Analysis of the Correlation of Microstructure, Instrumental Texture, and Consumer Acceptance of Shortbread Biscuits with Selected Sweeteners and Fibre

Abstract

Biscuits are characterized by their popular sweet taste, but they have a poor nutritional profile due to their high sugar and saturated fat content, along with low fibre levels. Their sweetness primarily comes from sucrose, which not only determines the flavour but also performs several technological functions, making it difficult to replace in pastry products. Commercial sweeteners and soluble fibres designed for pastry products are available. Therefore, it is necessary to test the feasibility of using these ingredients in biscuit formulations and assess their impact on biscuit quality. Concurrently, the correlation analysis of dough rheological parameters, structure, and instrumental texture parameters with sensory characteristics will help identify which parameters are strongly correlated and can be used to predict biscuit quality. The purpose of this study was to investigate the dough rheological properties, structure, texture, and sensory characteristics of biscuits in which sucrose was replaced by the commercial sweeteners Tagatesse, maltitol, and erythritol–stevia, with the addition of soluble fibres Nutriose^® FB (wheat fibre) and PromOat 35 (oat fibre). At the same time, a correlation analysis was conducted between dough rheological parameters (stickiness, work of adhesion, dough strength) and biscuit quality parameters, such as water activity, water content, colour, texture (pore area, pore shape, pore elongation), and instrumental texture properties (hardness, brittleness, number of acoustic emission (AE) events, AE event energy), with sensory discrimination evaluated through a consumer test. The use of wheat and oat fibres in combination with sucrose resulted in biscuits with lower apparent density, increased porosity, and weaker texture (fracturability, hardness, number of AE events), yet they had better sensory properties compared to biscuits containing sucrose alone. Replacing sucrose with sweeteners combined with fibres led to a deterioration in the sensory quality of the biscuits and a significant change in the dough’s rheological properties. Regardless of the type of sweetener, biscuits with wheat fibre were rated better than those with oat fibre. Of the tested sweeteners, only maltitol combined with wheat fibre resulted in a sensory quality similar to that of sucrose biscuits. Correlation analysis of all measured biscuit quality parameters showed that only the number of AE events had a strong positive correlation with all tested sensory attributes. Porosity was only correlated with sensory crispness, and fracturability was correlated with sweetness, taste, and overall acceptability. Therefore, it appears that the number of AE events recorded at the time of breaking may be a reliable parameter for predicting biscuit quality.

1. Introduction

Due to their long shelf life, attractive sensory characteristics, variety of taste and texture, and widespread consumption, biscuits are an important snack that consumers eagerly reach for [1,2,3]. In cookie products, sucrose contributes to the appropriate viscosity of the dough, competes with gluten for water molecules, and delays the gelation process of starch, affecting the formation of a fine structure and texture. It also participates in Maillard reactions and caramelization, ensuring the desired colour, and can act as a preservative by effectively reducing water activity [4,5,6,7,8,9]. However, sucrose is consumed in excessive amounts due to its prevalence in modern diets and should account for less than 10% of daily caloric intake [8,10]. Excessive sucrose intake contributes to an increased risk of cardiovascular disease, obesity, dental caries, and type II diabetes [10,11]. As a result, consumers are seeking products with adequate nutritional value but reduced sucrose content. This has led to the proposal of new formulation solutions using different types of sweeteners (saccharides, polyols) and fibres to provide adequate sweetness, texture, and structure in biscuits [6,12,13,14]. This is one strategy for reducing the sucrose content in pastry products [6]. However, due to the unique properties of sucrose, replacing it is challenging without compromising the colour, texture, structure, or sensory properties of biscuits [6,10,15,16]. Therefore, it appears practical to use commercial sweeteners specifically designed for pastry products. Tagatesse, which has a caloric value of 7 kcal, contains isomalt, polydextrose, inulin, D-tagatose, and sucralose. It is recommended as a 1:1 substitute for sucrose in biscuits. A sweetener based on steviol glycosides, with erythritol as the carrier, contains 97 g of carbohydrates per 100 g, with a caloric value of 0 kcal, including 1.5 g of stevia. According to the manufacturer, this sweetener is six times sweeter than sucrose, so the amount used in pastry products should be less than that of sucrose. Commercially available soluble fibres vary in botanical origin and are intended to improve the nutritional value of food while also serving technological functions [17]. They can be used to replace part of the sugar or fat in food products. One such commercial preparation is Nutriose^®FB, derived from wheat grain, which is a form of resistant dextrin. It is recommended, among other uses, to improve the nutritional profile of cakes and reduce their caloric content [17]. Nutriose^®FB is a fermentable carbohydrate that is well tolerated at a daily intake not exceeding 45 g [18]. Nutriose consists of irregular, coarse particles, some of which are hollow [19].

The preparation extracted from oat grain is PromOat 35, a soluble fibre containing 35% (g/100 g d.m.) of β-glucan [20]. β-glucan has exceptional water-binding properties and the ability to form texture, and it positively affects the taste of products [21]. According to the manufacturer, PromOat 35 can be used to replace a portion of the fat in food product formulations [20]. Nutriose^® FB, when present in the diet, has been shown to have a positive effect on the microbiota of healthy humans [22]. In hamster studies, Nutriose^® FB06 was proven to lower cholesterol levels and help maintain normal blood cholesterol concentrations [23]. Similar effects of β-glucan have been proven in clinical trials [24]. In addition, the anticancer properties of β-glucan have been shown [25]. Dietary supplementation with soluble fibre (inulin, resistant maltodextrin) has been shown to significantly reduce blood pressure and improve gut microbiome function [26]. Despite the wide variety and availability of fibre-containing plant products, the Western diet remains deficient in this important ingredient [27]. The literature does not provide a clear explanation as to whether the addition of fibre affects the sensory perception of sweetness. According to Kanemaru et al. [28], the combination of sucrose and soluble starch has a synergistic effect, resulting in increased sweetness. Adding inulin to cookies enhanced their sweetness, while the addition of oat fibre did not affect the perception of sweet taste [29]. Laguna et al. [30] studied the effects of polyols and commercial soluble fibres on the sensory and textural properties of biscuits. They found that reducing the amount of sucrose and adding 25% native inulin resulted in biscuits with flavour, sweetness, and texture similar to the control biscuits. However, the use of erythritol had a negative effect, as the biscuits were not sensorially acceptable. Therefore, it is beneficial to use alternative sweeteners and soluble fibres that can provide the appropriate texture, structure, and sensory properties in biscuits. Texture plays a crucial role in the sensory perception of biscuits and can be evaluated both sensorially and instrumentally [1]. Key texture characteristics, especially hardness and crispness, can be measured using mechanical methods and acoustic emission [31,32,33,34,35]. Since assessing crispness by panellists is challenging [31], instrumental methods, which offer rapid and objective assessments, are particularly useful during product development or reformulation. Both mechanical and acoustic characteristics are strongly influenced by the product’s structure, which results from the formulation composition [1,34,36] and manufacturing technology [37].

The purpose of this study was to evaluate the dough’s rheological properties, structure, and texture (measured both instrumentally—mechanically and acoustically) as well as consumer acceptance of biscuits made with selected sweeteners and the addition of wheat and oat fibres. In addition, a correlation analysis between the instrumental evaluations and consumer acceptance was conducted to identify which instrumentally measured parameters most strongly influence consumer acceptance and can be used to predict the quality of the biscuits.

2. Materials and Methods

2.1. Materials

The research material consisted of shortbread biscuits in which sucrose (S) was replaced with the commercial sweeteners Tagatesse (T), maltitol (M), and erythritol–stevia (ESt), as well as added Nutriose^® FB06 wheat fibre (WF) and PromOat 35 oat fibre (OF). The wheat dextrin soluble fibre Nutriose^® FB06, as specified, contained 83% fibre (AOAC db), 0.1% monosaccharides, 0.2% disaccharides, and 3.9% moisture (Roquette, Beinheim Poland). PromOat^® oat fibre (Brenntag Polska Sp. zo.o., Kędzierzyn-Koźle, Poland) contained 93% d.m., including 56% carbohydrate (with less than 0.5% sugar), 35 ± 2.0% β-glucan, 4% protein, and 0.5% fat. Other ingredients used to make the biscuits included wheat flour type 650 (13% water, 72% carbohydrates, 10% protein, 1.2% fat, 2.3% fibre), sucrose—powdered sugar (Diamant, Glinojeck, Poland), Tagatesse (Dambert Nutrition, Heusden-Zolder, Belgium), maltitol SweetPearl® (Roquette, Beinheim, France), erythritol and stevia (97% d.m., with 1.5% steviol glycosides content) (Stevifit Silver, Zdrofarm, Lublin, Poland), Rubin 20 rapeseed fat (Bunge Polska Sp.zo.o., Kruszwica, Poland), wheat starch (Hortimex, Konin, Poland), milk powder (Mlekowita, Wysokie Mazowieckie, Poland), salt (Kłodawa, Poland), ammonium bicarbonate and sodium bicarbonate (Colian, Opatówek, Poland), rapeseed lecithin (Brenntag Polska Sp. zo.o., Kędzierzyn-Koźle, Poland), and butter flavour (Jar Aromaty, Klaudyn k/Warszawy, Poland).

Table 1. Ingredients of shortbread biscuits (% flour basis). S—biscuits with sucrose, SWF—biscuits with sucrose and wheat fibre, SOF—biscuits with sucrose and oat fibre, TWF—biscuits with Tagatesse and wheat fibre, TOF—biscuits with Tagatesse and oat fibre, MWF—biscuits with maltitol and wheat fibre, MOF—biscuits with maltitol and oat fibre, EStWF—biscuits with erythritol–stevia and wheat fibre, EStOF—biscuits with erythritol–stevia and oat fibre.

Shortbread Biscuits Variant	S	SWF	SOF	TWF	TOF	MWF	MOF	EStWF	EStOF
Sweeteners	Sucrose			Tagatesse		Maltitol		Erytrithol–stevia
	36.7	36.7	36.7	36.7	36.7	36.7	36.7	17.0	17.0
Flour	100	100	100	100	100	100	100	100	100
Nutriose® (WF)	0	11.2	0	11.2	0	11.2	0	9.3	0
PromOat (OF)	0	0	11.2	0	11.2	0	11.2	0	9.3
Fat	11.7	11.7	11.7	11.7	11.7	11.7	11.7	11.7	11.7
Wheat starch	2.8	3.1	3.1	3.1	3.1	3.1	3.1	2.8	2.8
Powder milk	2.8	3.1	3.1	3.1	3.1	3.1	3.1	2.8	2.8
Sodium bicarbonate	1.40	1.6	1.6	1.6	1.6	1.6	1.6	1.6	1.6
Ammonium bicarbonate	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5
Rapeseed lecithin	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5	0.5
Salt	0.3	0.3	0.3	0.3	0.3	0.3	0.3	0.3	0.3
Butter flavour	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1	0.1
Water	21	21	21	25	25	25	25	25	25

shows the composition of shortbread biscuits.

The process of making biscuits began by sifting the flour through a sieve and weighing the ingredients on an analytical balance (Radwag, Radom, Poland) to the nearest 0.01 g, according to the recipe provided in Table 1. The fat was then melted to a temperature not exceeding 38 °C. The prepared ingredients were combined by mixing for 10 min using a food processor (KitchenAid Artisan, Greenville, OH, USA). The dough was rolled out to a thickness of 2 mm using a rolling pin with guides, and biscuits were cut out using a rectangular mould with dimensions of 50 × 100 mm. The biscuits were baked in an electric oven (Amica, Wronki, Poland) at 160 °C for 5 min, followed by an additional 3 min at 180 °C. After baking, the biscuits were cooled to room temperature (24 °C) and then packaged in 55 MY thick polypropylene film (Pakmar, Warszawa, Poland). Analyses were conducted 7 days after baking.

2.2. Rheological Properties of the Dough

The rheological properties of the dough were measured using a TA.HDplus texturometer (Stable Micro Systems Ltd, Godalming, UK) with up to 5 kg in force with the A/DSC attachment (SMS/Chen-Hoseney Dough Stickiness Rig). The texturometer’s instructions were followed according to the methodology used by Grausgruber et al. [38]. First, 1 mm of dough was extruded from the attachment and covered with acrylic glass. After 30 s, a compression test was performed at a speed of 0.5 mm/s and a compression force of 80 g. The contact time of the probe with the dough was 0.1 s. From the force–time curves, the rheological parameters were determined: the stickiness (maximum force (g)), work adhesion (area under the curve (g·s)), and strength of the dough (the distance the dough was stretched after the probe return (mm)).

Each type of dough was prepared twice, and the tests were conducted in ten repetitions, 30 min after production.

2.3. Measurement of Physical, Structural, and Textural Properties of Shortbread Biscuits

The water content was determined using the dryer method, and water activity was measured with an AquaLab apparatus (Decagon Devices Inc., Pullman, WA, USA) [1]. Measurements were performed in duplicate.

The porosity (ε) of the biscuits was calculated from the apparent density and true density [1]. The results were averaged from ten replicates.

The microstructure of the biscuits was evaluated using a TM-3000 scanning electron microscope (HITACHI, Tokyo, Japan), which allowed for observation in ESEM mode (without preliminary sample preparation). Samples with specific dimensions (5 × 5 × 10 mm) were cut from the biscuits, and their cross-sectional images were captured at 50× magnification. Image processing of the cookie structure was performed using the computer image analysis program MultiScanBase v. 18.03. On the projection surface, pores were manually marked, and fields corresponding to their area were obtained. The following parameters were determined: S—pore area (mm²), L—perimeter (mm), and Feret’s diameters. Analyses were conducted for 200 cells of each type of material. The shape coefficient (Φ) was calculated using the formula Φ = 4πS/L², and the elongation factor was determined as the ratio of Feret’s diameters, F_max/F_min [32].

Instrumental colour measurement was performed using a CR-300 Chroma Meter colourimeter (Konica Minolta) with the L*a*b* colour system. The determination was conducted in five replicates for each sample. Based on the determined L*a*b* colour parameters, the relative colour difference (ΔE) was calculated [39]. The reference sample was biscuits made with sucrose alone.

A three-point breakage test of the biscuits was conducted using a TA.HD.plus Texture Analyzer (Stable Micro Systems Ltd., Godalming, UK) with a 5 kg measuring head. The biscuits were placed on wedge-shaped supports, spaced 24 mm apart. The head with the breaking probe moved perpendicular to the biscuits at a speed of 20 mm/min.

An accelerometer (type 4381 from Brüel & Kjær, Nærum, Denmark) and an analogue-to-digital processing board (type 9112, sampling frequency 44.1 kHz, Adlink Technology Inc., New Taipei City, Taiwan) were used to record the acoustic emission (AE) generated during the three-point biscuit breaking test [1]. The Calculate44 Hz_auto program was used to determine basic acoustic emission descriptors, including the energy of a single AE event and the number of AE events [37]. The measurements were performed in ten repetitions.

2.4. Measurement of Sensory Properties of Shortbread Biscuits

Sensory evaluation of the biscuit quality was conducted by 90 consumers, who were students from the Faculty of Food Technology at the Warsaw University of Life Sciences. The group of consumers was divided into three teams of 30 people each. Each team received three coded biscuit samples for evaluation. During the evaluation of each sample, still water was used as a neutralizer. A five-point scale was used to assess the quality of the biscuits. The following attributes were evaluated: (i) appearance and colour of the biscuit surface (5—evenly coloured, smooth, non-cracked surface; 2—uneven colour, clearly visible cracks); (ii) texture (5—very crispy and porous texture; 2—insufficiently crispy, hard texture, and little porous texture); (iii) sweetness, defined as the intensity of the sweet taste in the mouth (5—very sweet; 2—not sweet enough); (iv) biscuit characteristic taste (5—very good; 2—not very characteristic of biscuits, not harmonious); (v) overall desirability, defined as appearance, texture, taste, and smell (5—very good; 2—not good enough).

2.5. Statistical Analysis

Statistical calculations were performed using Statistica 13 software. The tables present the mean values of the measured parameters along with the standard deviation (±SD). A one-way analysis of variance (ANOVA) was conducted to analyse the effect of adding fibre to biscuits with sucrose. Significant differences between the mean values of all parameters were evaluated using Tukey’s test, with homogeneous groups indicated by lowercase letters (a, b, c). Next, the effect of the type of sweetener and fibre on biscuit quality was analysed. A two-factor analysis of variance (ANOVA) was performed, and Tukey’s test was used to compare the mean values of the studied parameters. Homogeneous groups were denoted by capital letters (X, Y, Z, U, W). To explore the relationship between the sensory characteristics and the instrumentally measured quality parameters of the biscuits, Pearson correlation analysis was conducted. Finally, principal component analysis with classification (PCA) was performed to determine which biscuit variants were most similar to each other. All analyses were conducted at a significance level of α = 0.05.

3. Results and Discussion

3.1. Rheological Properties

The addition of wheat fibre to a recipe with sucrose (SWF) increased the stickiness, work adhesion, and strength of the dough, while the addition of oat fibre (SOF) resulted in a significant reduction in rheological parameters compared to dough with sucrose alone (

Table 2. Means and ±SDs of the rheological parameters of shortbread dough and the colour parameters of shortbread biscuits.

Shortbread Biscuits Variant	Dough			Shortbread Biscuits
	Stickiness (g)	Work of Adhesion (g·s)	Dough Strength (mm)	L*	a*	b*	ΔE
S	7.75 ± 1.70 b	0.22 ± 0.05 b	0.46 ± 0.03 b	71.24 ± 0.28 c	0.40 ± 0.07 a	17.60 ± 0.22 a	-
SWF	14.09 ± 2.56 cZ	0.44 ± 0.09 cY	0.54 ± 0.02 cU	67.87 ± 0.31 bU	0.51 ± 0.13 bX	17.78 ± 0.24 aX	3.4 ± 0.2 aY
SOF	4.23 ± 0.33 aX	0.11 ± 0.01 aX	0.38 ± 0.02 aXY	59.59 ± 0.59 aX	3.71 ± 0.03 cU	23.6 ± 0.67 bU	13.5 ± 0.2 bW
TWF	6.67 ± 2.49 XY	0.19 ± 0.09 X	0.43 ± 0.05 YZ	68.37 ± 0.43 U	1.24 ± 0.09 Y	19.27 ± 0.32 Y	3.4 ± 0.3 Yy
TOF	4.40 ± 0.58 X	0.11 ± 0.02 X	0.38 ± 0.02 X	61.49 ± 0.12 Y	3.52 ± 0.11 U	24.82 ± 0.82 W	12.5 ± 0.4 W
MWF	7.89 ± 1.91 Y	0.23 ± 0.06 X	0.46 ± 0.05 Z	64.53 ± 0.68 Z	0.99 ± 0.19 Y	19.06 ± 0.38 Y	6.9 ± 2.9 Z
MOF	14.36 ± 2.64 ZU	0.45 ± 0.09 Y	0.54 ± 0.02 U	62.32 ± 0.36 Y	2.81 ± 0.17 Z	22.47 ± 0.33 Z	10.5 ± 0.4 U
EStWF	16.68 ± 3.37 ZU	0.54 ± 0.13 YZ	0.57 ± 0.03 UW	70.68 ± 0.64 W	0.96 ± 0.06 Y	19.42 ± 0.51 Y	2.0 ± 0.2 X
EStOF	17.43 ± 5.47 U	0.60 ± 0.24 Z	0.60 ± 0.09 W	64.05 ± 0.83 Z	3.01 ± 0.16 Z	24.98 ± 0.33 W	10.6 ± 1.7 U
One-way analysis of variance (ANOVA)
Effect added fibre	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Two-way analysis of variance (ANOVA)
Effect of sweeteners	<0.001	<0.001	<0.001	<0.001	0.014	<0.001	0.014
Effect of fibre	0.021	0.114	0.005	<0.001	<0.001	<0.001	<0.001
Sweeteners × fibre	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001

The averages in the columns for biscuits with sucrose and added fibre, which do not share a common lowercase letter (a, b, c) in the superscript, are significantly different between groups. Similarly, those without a common uppercase letter (X, Y, Z, U, W) in the superscript are significantly different between the sweetener and fibre groups (p < 0.05).

). In our study, the wheat (Nutriose) and oat (PromOat) fibres we used differed in composition (Section 2.1). Moreover, Nutriose 06 is a well-soluble fibre consisting of coarser, irregular particles, some of which are hollow [19]. Martínez et al. [19] showed that the addition of soluble fibres such as Nutriose and polydextrose to gluten-free dough resulted in lower consistency and elasticity. The authors found that the size of the fibre particles significantly affected the rheological properties of the gluten-free dough. In addition, they explained that fibres are water-soluble, helping to surround the starch granules, which limits the amount of starch available for water absorption. In another study, it was proven that flour mixtures with Nutriose soluble fibre, inulin, and polydextrose exhibited weaker water-binding capacity than flour alone. The addition of soluble fibre slightly modified the rheological properties of the dough, causing a decrease in texture [40]. β-glucan strongly binds water and, when added to food, modifies its rheological properties and texture [21].

An interaction between the added fibres and sweeteners was observed (p < 0.001) (Table 2). The use of Tagatesse and maltitol sweeteners in combination with wheat fibre (TWF and MWF biscuits) and Tagatesse with oat fibre (TOW) resulted in a decrease in stickiness, although the work of adhesion and dough strength did not change significantly. In biscuits with erythritol–stevia (ESt) combined with both wheat fibre and oat fibre, an increase in all tested dough rheological parameters was observed. A similar effect was seen when maltitol was combined with oat fibre. It appears that the lower ratio of erythritol–stevia to flour, combined with the low solubility of erythritol, contributed to higher values for the rheological parameters. Tagatesse, when combined with both fibres, yielded dough with similar rheological characteristics, but weaker than those with maltitol and erythritol–stevia (Table 2). The literature suggests that the different solubilities of sweeteners, along with the ratio of water to sweetener or sweetener to flour in the dough, significantly affect rheological properties [9,40].

3.2. Physical, Structural, and Textural Properties of Shortbread Biscuits

Biscuits containing sucrose had the lightest colour (Table 2). The addition of fibres and the replacement of sucrose with sweeteners resulted in a significantly darker colour. Greater colour changes were observed with oat fibre compared to wheat fibre. Biscuits with oat fibre were darker, and this colour change was noticeable to the observer, as indicated by the ΔΕ value (Table 2). The type of sweetener that was used significantly influenced the colour of the biscuits (p < 0.001). At the same time, an interaction between the sweeteners and fibres was observed (p < 0.001). Tagatesse with wheat fibre did not affect the colour of the biscuits, whereas a significantly darker colour was observed with oat fibre. In contrast, maltitol and erythritol–stevia with wheat fibre resulted in a darker colour, but much greater differences were seen when these substances were used with oat fibre (Table 2). Wheat fibre is a white powder, while oat fibre is a fine powder with a cream colour. Therefore, biscuits containing oat fibre were darker, and the colour change was noticeable to the observer. The chemical properties of the sweeteners likely influenced the colour of the baked goods. Sucrose, being a nonreducing sugar, breaks down into glucose and fructose during baking, which then participate in Maillard reactions and caramelization. Maltitol, however, does not caramelize at high temperatures [6].

The inclusion of fibres had no effect on the water content (p = 0.745) or water activity (p = 0.171) of biscuits containing sucrose (

Table 3. Means and ± SDs of the water content, water activity, density, porosity, and structure parameters of shortbread biscuits.

Shortbread Biscuits Variant	Water Content (g/100 g)	Water Activity (-)	Apparent Density (kg/m3)	True Density (kg/m3)	Porosity (%)	Pores Area (mm2)	Shape Coefficient (-)	Elongation Factor (-)
S	4.87 ± 0.30	0.354 ± 0.004	319 ± 38 a	1595 ± 200	79.99 ± 2.36 c	0.010 ± 0.014 a	0.502 ± 0.269 a	1.197 ± 0.387 ab
SWF	4.71 ± 0.75 X	0.317 ± 0.004 XY	383 ± 32 bX	1461 ± 262	76.00 ± 2.00 bZ	0.011 ± 0.019 ab	0.501 ± 0.327 aX	1.153 ± 0.436 aX
SOF	4.48 ± 0.28 X	0.289 ± 0.043 X	394 ± 17 bXY	1474 ± 339	75.33 ± 1.04 aZ	0.015 ± 0.014 b	0.598 ± 0.282 bXYZ	1.271 ± 0.478 bXY
TWF	4.73 ± 1.48 X	0.540 ± 0.072 ZU	495 ± 15 Z	1458 ± 267	69.00 ± 0.90 Y	0.013 ± 0.011	0.568 ± 0.245 XYZ	1.414 ± 0.539 YZ
TOF	5.07 ± 0.09 XY	0.648 ± 0.013 U	595 ± 20 U	1498 ± 381	62.68 ± 1.28 X	0.018 ± 0.021	0.636 ± 0.306 YZ	1.603 ± 0.598 ZU
MWF	6.32 ± 0.16 X	0.365 ± 0.020 XYZ	420 ± 26 Y	1422 ± 295	73.69 ± 1.64 Z	0.029 ± 0.080	0.694 ± 0.601 Z	1.360 ± 0.540 YZ
MOF	6.20 ± 0.84 X	0.390 ± 0.008 XYZ	402 ± 20 XY	1487 ± 322	74.80 ± 1.26 Z	0.013 ± 0.030	0.541 ± 0.355 XY	1.267 ± 0.417 XY
EStWF	8.29 ± 0.25 Y	0.503 ± 0.121 YZU	488 ± 29 Z	1512 ± 270	69.42 ± 1.80 Y	0.032 ± 0.147	0.673 ± 0.677 Z	1.445 ± 0.682 ZU
EStOF	7.51 ± 0.31 XY	0.445 ± 0.025 XYZU	594 ± 36 U	1558 ± 397	62.76 ± 2.27 X	0.018 ± 0.019	0.567 ± 0.294 XYZ	1.629 ± 0.632 XYZU
One-way analysis of variance (ANOVA)
Effect added fibre	0.745	0.171	<0.001	0.081	<0.001	0.015	0.020	<0.001
Two-way analysis of variance (ANOVA)
Effect of sweeteners	0.002	<0.001	<0.001	0.172	<0.001	0.259	0.115	<0.001
Effect of fibre	0.660	0.676	<0.001	0.135	0.003	0.343	0.537	0.034
Sweeteners × fibre	0.837	0.216	<0.001	0.919	0.015	0.068	<0.001	0.001

The averages in the columns for biscuits with sucrose and added fibre, which do not share a common lowercase letter (a, b, c) in the superscript, are significantly different between groups. Similarly, those without a common uppercase letter (X, Y, Z, U, W) in the superscript are significantly different between the sweetener and fibre groups (p < 0.05).

). The lack of an effect of soluble fibres on the moisture content of biscuits has been reported by several authors [34,40,41]. In other studies of biscuits with added psyllium, which contains 67% soluble fibre, it was found that water activity significantly decreased. The authors attributed this to the greater water absorption by soluble fibre [42].

The use of sweeteners other than sucrose increased both water content and activity (Table 2). Biscuits with Tagatesse and erythritol–stevia had the highest water content and activity (Table 2). It is likely that other substances, not just Tagatesse itself, in the Tagatesse sweetener used in our study may have affected the moisture content of the biscuits. Tagatesse contains a mixture of sweeteners (isomalt, polydextrose, inulin, D-tagatose, and sucralose), which differ in solubility and water absorption capacity. For biscuits to qualify as having a crumbly texture, their water content should be around 5%. In addition, to ensure microbiological safety, water activity should not exceed 0.6. This condition was not met by Tagatesse biscuits with oat fibre (a_w = 0.65). A previous study by Van der Sman and Renzetti [9] demonstrated that the usefulness of erythritol in biscuits may be limited due to its poorer solubility compared to sucrose. The functional properties of the sugars in biscuits can be attributed to their roles as either plasticizers or humectants [9].

Biscuits with sucrose had the lowest apparent density and the highest porosity, around 80% (Table 3). Both the addition of fibres and the replacement of sucrose with other sweeteners led to statistically significant increases in density and decreases in porosity. The actual density of the cakes was not significantly different (p = 0.081). Similar effects of soluble fibres (Nutriose and polydextrose) on density and porosity were observed in gluten-free bread [19]. Replacing sucrose with maltitol did not affect apparent density, but the use of Tagatesse and erythritol–stevia, in combination with both fibres, increased apparent density. The effects of fibre, sweetener, and the interaction of these factors on apparent density and porosity were demonstrated. Among the biscuits with both wheat and oat fibre, those with sucrose and maltitol had the highest porosity, while those with Tagatesse and erythritol–stevia had the lowest. At the same time, biscuits with wheat fibre and the tested sweeteners exhibited significantly higher porosity than those with oat fibre. The biscuits exhibited a variety of structural characteristics. Example images of the internal structure are provided in Figure S1 (Supplementary Materials). Variations were observed in the shape and distribution of the pores. All the biscuits studied displayed a porous structure with numerous irregularly shaped pores. These observations were further confirmed by calculations. While the average pore area size in the biscuits did not show significant differences (Table 3), the cumulative pore size distribution, shown in Figure S2 (Supplementary Materials), indicates that the addition of fibre and the replacement of sucrose with alternative sweeteners resulted in differences in pore size.

The shape factor of the pores in the biscuits ranged from 0.539 to 0.619, indicating that the pores were not round in shape. The addition of fibres and sweeteners contributed to a reduction in this parameter (Table 3). The type of fibre did not affect the pore elongation factor, but the type of sweetener had a significant impact (Table 3, Figure S3). In addition, an interaction between the sweetener and fibre was observed. The use of sweeteners with wheat fibre resulted in lower values for the pore elongation factor compared to oat fibre (Table 3).

The fracture properties are closely related to the degree of inhomogeneity in the material, with greater structural inhomogeneity resulting in lower fracture stress [43]. This observation aligns with the correlations obtained in our study. The inhomogeneity of the structure (shape factor, elongation factor) and reduced porosity, resulting from the addition of fibres and the replacement of sucrose with sweeteners, led to a decrease in fracturability (

Table 4. Means and ±SDs of the mechanical and acoustic properties of shortbread biscuits.

Shortbread Biscuits Variant	Fracturability (mm)	Hardness (N)	Energy of Acoustic Event (mJ)	Number of AE Events
S	41.21 ± 0.80 b	30.39 ± 10.32 b	2025 ± 510 b	443 ± 210 a
SWF	40.14 ± 0.67 aZ	16.84 ± 6.11 aX	1089 ± 321 aX	631 ± 299 bU
SOF	40.41 ± 0.32 aZ	25.93 ± 9.09 abXY	1468 ± 576 abX	610 ± 198 bUZ
TWF	40.23 ± 0.32 Z	42.35 ± 9.56 Z	898 ± 300 X	20 ± 7 X
TOF	39.15 ± 0.55 Y	30.61 ± 4.42 Y	1274 ± 198 X	11 ± 5 X
MWF	38.99 ± 0.32 Y	44.36 ± 8.83 Z	3742 ± 700 Z	476 ± 80 Z
MOF	39.00 ± 0.54 Y	26.22 ± 4.53 XY	2347 ± 352 Y	316 ± 70 Y
EStWF	38.65 ± 0.44 Y	24.27 ± 8.35 XY	940 ± 231 X	29 ± 14 X
EStOF	37.47 ± 0.36 X	29.55 ± 8.69 Y	1181 ± 430 X	34 ± 11 X
One-way analysis of variance (ANOVA)
Effect added fibre	0.002	0.005	0.018	<0.001
Two-way analysis of variance (ANOVA)
Effect of sweeteners	<0.001	<0.001	0.045	<0.001
Effect of fibre	<0.001	0.027	0.030	<0.001
Sweeteners × fibre	<0.001	<0.001	0.025	<0.001

The averages in the columns for biscuits with sucrose and added fibre, which do not share a common lowercase letter (a, b, c) in the superscript, are significantly different between groups. Similarly, those without a common uppercase letter (X, Y, Z, U, W) in the superscript are significantly different between the sweetener and fibre groups (p < 0.05).

).

In biscuits containing sucrose, the addition of wheat fibre significantly reduced hardness, while oat fibre had no effect (Table 4). An interaction between the sweeteners and fibres was observed. Tagatesse and maltitol, when combined with wheat fibre, caused a significant increase in biscuit hardness (Table 4), whereas sweeteners combined with oat fibre had no effect on hardness (Table 4). Similarly, Lee et al. [21] showed that replacing 10% of the flour with a hydrocolloid containing 20% β-glucan had no effect on biscuit hardness. Sayar et al. [44] demonstrated that the molecular weight of β-glucan is important; adding β-glucan of varying molecular weights to a muffin recipe increased hardness and decreased springiness. During baking, the texture formation of cakes is strongly influenced by the thermosetting of gluten. This relationship, demonstrated by Pareyt et al. [45], was confirmed by van der Sman et al. [9]. In addition, Sman et al. [9] proved that redox factors modulate gluten aggregation during biscuit baking, affecting the dimensions, internal structure, and texture of the biscuits. The low water content, along with high sugar and fat levels in the dough, hinder gluten development [21].

The texture is a sensory property of food and determine consumer acceptability [46]. In shortbread biscuits, textural features such as crispiness/crispness are particularly important for the consumer. Crispy/crunchy products generate characteristic sounds during chewing, which are important for the sensory perception of crispness [43,47,48]. The crispness of biscuits can be assessed by analysing the sounds recorded during mechanical tests using a microphone [48] or a vibration sensor [1,34,36]. Acoustic parameter analysis revealed that the addition of fibre significantly weakened the energy of AE events and increased the number of AE events (Table 4). Earlier studies have shown that the addition of inulin to biscuits reduced acoustic parameters [36]. The authors attributed these findings to the different porous structure of the cakes and the high fat content, which causes sound attenuation. It has been confirmed that sound intensity is strongly influenced by water activity [33,48]. Therefore, cakes with Tagatesse and erythritol–stevia generated the fewest low-energy AE events (Table 4), with the exception of maltitol, which, when combined with both fibres, caused strong acoustic emissions (high energy of acoustic event and a high number of AE events). It is likely that the acoustic emission generated by the biscuits with maltitol was due to their structure, mainly their porosity (Table 3).

3.3. Sensory Properties of Shortbread Biscuits

Sensory analysis showed that the addition of wheat and oat fibre positively influenced all sensory attributes of the biscuits containing sucrose (Figure 1).

Among the biscuit variants we analysed, those with sucrose combined with wheat fibre were the sweetest and received the highest ratings for appearance, taste, and overall acceptability (Figure 1a). It is probable that differences in the chemical composition of both fibres influenced the perception of sweet taste. Overall, in the sensory test, biscuits with wheat fibre received higher ratings and were judged to be sweeter than those with oat fibre (Figure 1).

The exceptions were the biscuits with Tagatesse and those with oat fibre, which stood out for having the highest-rated appearance (Figure 1b). Studies on the effect of β-glucans replacing baker’s fat in shortbread biscuits have shown that oat β-glucan can partially replace 5% of baker’s fat without negatively affecting the physical or sensory quality [39].

Replacing sucrose with sweeteners negatively affected the sensory characteristics of the biscuits. At the same time, biscuits containing maltitol and wheat fibre had sensory qualities similar to those with sucrose alone, but were inferior to those with both sucrose and fibre (Figure 1a). Both Tagatesse and erythritol–stevia resulted in a deterioration of the sensory quality of the biscuits. No interaction was observed between the tested fibres and sweeteners concerning the perception of sweetness, taste, crispness, or overall acceptability of the biscuits.

3.4. Correlation Analysis

The results of Pearson correlations between the instrumentally measured parameters and the sensorial assessments are shown in Table S1 (Supplementary Materials). Strong correlations were found between the number of AE events and sensory attributes: crispness (r = 0.903), sweetness (r = 0.765), taste (r = 0.832), and overall acceptability (r = 0.836). Fracturability also showed a strong positive correlation with sweetness (r = 0.714), taste (r = 0.704), and overall acceptability (r = 0.723). Of the structural parameters, only porosity showed a positive correlation with crispness (r = 0.796). Additionally, a noteworthy negative correlation was observed between water activity and crispness (r = −0.754). These correlations demonstrate the value of measuring the number of AE events, as it strongly correlates with all the sensory attributes analysed. These results confirm the relationships between the sensory and acoustic properties of biscuits previously described in the literature [1,49,50].

To determine the extent to which the biscuits differ from each other and identify which of the analysed parameters have the greatest influence, PCA was performed (Figure 2). The first two principal components (PC1 and PC2) together accounted for 82.55% of the variance, with PC1 explaining 56.95% and PC2 explaining 25.60% of the variation in the analysed biscuit quality parameters. As shown in Figure 2a, the vectors for cookie quality parameters vary in length—the shorter the vector, the smaller the contribution to PC1 and PC2. The parameter vectors shown in red were not included in the calculation of PC1 and PC2. PC1 was formed by all sensory parameters except appearance, along with porosity, apparent density, the number of AE events, brittleness, and water activity, while PC2 consisted of dough rheological parameters (stickiness, work of adhesion, and dough strength). The positioning of biscuit variants next to each other in Figure 2b indicates the similarity of their comprehensive quality, as measured instrumentally and sensorially, with the distance between them representing the magnitude and nature of the differences. The biscuits in Figure 2b formed three groups. Group 1 was distinguished by the best sensory properties, the highest number of AE events, high porosity, and intermediate—neither the largest nor the smallest—rheological parameters. Group 1 included biscuits with sucrose alone, sucrose with both fibres, and maltitol with wheat fibre. Of the sweeteners used in the biscuits, maltitol is considered to have the most similar effect to sucrose [8], which was confirmed in this study, but only in combination with wheat fibre. Group 2 included Tagatesse biscuits with both fibres, which had significantly higher rheological parameters and lower sensory evaluations than Group 1, but values similar to those of Group 3. Group 3 included biscuits with maltitol and oat fibre, as well as erythritol–stevia with both fibres. This group had the lowest sensory evaluation, low porosity, and the highest rheological parameter values.

4. Conclusions

Research has proven that performing instrumental texture measurements on biscuits using the acoustic emission method is beneficial due to the strong correlations between the acoustic parameter (number of AE events) and all sensory discriminants. This is particularly important when developing new biscuit recipes because it can reduce the number of sensory tests. Shortbread biscuits with sucrose had the lowest apparent density at 319 kg/m³; had the highest porosity (80%), fracturability (41 mm), and hardness (30 N); and generated 443 AE events. The use of commercial fibres (Nutriose^® and PromOat 35) in combination with sucrose positively influenced the sensory quality of shortbread biscuits. These fibres, at a constant amount of sucrose, contributed to an increase in the perception of sweetness in biscuits of approximately 1 point, which were characterized by the best appearance (4.3 points), taste (4.5 points), crispness (4.3 points), and overall acceptability (4.4 points). The fracturability (40 mm) and hardness of biscuits with fibre wheat (17 N) and fibre oat (26 N) decreased, but the number of AE events was higher and amounted to approximately 630.

Our study confirmed the difficulty of replacing sucrose in biscuits, even with commercial sweeteners. Of the sweeteners we tested, only the combination of maltitol with wheat fibre produced biscuits with satisfactory sensory quality, texture, and structure similar to those with sucrose alone. Other biscuit variants, including maltitol with oat fibre, Tagatesse, and erythritol–stevia in combination with wheat and oat fibre, compromised sensory quality. In dough with sucrose, rheological properties such as stickiness, work of adhesion, and dough strength were 7.7 g, 0.22 g·s, and 0.46 mm, respectively. The addition of wheat fibre resulted in higher stickiness and work of adhesion about twice, and oat fibre reduced it about twice. Only biscuits with maltitol and wheat fibre had rheological properties similar to biscuits with sucrose alone. Our research shows that it is possible to use 11.2% (g/100 g flour) of wheat and oat soluble fibres in combination with sucrose, or wheat fibre with maltitol, to improve the nutritional value of biscuits. However, further research is needed to confirm their nutritional benefits.