3.1. Textural Attributes of Plant-Based Purées Intended for OD Diet
The firmness of the plant-based purée samples (
Figure 1) was significantly affected by the type of protein source and the concentration used in the study. The higher the value, the higher the firmness of the product [
16]. The type of secondary treatment did not have a significant effect on the firmness of the plant-based purée. Protein was found to harden the texture of the purées. High levels of firmness for the samples with pea protein concentrate made them unsuitable for plant-based purée preparation because they exceeded the target values set by medical experts for patients with swallowing disorders, who need products which can be easily chewed and swallowed. The increased concentration of pea protein from 5 to 9 g/100 g resulted in firmness approximately four-fold higher; the same increase in soy protein additive almost doubled the firmness of the purée, which would cause it to require greater tongue muscle power to initiate swallowing. An increase in whey protein concentration did not have a significant effect on the firmness of plant-based purée.
The purées made with soy protein isolate and whey protein isolate at 5 g/100 g were within acceptable limits (20.0–40.0 N s) regarding purée consistency (
Figure 2), whereas the same proteins at 9 g/100 g slightly exceeded the set values (
Table 1). The higher the consistency value, the thicker the sample [
16]. A dramatic increase in consistency was observed for purées made with pea protein concentrate, irrespective of the secondary heat treatment applied. This could be attributed to the higher water binding ability and possible swelling of protein particles and starch, which behave like filling agents and, thus, reduce the food matrix motion, which may result in greater consistency [
11,
27].
Increased cohesiveness was observed in purées made with protein additives at 9 g/100 g, irrespective of protein type and heat treatment applied (
Figure 3). Purées with soy protein isolate and pea protein concentrate made with 9 g/100 g exhibited two-fold higher cohesiveness compared to the respective purées made with 5 g/100 g protein additive. An increase in whey protein isolate concentration did not cause a drastic rise (
p > 0.05) in cohesiveness, but for this purpose, the type of heat treatment could be important.
Cohesiveness is related to the product’s ability to keep its structure due to internal stickiness, and it plays an important role in bolus formation [
28]. Some products, such as mashed potatoes and heated cereals, exhibit good cohesiveness, whereas others, such as purées, may undergo separation. Many studies have demonstrated the application of various hydrocolloids for the improvement of textural properties [
29]. Products with low cohesiveness may cause aspiration due to the formation of more boluses during swallowing [
30], whereas highly cohesive products may leave residues in the oro-pharynx, which could also contribute to the risk of aspiration.
3.2. Rheological Characteristics of Plant-Based Purées
The viscosity of purées made with whey protein isolate was stable irrespective of its concentration and the type of heat treatment applied (
Figure 4). Plant proteins (soy and pea) at higher concentrations demonstrated a rise in purée viscosity. This may be due to the findings by Sim et al. [
31] and Giura et al. [
16] that plant proteins, in comparison with animal proteins, have poor solubility, emulsifying, gelling and foaming properties.
Flow behavior data (
Figure 5) allowed for the determination of the yield point, which, according to Nishinari et al. [
28], plays an important role in the swallowing process. The yield point for samples containing soy or pea protein was significantly higher at protein additive rates of 9 g/100 g compared to 5 g/100 g. The yield point of plant-based purées enriched with whey protein did not show significant dependency on the protein concentration (
Table 4).
The Bingham and Herschel–Bulkley models were used to describe the flow behavior of control and protein-enriched plant-based purée samples. All samples demonstrated shear thinning behavior, which coincides with the findings of Ribes et al. [
11], who attributed it to the structural breakdown and increased lining up of molecules caused by applied shear force. The Bingham model better fitted to the experimental data, because in the case of the Herschel–Bulkley model, it was not possible to determine the required parameters for some samples. The use of rheological models allowed us to obtain values of yield stress, which demonstrate the force required to start the flow of the product. The yield stress determined in our study was higher than that reported by earlier studies [
32].
The findings are in agreement with the study of Štreimikyte et al. [
12], who reported that protein-based beverages suitable for the dysphagia diets of the elderly consisted of yielding, pseudoplastic fluids with σ
0 > 1 and n < 1. The highest yield stress and viscosity factors were attributed to beverages containing pea protein in comparison to milk protein, possibly due to the excellent water binding capacity of pea protein.
Frequency sweep tests (
Figure 6) indicated higher values of storage modulus G’ than those of loss modulus G’’ at all conditions, which confirms the gel-like behavior of plant-based purées. Similar to Cuomo et al. [
17], who performed rheological assessment of dysphagia-oriented new food preparations, our study revealed the low dependency of the elastic modulus (G’) on frequency, whereas the viscous modulus (G’’) was slightly more frequency-dependent.
The evaluation of textural and rheological properties of protein-enriched plant-based purées led to the decision to select whey protein isolate as a protein additive in the development of product prototypes for OD patients’ diets. Although the results showed that using smaller amounts of soy protein could be just as appropriate as using whey protein isolate to reach textural property goals, it was decided to continue the prototype development using whey protein isolate.
3.3. Characteristics of the Industrially Produced Product Prototypes Intended for OD Diets
Dysphagia patients often eat smaller portions of food, and, thus, receive insufficient nutrients. Therefore, the main target in the development of new product prototypes was adjustment of nutritive value, along with achieving satisfactory rheological attributes. This resulted in two prototypes of soups and two prototypes of desserts being made from local raw materials, intended for OD patients.
The values of viscosity and textural properties (firmness, consistency and cohesiveness) of the industrially produced product prototypes (
Table 5) were within the limits set in the first stage of this study by medical experts who work daily with dysphagia patients. Thus, the products would be suitable for patients with swallowing disorders.
The physical and chemical characteristics of industrially produced plant-based purées are summarized in
Table 6. The protein content ranged from 9.61 g (D2) to 13.38 g (S2) per 100 g of product. For comparison, the protein content in commercially available adult medical nutrition products is, on average, 12 g per 125 mL (133 g on average) of product, which makes up roughly 9 g per 100 g of products. However, this may vary depending on the intended use. The fat content in the developed product prototypes was, on average, lower than that in commercial products. The total dietary fiber content in the developed products was twice as high as in commercial products. The majority of commercial products do not contain fiber. The content of titratable acids in the produced products ranged from 0.66 g (S2) to 0.88 g (S1) per 100 g of product. The determined chemical composition allowed for the calculation of developed product energy value, which was found to be between 153 kcal (S2) and 162 kcal ((D1) per 100 g. The energy value of commercial products, on average, reaches 300 kcal per 100 g, mainly due to their higher sugar and fat content, making them energy-dense.
The moisture content in the developed products ranged from 71.45 ± 0.05% (D1) to 73.73 ± 0.05% (S2). The samples S1 and S2 had two-fold higher added protein contents, thus requiring more juices instead of purées to be used in the basic formulation (
Table 2) to provide suitable rheological properties. The sample S2 had the highest pH (6.22 ± 0.03) and the lowest content of soluble solids (18.2 ± 0.2 Brix%), which, again, can be explained by the formulation of the basic recipe.
The analysis revealed significantly higher total carotene content per 100 g of product in the samples D1 and D2, thanks to higher carrot purée and/or sea buckthorn pulp juice content, compared to other samples. The total phenol content differed among all samples, being the highest in product S2 (199.64 mg TE/100 g). Sample D1 exhibited the highest level of radical scavenging activity of DPPH radical and ABTS radical cations. There were no significant differences (p > 0.05) in any of antiradical activities for samples D2 and S1.
Adding the vitamin and mineral compound premix allowed sufficiently high values to be reached, which was not possible using only plant material, as indicated in the previous study [
33]. The contents of vitamins B
6, B
9, B
12, C, D
3 and E and selected minerals are shown in
Table 7.
Despite the equal amount of added premix, slight differences were observed among the samples. This could be explained by the differences in the product formulations, as some ingredients are richer in specific vitamins or minerals. Thus, the highest content of vitamin B12 was found in sample D2, which also had the highest vitamin B6 and D3 content. Sample S1 possessed the highest vitamin B9 content.
The mineral compound content also varied between samples (
Table 7). Overall, the soup samples (S1 and S2) exhibited the highest amount of sodium due to the added salt. The sample S2 differed from other samples, with a higher content of Na, Zn, Ca and Mg.