, %
Raw puree	0.64 ± 0.03 a	79 ± 0.2 a
Cooked puree	0.57 ± 0.02 a	68 ± 0.1 b

DE—degree of esterification. Values followed by different superscript letters (a,b) are statistically different at 95% confidence level.

.

To replace the sugar and fat in the muffins, cooked puree was chosen, obtained according to the method described by [16], due to its microbiological stability, respecting the limits established by the Food Safety and Standard Regulations (FSSAI), European legislation and Food and Drug Administration (FDA), regarding the presence of microorganisms, such as Salmonella, Clostridium botulinum, Enterococcus, yeasts, molds, total germ count, Bacillus cereus, Escherichia coli and coliforms [16]. Hydrophobic moieties, such as methyl esters and acetyl groups from HM pectin of apple puree influence its adsorption at the oil–water interface and improve the texture of the muffins, providing uniform and pleasant consistency [51]. Importantly, pectin is practically odorless and tasteless, which means that it does not add a taste or aroma of its own to the food products in which it is found. The formulation of muffins prepared with different levels of sugar and fat replacement with applesauce is shown in

Table 4. Formulation of muffins prepared with different levels of sugar and fat replacement by apple puree.

Run	Ingredients (g)
	Eggs	Sugar	Oil	Milk	Wheat Flour	Baking Powder	Salt	Apple Puree (0.57% Pectin)
1	75	25	75	35	125	3.5	1.5	100
2	75	75	25	35	125	3.5	1.5	100
3	75	25	75	35	125	3.5	1.5	100
4	75	25	25	35	125	3.5	1.5	150
5	75	25	25	35	125	3.5	1.5	150
6	75	-	75	35	125	3.5	1.5	125
7	75	-	-	35	125	3.5	1.5	200
8	75	50	50	35	125	3.5	1.5	100
9	75	75	75	35	125	3.5	1.5	150
10	75	-	50	35	125	3.5	1.5	150
11	75	75	75	35	125	3.5	1.5	150
12	75	75	75	35	125	3.5	1.5	150
13	75	75	25	35	125	3.5	1.5	100
14	75	75	-	35	125	3.5	1.5	125
15	75	-	50	35	125	3.5	1.5	150
16	75	50	-	35	125	3.5	1.5	150

.

3.2. Determination of Muffin Texture

A non-significant lack of fit test indicates good model fit, a small p-value (p < 0.05) indicates that there is a significant difference between groups or factors, and a high R² suggests that the model explains most of the variability. Thus, to represent the evolution of the hardness, chewiness and cohesiveness of the muffins, the quadratic polynomial model was chosen. Based on the analysis of variance (ANOVA) results in

Table 5. Analysis of variance (ANOVA) for the polynomial model in terms of muffin hardness, chewiness and cohesiveness.

Hardness (N)
Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	3.85	9	0.4281	23.05	0.0006
A—Subtituted sugar	1.45	1	1.45	78.00	0.0001
B—Substituted oil	0.0099	1	0.0099	0.5314	0.4935
C—Temperature	0.6833	1	0.6833	36.80	0.0009
AB	0.2764	1	0.2764	14.89	0.0084
AC	0.1647	1	0.1647	8.87	0.0247
BC	0.0690	1	0.0690	3.72	0.1021
A2	1.18	1	1.18	63.41	0.0002
B2	0.7080	1	0.7080	38.13	0.0008
C2	0.0165	1	0.0165	0.8880	0.3824
R2	0.971
Chewiness (N)
Model	2.12	9	0.2355	36.75	0.0001
A—Subtituted sugar	0.0312	1	0.0312	4.88	0.0693
B—Substituted oil	0.0164	1	0.0164	2.56	0.1604
C—Temperature	0.0834	1	0.0834	13.01	0.0113
AB	0.0066	1	0.0066	1.03	0.3500
AC	0.2034	1	0.2034	31.74	0.0013
BC	1.43	1	1.43	223.10	<0.0001
A2	0.0197	1	0.0197	3.08	0.1297
B2	0.5772	1	0.5772	90.06	<0.0001
C2	0.0123	1	0.0123	1.92	0.2156
R2	0.982
Cohesiveness (/)
Model	0.0307	9	0.0034	32.15	0.0002
A—Sugar	0.0003	1	0.0003	2.44	0.1691
B—Oil	0.0037	1	0.0037	35.29	0.0010
C—Temperature	0.0026	1	0.0026	24.38	0.0026
AB	0.0076	1	0.0076	71.78	0.0001
AC	0.0086	1	0.0086	80.99	0.0001
BC	0.0030	1	0.0030	28.76	0.0017
A2	0.0069	1	0.0069	65.29	0.0002
B2	0.0000	1	0.0000	0.4502	0.5272
C2	0.0002	1	0.0002	1.64	0.2474
R2	0.9679

, the model is effective in explaining and predicting the variation in muffin textural parameters, namely hardness, chewiness and cohesiveness.

The correlation between the experimental (actual) and predicted values of the response variables are shown in

Table 6. Optimal (Custom) model with experimental and predicted values for muffin hardness, chewiness and cohesiveness.

Run	Independent Variables			Measured Response			Predicted Response
	SS, %	SO, %	T, °C	Hardness, N	Chewiness, N	Cohesiveness, -	Hardness, N	Chewiness, N	Cohesiveness, -
1	75	25	200	7.86	5.51	0.63	7.97	5.54	0.63
2	25	75	200	8.3	6.01	0.71	8.41	5.90	0.71
3	75	25	200	8.08	5.59	0.64	7.97	5.54	0.63
4	75	75	220	8.85	5.62	0.69	8.70	5.59	0.68
5	75	75	220	8.52	5.53	0.67	8.70	5.59	0.68
6	100	25	220	7.13	6.01	0.79	7.11	5.98	0.79
7	100	100	200	7.49	5.57	0.67	7.46	5.52	0.67
8	50	50	180	8.44	5.85	0.63	8.36	5.78	0.63
9	25	25	180	7.51	4.88	0.65	7.53	4.89	0.64
10	100	50	180	7.36	6.23	0.63	7.38	6.23	0.63
11	25	25	200	8.19	5.47	0.63	8.19	5.49	0.63
12	25	25	220	8.73	6.22	0.65	8.72	6.21	0.4
13	25	75	200	8.52	5.83	0.73	8.41	5.90	0.71
14	25	100	220	8.13	5.09	0.71	8.11	5.08	0.71
15	100	50	200	7.59	5.99	0.7	7.59	6.05	0.69
16	50	100	180	7.99	6.01	0.71	8.01	6.05	0.70

SS—substituted sugar; SO—substituted oil; T—temperature.

.

To make predictions about the hardness, chewiness and cohesiveness of muffins studied for the levels of each factor, the coded factor Equations (3)–(5) can be used. Analyzing Equations (3)–(5) corresponding to the model for each factor, a ranking of the significant factors is observed in the order of importance of their impact on the hardness, chewiness and cohesiveness of the muffins.

Hardness (N) = 8.81444 − 0.399673 × SS + 0.0348701 × SO + 0.288479 × T + 0.218658 × SS × SO − 0.186782 × SS × T − 0.120904 × SS × T − 0.68025 × SS² − 0.52308 × SO² − 0.0662622 × T²

(3)

Chewiness (N) = 5.93033 + 0.0587074 × SS − 0.0450006 × SO − 0.100777 × T − 0.0337358 × SS × SO − 0.207557 × SS × T − 0.550314 × SS × T + 0.0881037 × SS² − 0.472288 × SO² + 0.0571753 × T²

(4)

Cohesiveness (/) = 0.643195 − 0.00534385 × SS + 0.0214696 × SO + 0.0177404 × SS − 0.0362734 × SS × SO + 0.0426371 × SS × T − 0.0254079 × SO × T + 0.0521497 × SS² − 0.00429425 × SO² + 0.00680574 × T²

(5)

The linear term T has the greatest positive effect on hardness (Equation (3)), followed by the linear terms SS (substituted sugar) > the interaction of the linear terms SS × SO (substituted sugar × substituted oil) > SO (substituted oil). On the other hand, responsible for the greatest negative effect on hardness is followed by the quadratic terms SO² (substituted oil) > linear term SS > SS² (substituted sugar) and, respectively, the interaction of the linear terms SS × T, SO × T and the quadratic term T².

Regarding chewability (Equation (4)), the quadratic term SS² has the greatest positive effect, followed by the linear term SS (substituted sugar) > T (temperature), and the interaction of the terms has the greatest negative effect linear SS > T² (substituted sugar × temperature). In the case of cohesiveness (Equation (5)), the positive effect with the greatest impact is represented by quadratic term ZS², followed by the linear terms US > ZS > ZS × T.

Hardness, the force required to deform or crush a muffin, can be influenced by the ingredients or the proportion of the ingredients and it is an important parameter that can affect the acceptability and quality of the final product. Regarding the substitution of sugar and oil with apple puree from muffins, the hardness did not show significant differences (p < 0.01), and a similar result was obtained by Psimouli and Oreopoulou (2013) following the substitution of fat from cakes with inulin and pectin [51]. The hardness showed an increase in the case of samples where the percentage of substitution was around 50%, and a similar variation was obtained by Belorio et al. (2019), in the case of cakes where fat was substituted with psyllium fiber, and Majzoobi et al. (2018), who substituted sugar and fat in cakes with rebaudioside A and inulin [36,44]. Pectin from apple puree, used as a substitute for sugar and fat, contributed to obtaining a hardness close to that of the control sample, similar to results obtained by Lim et al. (2014), by using pectin as a substitute [10].

Chewiness increased with the increase in substitution percentage, similar to results obtained by Arifin et al. (2019) and Eslava-Zomeño et al. (2016) [52,53].

In the case of muffins in which both sugar and fat were partially or totally substituted, due to the pectin content of the apple puree, the cohesiveness did not show significant changes (p < 0.01), similar results obtained by Zahn et al. (2010) in muffins in which fat was substituted with inuli [54]. Also, by substituting the fat from the cakes with a natural ingredient derived from flaxseed, Eslava-Zomeño et al. obtained insignificant differences (p < 0.01) in terms of cohesiveness [53]. In addition, Psimouli and Oreopoulou (2013) substituted the fat in the cakes with inulin and pectin, and the cohesiveness did not show significant changes (p < 0.01) [51]. In 2019, Arifin et al. obtained similar results by substituting fat from muffins with pumpkin puree [52]. The highly methylated pectin in apple puree contributes to a fluffier texture and increased volume of baked products, as the gel network formed can retain air bubbles created during the baking process [51]. Also, pectin interacts with the proteins and starch in the flour, influencing their behavior during baking, forming a complex with the proteins, modifying the gluten network and influencing the final texture of the product [51].

By making and examining the three-dimensional graphic representations (Figure 2), the factors on the hardness, chewiness and cohesiveness of the muffins are observed.

3.3. Study on the Effects of Replacing Sugar and Fat with Apple Puree on the Rheological Behavior of Muffin Dough

The rheological characteristics of muffin doughs, obtained through dynamic rheological measurements, are essential for understanding their behavior during the manufacturing process, as well as for evaluating the texture and quality of the finished product. The dough must be optimally viscous to capture gas bubbles during mixing and retain them during baking [55]. Dynamic rheological measurements allow the analysis of dough viscoelasticity, that is, its properties behave both as an elastic solid and as a viscous fluid. The main rheological characteristics analyzed through dynamic measurements were the modulus of elasticity (G’) and the modulus of viscosity (G”) with frequency or, respectively, the modulus of elasticity (G’) and the modulus of viscosity (G”) with temperature (Figure 3 and Figure 4) [55,56].

The modulus of elasticity (G’) represents the elastic component of the dough, i.e., the energy stored in the material during deformation that can be recovered later. Higher values of the modulus of elasticity (G’) indicate a more elastic dough with a well-defined and stable structure. The viscosity modulus represents the viscous component, i.e., the energy dissipated as heat during deformation. Higher values of the modulus of elasticity (G”) suggest a more fluid behavior of the dough, which means that the material deforms more easily and has a greater degree of relaxation.

In all samples, both G’ and G” values increased with frequency (Figure 5 and Figure 6). All muffin doughs in which sugar and fat were partially or totally substituted showed a modulus of elasticity and viscosity higher than that of the control sample. The substitution of sugar and fat with apple puree, with a high fiber content (pectin), had a significant effect on the viscoelastic properties of the dough. As the fiber content increased, the values of both moduli increased. Fibers have high water binding capacity, so water availability has decreased. Similar results were obtained by Aydogdu et al. (2018) by adding fiber to cookie dough [56]. Also, Singh et al. (2016) demonstrated that when dietary fiber from carrot pomace was added to muffin dough, the modulus of elasticity and viscosity of the dough, due to the reduced amount of free water available in the dough, increased [57,58].

Rheological analysis can identify dough transition points, such as changes that occur during the heating process. These points are important to understand how the dough loses its fluidity and acquires a stable structure during baking.

To investigate the structural changes that occur in different muffin doughs during heating, the viscoelastic properties were studied from 20 °C to 90 °C, in an attempt to simulate the behavior of the dough in the oven. The structural changes that occur in muffin dough during baking are determining factors in the formation and stability of bubbles and determine the final structure and texture of the baked product. In particular, the role of sucrose is crucial during heating, as it increases the temperature of starch gelatinization and protein denaturation. This allows for the correct formation of water vapor, and CO₂ allows the air cells to expand sufficiently before the dough hardens [23,59].

The increase in the value of the elasticity and viscosity modules after the inflection point are related to the increase in dough consistency associated with the processes of starch gelatinization and protein coagulation, and similar were results obtained by Martínez-Cervera et al. by replacing sucrose in muffins with polyols [23].

The quadratic polynomial model was chosen to represent the evolution of the height, volume and baking loss of the muffins. Based on the analysis of variance (ANOVA) results in

Table 7. Analysis of variance (ANOVA) for the polynomial model in muffin height, volume and baking loss.

Height (mm)
Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	23.83	9	2.65	23.41	0.0005
A—Subtituted sugar	5.96	1	5.96	52.71	0.0003
B—Substituted oil	0.7707	1	0.7707	6.82	0.0401
C—Temperature	0.2758	1	0.2758	2.44	0.1694
AB	10.22	1	10.22	90.38	<0.0001
AC	3.16	1	3.16	27.98	0.0018
BC	0.7924	1	0.7924	7.01	0.0382
A2	0.1122	1	0.1122	0.9926	0.3576
B2	4.31	1	4.31	38.16	0.0008
C2	1.64	1	1.64	14.54	0.0088
R2	0.972
Volume (cm3)
Model	76.51	9	8.50	12.33	0.0032
A—Subtituted sugar	58.77	1	58.77	85.25	<0.0001
B—Substituted oil	3.97	1	3.97	5.75	0.0534
C—Temperature	0.1427	1	0.1427	0.2069	0.6652
AB	1.29	1	1.29	1.87	0.2203
AC	2.52	1	2.52	3.65	0.1045
BC	4.80	1	4.80	6.97	0.0386
A2	0.1615	1	0.1615	0.2343	0.6455
B2	4.97	1	4.97	7.21	0.0363
C2	0.0751	1	0.0751	0.1089	0.7526
R2	0.948
Weight Loss during Baking (%)
Model	29.95	9	3.33	13.51	0.0012
A—Substituted sugar	14.14	9	1.57	7.98	0.0100
B—Subtituted oil	0.4596	1	0.4596	2.34	0.1772
C—Temperature	1.14	1	1.14	5.82	0.0524
AB	0.0000	1	0.0000	0.0002	0.9880
AC	0.0063	1	0.0063	0.0320	0.8640
BC	1.06	1	1.06	5.37	0.0597
A2	0.0035	1	0.0035	0.0176	0.8987
B2	4.15	1	4.15	21.11	0.0037
C2	5.61	1	5.61	28.50	0.0018
R2	0.922

, the model is suitable for explaining and predicting the variations in muffin height, volume and baking loss.

Pastries with high height and volume tend to be perceived as having a more porous and soft texture. These characteristics contribute to their greater sensory acceptability [36]. Sucrose and fat can delay starch gelatinization during baking. This delay allows the air bubbles to expand properly before the cake structure fully hardens. It contributes to maintaining a soft and light texture, preventing the formation of a too-stiff starch network too early during baking [36].

Due to the ability of pectin to maintain the volume of the products in which it is incorporated, the substitution of sugar and fat with apple puree did not show significant changes (p < 0.01) on the height and volume of the muffins, similar to results obtained by Majzoobi et al. (2018) by replacing sugar and fat with rebaudioside A and inulin [36]. Also, similar results for muffin height were obtained by Zahn et al. (2010) and Arifin et al. (2019) replacing the fat in the composition of the muffins with inulin, i.e., pumpkin puree [52,54].

The effect with the greatest positive impact on muffin height (Equation (6)) was the interaction of the linear terms SS × T > SO × T, followed by the quadratic term SS² > the linear term T. The effect with the greatest negative result on the height of the muffins was a result of ZS × US > US² > ZS > T² > US.

Regarding the volume of muffins (Equation (7)), the effect with the greatest positive impact was from the linear terms SS × T > SS × SO followed by SS² > T².

The baking loss increased significantly (p < 0.01) with the increase in the percentage of substituted sugar and fat, and a similar increase was obtained by Belorio et al. (2019), who replaced fat with psyllium fiber, and Majzoobi et al. (2018) [36,44]. In the case of ripening loss (Equation (8)), the effect with the greatest positive impact was achieved by the linear term SO, followed by the terms SO² > SS² > SO > T² > SS > ZS × US. On the other hand, the effect with the greatest negative impact was achieved through the interaction of the linear terms ZS × T > US × T, followed by T.

Height (mm) = 47.2798 − 0.810815 × SS − 0.308171 × SO + 0.183287 × T − 1.3296 × SS × SO + 0.818677 × SS × T + 0.409682 × SO × T + 0.21004 × SS ² − 1.29135 × SO² − 0.661732 × T²

(6)

Volume (cm³) = 54.8599 − 2.54607 × SS − 0.699046 × SO − 0.131807 × T + 0.472414 × SS × SO + 0.730254 × SS × T − 1.00846 × SO × T + 0.251973 × SS² − 1.38612 × SO² + 0.141383 × T²

(7)

Weight loss during baking (%) = 7.09273 + 0.22516 × SS + 0.375521 × SO − 0.00243187 × T + 0.0329812 × SS × SO − 0.472954 × SS × T − 0.027098 × SO ×T + 1.2777 × SS² + 1.47193 × SO² + 0.259555 × T²

(8)

Table 8. Optimal (Custom) model with experimental and predicted values for muffin height, volume and baking loss.

Run	Independent Variables			Measured Response			Predicted Response
	SS, %	SO, %	T, °C	Height, mm	Volume, cm3	Weight Loss Baking, %	Height, mm	Volume, cm3	Weight Loss Baking, %
1	75	25	200	46.72	54.00	7.96	46.49	53.19	8.40
2	25	75	200	48.82	58.00	7.91	48.50	57.11	8.42
3	75	25	200	46.33	53.00	8.73	46.49	53.19	8.40
4	75	75	220	47.02	54.00	7.41	46.57	53.62	7.69
5	75	75	220	46.15	53.00	8.04	46.57	53.62	7.69
6	100	25	220	46.91	53.00	9.55	46.96	53.15	9.47
7	100	100	200	43.73	51.00	10.46	43.75	50.95	10.48
8	50	50	180	46.83	55.20	7.33	46.95	56.05	7.30
9	25	25	180	46.46	58.00	8.89	46.37	57.44	9.04
10	100	50	180	45.56	52.00	9.46	45.55	51.69	9.35
11	25	25	200	45.86	56.50	9.6	45.99	57.44	9.27
12	25	25	220	44.33	58.00	9.88	44.28	57.73	10.03
13	25	75	200	48.23	57.00	8.89	48.50	57.11	8.42
14	25	100	220	47.11	53.00	10.67	47.14	53.37	10.66
15	100	50	200	47.11	52.00	8.46	47.08	52.49	8.62
16	50	100	180	45.46	55.00	9.13	45.44	55.02	9.13

SS—substituted sugar; SO—substituted oil; T—temperature.

shows the correlation between the experimental (actual) and predicted values of the response variables.

For an advanced understanding of the influence of the factors on the height, volume and baking loss of the muffins, the three-dimensional graphical representations in Figure 7 were made and analyzed.

3.4. Optimization

Optimization was performed to obtain muffins with optimal textural and physical properties. A numerical optimization function was used to find a composition that provides maximum desirability for the chosen attributes, i.e., maximum values for sugar and fat percentage subtitled (Figure 8).

Taking into account all quality characteristics and following the optimization procedure of the Design Expert program in which fat and sugar inclusion levels were minimized and apple puree inclusion levels were maximized, a muffin formula was predicted in which sugar and fat are substituted at percentages of 34.04% and 43.78%, respectively, the baking was temperature is 193.41 °C (Figure 8).

3.5. Analysis of Muffin Samples by Fourier Transform Infrared Spectroscopy (FTIR)

FTIR (Fourier transform infrared spectroscopy) analysis is a technique used to identify the chemical composition of a material based on the absorption of infrared radiation by the functional groups in the molecules. It is used to characterize ingredients and structural changes in dough, baked goods or other manufacturing processes. In the case of muffin dough, FTIR can provide information about the interactions between proteins, carbohydrates, lipids and other ingredients. The FTIR spectral information described for the ingredients and muffin samples highlights the presence of certain important functional groups, identified by the analysis of the corresponding spectral peaks. FTIR spectroscopy focuses on the specific absorption regions of these bonds, which confirm the presence of chemical groups characteristic of muffin samples [60].

Most of the spectra were observed in two regions (3900–2800 and 1700–1000 cm⁻¹) confirming the presence or absence of specific functional groups. The FTIR spectra of the muffin samples, with visible peaks at frequencies of 3853, 3735, 3282, 2925, 1636, 1540, 1456, 1020, 1150, 1020, 995 and 926 cm⁻¹, are presented in Figure 9. Peaks at 3853 cm⁻¹ and 3735 cm⁻¹ correspond to stretching vibrations of O-H (hydroxyl) groups, indicating the presence of water or residual moisture in the samples, as well as possible interactions of proteins and carbohydrates with water. The peak at the 3282 cm⁻¹ frequency can be associated with the stretching vibrations of the N-H groups in flour proteins (especially gluten), as a result of the interaction with water (protein hydration). The 2925 cm⁻¹ peak corresponds to the asymmetric stretching vibrations of C-H bonds in aliphatic chains (CH₂, CH₃). These groups are typical of fats (lipids) and triglycerides that are present in the dough through oil or other added fats [60].

The absorption peak at 1745 cm⁻¹ is characteristic of the stretching vibrations of the C=O (carbonyl) bond in esters, associated with fats and oils, especially triglycerides. This peak indicates the presence of fats used in the dough, such as vegetable oils. This peak is important because it confirms the presence of lipid compounds, which play an essential role in the formation of the texture and structure of the muffins, contributing to the tenderness and stability of the final product. However, this peak may also indicate the presence of pectin, since pectin is a polysaccharide that contains esterified groups in its structure. Pectin has galacturonic acid units, which can be esterified with methyl groups. In the case of methoxylated pectin, the C=O ester bond stretching vibration also appears around 1745 cm⁻¹. This absorption indicates the methoxylated ester groups in pectin, so the presence of pectin can be detected in the same frequency range. The peak at 1636 cm⁻¹ represents the stretching vibrations of C=O (carbonyl) bonds from amide (proteins) or absorbed water (H-O-H strain band). This peak indicates protein interaction with water [60].

The peak at 1540 cm⁻¹ is specific for the amide band II, indicating the presence of proteins. The torsional vibrations of the N-H bonds and the stretching vibrations of the C-N bonds are typical of protein structures such as gluten proteins in flour. The 1456 cm⁻¹ peak is associated with the torsional vibrations of C-H bonds in aliphatic CH₂ and CH₃ groups, typical of lipid components and long aliphatic chains in fats. The 1150 cm⁻¹ peak corresponds to stretching vibrations of C-O-C bonds in polysaccharide structures such as starch. This is also a region of the spectrum characteristic for the C-O-C stretching vibrations of the glycosidic bonds, which link the monosaccharides together within the polysaccharide structure of pectin. The 1020 cm⁻¹ peak is specific to the stretching vibrations of C-O bonds in carbohydrates and indicates the presence of starch or other sugars. Starch plays an essential role in muffin batter, contributing to the binding of the ingredients, the texture and the final structure of the product. Peaks 995 cm⁻¹ and 926 cm⁻¹ are related to C-O and C-C vibrations in polysaccharides and starch, indicating changes in starch structure during the baking process. FTIR spectra of muffin samples reveal the presence of their major components: protein (gluten), carbohydrates (starch and sugars), fat and water. Peaks in the 3853–2925 cm⁻¹ regions are associated with protein–water interactions, and those in the 1636–926 cm⁻¹ regions are related to carbohydrate and fat structures essential for muffin dough formation and stability [60].

3.6. Muffin Crumb Microstructure

The analysis of the images of the core (Appendix A) highlighted the effect of replacing sugar and fat in muffins with apple puree on the uniformity of their structure. A good-quality muffin is characterized by small and uniform pores, which is desirable for good porosity. From Figure A1, it can be seen that the substitution of sugar and fat in the muffins resulted in an increase in the average pore diameter. With the increase in the percentage of substituted fat, the core has become more uneven, the pores are fewer but larger, so the volume of the product is not affected. A 50% substitution of fat does not seem to affect the structure and uniformity of the muffin core compared to the control sample. The structure and uniformity of the muffin core in which total sugar and fat were substituted differed significantly compared to the control sample. Also, the color difference between the samples can be attributed to the Maillard reaction, which occurs in the presence of sugar and contributes to the development of the characteristic brown color [61].

4. Conclusions

Apple puree, by its composition, is a healthy alternative to sugar and fat in muffins, offering benefits such as reducing calories and improving the texture and stability of baked goods. The percentage used was established following the optimization scheme of the technological process of obtaining muffins.

The impact of apple puree as a substitute for both sugar and oil on the physical and textural of muffins was evaluated. Determinations of texture (hardness, chewiness, cohesiveness) and physical properties (height, volume, weight loss during baking) of the muffins were performed to determine the optimal combinations and proportions of this substitute. The use of the response surface methodology made it possible to optimize the technological process of obtaining muffins by substituting sugar and fat with apple puree.

The result of this study showed that in order to obtain optimal textural and physical properties, the sugar and fat in the muffins can be substituted with applesauce in percentages of 34.04% and 43.78%, respectively.