Development and Characterization of Hybrid Burgers Made from Pork and Multi-Ingredient Plant Mixtures and Protected with Lactic Acid Bacteria

Abstract

Hybrid (meat–plant) products can be a sustainable response to the increased interest in flexitarianism. Their development requires designing appropriate composition and functional properties and ensuring microbiological safety. This study aimed to determine the influence of using various multi-ingredient plant mixtures and pork in different proportions and two recipe variants on the characteristics of baked burgers. Additionally, the impact of lactic acid bacteria (LAB) application on the microbiological quality of raw hybrid/plant burgers was assessed. All products were analyzed in terms of basic chemical composition (NIR spectroscopy) and physicochemical and textural properties (instrumental methods). The raw plant and hybrid burgers met the microbiological requirements. The baked burgers did not significantly differ in terms of their chemical composition, except for the fiber found in plant/hybrid samples. Moreover, the reformulation of burgers had no effect on their cooking yield or water activity. As the content of plant parts increased, the baked burgers were darker (lower L*) and had a more tender texture (lower shear force and penetration force). Overall, the multi-ingredient plant mixtures showed great potential in the development of high-quality hybrid burgers. However, the texture formed can, importantly, determine final consumer acceptance. Therefore, research should be continued, especially in terms of comprehensive textural and sensory analyses.

1. Introduction

Meat products are very popular in European countries. They are a good source of nutrients, but they are controversial in ecological, social, and health terms [1]. Consumers are increasingly aware of sustainability, health, and animal welfare [2,3], which increases their interest in flexitarian diets [4,5]. These diets include more products based on plant raw materials or hybrid products containing both meat and plant ingredients. Currently, the definition of hybrid meat products is not precisely formulated. They are sometimes defined as meat products containing various amounts of plant-origin ingredients (such as legumes, cereals, fruits, and vegetables) in various proportions (usually from about 25% to about 50%), which are not added as fillers, but for their positive technological, nutritional properties, and even sensory features [3,5,6]. Hybrid meat products can fill the gap among products targeted at consumers who want to reduce meat consumption without sacrificing the taste, convenience, and familiarity of traditional meat products [7,8]. Currently, consumers are increasingly aware of the impact of the food they eat on their health, as well as paying attention to the composition and degree of processing of a product. They are therefore more demanding of the products offered. Moreover, the current lifestyle of society, especially the younger generation, often limits the possibilities of preparing nutritious meals at home. This is the driving force behind the dynamic development of the convenience food sector [9].

Meat industry recipients are open to convenience food products, whose preparation is simple and not time-consuming [9,10,11]. Burgers are perceived by consumers as attractive convenience foods, despite their relatively high fat content. The burger market is constantly expanding, which allows it to meet the changing needs of consumers over time. Generally, these products have gained great popularity all over the world due to their quick preparation and high sensory acceptability. In the Polish meat processing sector, initiatives have been undertaken for many years to improve the health quality of meat products. They focus mainly on reformulation, i.e., modifying the recipe composition of these products [12,13]. Hybrid products such as burgers, nuggets, and sausages create both benefits and challenges for the food industry. The addition of plant raw materials affects the sensory and technological properties of hybrid products, such as appearance, texture, or taste. An additional benefit comes from the possibility of combining meat and plant raw materials, increasing the content of fiber or minerals, as well as reducing the caloric value of the finished product [14,15].

However, reformulation of food products, including combining plant- and animal-origin ingredients, involves not only a change in their composition and physicochemical properties, but can also significantly affect their microbiological quality at various stages of processing, storage, and distribution. Food safety is of great importance to both industry and consumers. It is common knowledge that pathogens such as Campylobacter, Salmonella, Escherichia coli, and Listeria can pose a significant threat to humans and the production environment. Diseases resulting from the presence of foodborne pathogens have become one of the most widespread public health problems in the world [16]. As a result, there is a need to look for solutions that will inhibit the transfer of harmful bacteria into the food chain. The application of lactic acid bacteria (LAB) cultures can be an effective method to prevent or reduce the occurrence of pathogens, thereby improving food safety and consumer health. Additionally, LAB can limit the growth of microorganisms responsible for food spoilage and, at the same time, contribute to extending the shelf life of products [17,18,19,20].

So far, scientific work on hybrid (meat–plant) products has focused on the introduction of single-plant raw materials [21,22,23], while the use of LAB protective cultures includes either animal products or vegetarian products [20]. There is still a lack of research on the development of products based on meat raw materials and multi-ingredient plant mixtures consisting of various raw materials, i.e., vegetables, cereals, legumes, and oil plants. The simultaneous use of LAB protective cultures to increase the microbiological safety of such products would expand the possibilities of designing new foods. Therefore, one of the purposes of the work was to determine the possibility of using various multi-ingredient plant mixtures and pork in the development of hybrid burgers. At the same time, the impact of the application of lactic acid bacteria cultures on the microbiological safety of raw hybrid and plant burgers was examined, compared to a meat burger. Moreover, the influence of the proportion of meat and plant parts in the recipe on the basic chemical composition and physicochemical properties of baked hybrid burgers was assessed.

2. Materials and Methods

2.1. Materials

Pork meat (pork shoulder) was obtained from Sokołów SA (Sokołów Podlaski, Poland), and curing salt (99.4% NaCl, 0.6% NaNO₂) was obtained from Quemetica SA (Janikow, Poland). Rapeseed pomace and camelina pomace, as by-products of cold pressing of oils from rapeseed and camelina seeds, respectively, were provided by Kropla Omega Company (Starachowice, Poland). Other plant raw materials (millet groats, dried red tomatoes, sunflower seeds, onion, lentils, carrots, and oatmeal), spices (salt, powdered black pepper, garlic, red paprika ginger, and dried Provençal herbs), and refined rapeseed oil were purchased in a local store. Lactic acid bacteria culture SafePro^® Flora Ctrl 01 (Lactobacillus sakei) was kindly provided by Chr. Hansen GmbH company (Nienburg, Germany).

2.2. Preparation of Burgers

Three different hybrid burgers were prepared with varying proportions of meat and plant parts (M75_P25, M50_P50, and M25_P75). The fourth sample was a plant burger (M0_P100). A meat burger (M100_P0) was considered as a control sample. Additionally, the hybrid and plant burgers were prepared in two recipe variants. In total, there were eight experimental samples and one control sample, which were obtained and analyzed in three independent repetitions. The experimental design, recipe variants, and compositions of meat, hybrid, and plant burger samples are shown in

Table 1. Types and recipe variants of meat, hybrid, and plant burger samples obtained.

Sample Type	Proportion of Meat and Plant Parts	Recipe Variant
		I	II
M100_P0	100:0	100% cured pork stuffing *
M75_P25	75:25	75% cured pork stuffing *; 13.7% millet groats, 5% dried red tomatoes, 2.3% sunflower seeds, 2% onion, 1.2% rapeseed pomace, 0.8% spices **	75% cured pork stuffing *; 11.2% lentils, 5% carrots, 2.5% sunflower seeds, 2.5% oatmeal, 1.8% onion, 1.2% camelina pomace, 0.8% spices **
M50_P50	50:50	50% cured pork stuffing *; 27.5% millet groats, 10% dried red tomatoes, 4.5% sunflower seeds, 4% onion, 2.5% rapeseed pomace, 1.5% spices **	50% cured pork stuffing *; 22.5% lentils, 10% carrots, 5% sunflower seeds, 5% oatmeal, 3.5% onion, 2.5% camelina pomace, 1.5% spices **
M25_P75	25:75	25% cured pork stuffing *; 41.3% millet groats, 15% dried red tomatoes, 6.7% sunflower seeds, 6% onion, 3.7% rapeseed pomace, 2.3% spices **	25% cured pork stuffing *; 33.7% lentils, 15% carrots, 7.5% sunflower seeds, 7.5% oatmeal, 5.3% onion, 3.7% camelina pomace, 2.3% spices **
M0_P100	0:100	55% millet groats, 20% dried red tomatoes, 9% sunflower seeds, 8% onion, 5% rapeseed pomace, 3% spices **	45% lentils, 20% carrots, 10% sunflower seeds, 10% oatmeal, 7% onion, 5% camelina pomace, 3% spices **

* Comminuted pork shoulder mixed with 10% water (pork stuffing) and 2% curing salt addition (in relation to the weight of the pork stuffing); ** mixture of salt, black pepper, garlic, and Provençal herbs in the case of recipe variant I; mixture of salt, black pepper, red paprika, and ginger in the case of recipe variant II.

. The burger recipe variants and the raw material composition of the plant parts were developed as a result of a series of preliminary tests (Table S1). Eight original plant mixtures were composed and examined. Finally, two of them were selected on the basis of the best results in terms of determined physicochemical, textural, and sensory parameters.

Pork meat was comminuted in a Zelmer ZMM4048B grinder (Zelmer S.A., Mielno, Poland) using a mesh of 4.5 mm diameter. Then, 10% water and 2% curing salt (in relation to the weight of the meat) were added and thoroughly mixed using a Kenwood KM 070 mixer (Kenwood Ltd., Havant, UK). Most plant raw materials have been subjected to preliminary thermal treatment: millet groats, lentils, and oatmeal were prepared in accordance with the manufacturer’s recommendations (instructions on the package labels); carrots were boiled in water; and onion was fried in a small amount of rapeseed oil until golden color. However, all of them were finally comminuted with a grinder. In addition, the rapeseed/camelina pomace was ground in a mortar. Prepared raw materials and spices were weighed according to the recipe compositions (Table 1) and mixed to obtain a stuffing with an even distribution of all ingredients. Approximately 100 g of stuffing was formed into a typical burger shape using a metal ring with a diameter of 75 mm and a height of 25 mm. In order to investigate the possibility of ensuring the microbiological safety of raw hybrid/plant burgers during refrigerated storage and distribution (before they reach the consumer), the SafePro^® Flora Ctrl 01 protective cultures were added to these burgers’ stuffing (according to the manufacturer’s recommendations).

In turn, to examine the properties of burgers prepared for consumption (after reheating), the formed semifinished products were baked in a Rational SCC WE61 electric combi oven (Rational Aktiengesellschaft, Landsberg am Lech, Germany) at a temperature of 160 °C and 60% humidity until a temperature of 70 °C was reached in the geometric center of the product. The baked burgers were chilled, covered with aluminum foil (to limit their drying out), and then stored for 24 h at a temperature of 4 °C. After this time, the final products were tested. The burgers were produced in accordance with the principles of good hygiene practice and good production practices.

2.3. Microbiological Analyses of Raw Burgers

Microbiological analyses of raw burgers included the determination of the total plate count of aerobic mesophilic microorganisms according to the PN-EN ISO 4833-1:2013-12 standard [24] and lactic acid bacteria (LAB) according to the PN-ISO 15214:2002 standard [25]. Samples for the analyses were prepared in accordance with the Polish procedure described in the PN-EN ISO 6887–2:2017 standard [26]. Briefly, the foil package containing the product was opened using a sterile scalpel, and 20 g of the sample was taken with a sterile spoon. Then, the sample was mixed with 180 mL of sterile peptone water (bioMérieux Polska Sp.z o.o., Warsaw, Poland), and the first 10-fold dilution was obtained. The sample was homogenized in a stomacher blender (Lab Blender 400 Circular, Seward Ltd., Worthing, UK) for 1 min at a maximum speed and 18 °C. Serial tenfold dilutions were prepared by transferring 1 mL of the first dilution to 9 mL of sterile peptone water.

Aerobic mesophilic colonies were inoculated on Plate Count Agar (BTL, Łódź, Poland) and then incubated in aerobic conditions at 30 °C for 72 h. In turn, the lactic acid bacteria were inoculated on MRS (de Man, Rogosa, and Sharpe Agar; Bio-Rad Laboratories, Inc., Hercules, CA, USA) and then incubated under anaerobic conditions at 30 °C for 72 h. In both cases, the pour-plate technique was used. All bacterial counts were expressed in colony-forming units per g of sample (cfu/g).

2.4. Determination of the Basic Chemical Composition of Baked Burgers

The basic chemical composition (water, protein, fat, fiber, and salt) of burgers was determined using near-infrared reflectance (NIR) spectroscopy, using FoodScanTM2 Meat Analyzer (FOSS Analytical, Hilleroed, Denmark), in accordance with a PN-A-82109 [27] standard. Burger samples weighing approximately 250 g were placed in a round measuring container made of optical glass and inserted into the apparatus chamber. The principle of the operation of the apparatus consists of 16 times measurement of the components of burgers with the use of a monochromator with a movable diffraction grating, which analyzes the near-infrared spectrum in the range of 850–1050 mm.

2.5. Determination of the Physicochemical Properties of Baked Burgers

2.5.1. Cooking Yield

The cooking yield of meat, hybrid, and plant burgers was determined by measuring the changes in their weight as a result of heat treatment [21,28]. It was calculated using the following equation:

C_Y = [(W₁ − ΔW)/W₁] × 100

(1)

where C_Y—cooking yield [%], W₁—initial sample weight (before thermal treatment) [g], ΔW = (W₁ − W₂)—difference in sample weight before and after thermal treatment [g], W₂—final sample weight (after thermal treatment and 24 h of cold storage) [g].

2.5.2. Water Activity

The determination of water activity (a_w) of burger samples was carried out using an AquaLab Series 3 water activity meter (METER Group, Inc., München, Germany), at a temperature of 23 ± 1 °C. Measurements were performed three times for each sample.

2.5.3. pH Level

The pH level of the burgers was determined by the reference method in accordance with the ISO 2917 [29] standard. A Testo 206-pH2 pH-meter (Testo SE & Co. KGaA, Titisee-Neustadt, Germany) with automatic temperature compensation and a combined glass–calomel electrode was used. Before measurements, the device was calibrated against standard buffers (pH = 4 and pH = 7). Measurements were performed in three repetitions for each sample.

2.5.4. Color Parameters

The color parameters of the burgers were determined using the reflection method in the CIEL*a*b* color space (L*—lightness, a*—red/green coordinate, b*—yellow/blue coordinate). For this purpose, a Konica Minolta Chroma Meter CR-400 (Minolta, Osaka, Japan) was used with the following settings: D65 light source, 2° observer, and 2 mm aperture size. Each time, before starting a series of measurements, the measuring head was calibrated against the white standard (Y = 84.2, x = 0.3202, y = 0.3373). Measurements were made on the surface and in the cross-section of the burgers (in six repetitions).

Moreover, based on the results of the basic color coordinates, the total color difference (ΔE₁) between hybrid/plant burgers and the meat burger was calculated according to the following equation [30]:

ΔE₁ = [(L₂* − L₁*)² + (a₂* − a₁*)² + (b₂* − b₁*)²]^1/2

(2)

where ΔE₁—total color difference in hybrid/plant burger samples compared to the meat burger sample; L₁*, a₁*, b₁*—color parameters determined for the meat burger sample; L₂*, a₂*, b₂*—color parameters determined for hybrid/plant burger samples.

In order to determine the degree of browning of the surface of burgers after heat treatment, the Browning Index (BI) was calculated using the following equations [31]:

BI = 100 × [(x − 0.31)/0.17]

(3)

where:

x = (a* + 1.75 L*)/(5.645 L* + a* − 3.012 b*)

(4)

2.6. Determination of the Textural Properties of Baked Burgers

Textural parameters of burgers were analyzed using an instrumental method using a Zwicki 1120 apparatus (Zwick GmbH, Ennepetal, Germany). A penetration test and a shear test were performed. Before measurements, the samples were conditioned for 24 h at a temperature of 22–23 °C. The penetration test was carried out on whole burgers with a diameter of 75 mm and a thickness of approximately 20 mm, using a flat pin with a diameter of 13 mm (head speed 50 mm/min; initial force 0.2 N). The penetration force [N], i.e., the maximum force needed to penetrate the sample to a depth of 10 mm, was determined [32]. The shear test was carried out on burger samples cut into rectangular blocks with dimensions of 7.5 × 1.0 × 1.0 cm. A Warner–Bratzler set with a flat-shaped cutting blade was used (head speed 50 mm/min; initial force 0.2 N). The shear force [N], i.e., the maximum force needed to cut the sample completely, was determined [33]. The measurements were performed in triplicate for each sample.

2.7. Statistical Analysis

In order to compare the results for different types of burger samples (M100_P0, M75_P25, M50_P50, M25_P75, M0_P100), a one-way analysis of variance (one-way ANOVA) test was used. The significance of differences between mean values was determined using the sigma-constrained parameterization, and homogeneous groups were distinguished using Tukey’s HSD post hoc test. In turn, to compare the results obtained for different recipe variants (I and II), a Student’s t-test was used. A significance level of α was set to 0.05. These differences were considered significant when p-value ≤ 0.05 [34,35]. Statistica 13.3 software (TIBCO Inc., Palo Alto, CA, USA) was used for all statistical analyses.

3. Results and Discussion

3.1. Microbiological Characteristics of Raw Meat, Hybrid, and Plant Burgers

A very useful and effective strategy for preventing or reducing the incidence of pathogens, thereby improving food safety and consumer health, can be the application of beneficial lactic acid bacteria (LAB), mainly from the following genera: Lactobacillus, Carnobacterium, Pediococcus, Lactococcus and Enterococcus [17,18]. LAB can be used either as protective cultures (live microorganisms that inhibit the development of pathogens and/or extend the shelf life of products) or as probiotic cultures (live microorganisms that, when consumed, can have a beneficial effect on health). The protective role of LAB has been proven in many non-fermented food products as part of scientific research [36,37,38] or industrial practice. The LAB cultures (Lactobacillus sakei) used in this research for the bioprotection of raw hybrid and plant burgers have the ability to multiply at low temperatures, which allows them to dominate the microbiological environment and effectively combat Listeria monocytogenes bacterium [39,40]. The results of microbiological analyses of raw burgers are presented in

Table 2. Results of microbiological analyses of raw meat, hybrid, and plant Burgers.

Recipe Variant	Sample Type
	M100_P0	M75_P25	M50_P50	M25_P75	M0_P100
	Aerobic colony count (ACC) [cfu/g]
I	1.5 ± 1.0 × 105 aA	1.4 ± 0.8 × 105 aA	1.4 ± 0.9 × 105 aA	1.3 ± 1.1 × 105 aA	9.2 ± 1.1 × 104 aA
II	1.3 ± 0.9 × 105 aA	1.3 ± 1.1 × 105 aA	1.3 ± 0.8 × 105 aA	1.6 ± 1.0 × 105 aA	1.6 ± 1.1 × 105 aA
	Lactic acid bacteria (LAB) count [cfu/g]
I	-	1.5 ± 0.8 × 107 aA	1.6 ± 1.2 × 107 aA	1.1 ± 0.9 × 107 aA	9.4 ± 1.6 × 106 aA
II	-	1.6 ± 0.9 × 107 aA	1.6 ± 1.2 × 107 aA	1.8 ± 1.1 × 107 aA	2.1 ± 1.2 × 107 aA

For the denomination of hybrid burger samples, see Table 1. Different lowercase letters between means values ± standard deviations in the row indicate significant differences (grouping variable: sample type), p ≤ 0.05. Different uppercase letters between means values ± standard deviations in the column for a given measurement indicate significant differences (grouping variable: recipe variant), p ≤ 0.05.

.

The aerobic colony count (ACC) varied from 9.2 × 10⁴ to 1.6 × 10⁵ cfu/g. Different proportions of meat and plant parts and the recipe variants did not have a statistically significant impact on ACC variability. According to international guidelines [41], ACC for comminuted raw meat should be lower than 10⁷ cfu/g for cooked vegetables and cereals < 10⁵ and < 10⁶ cfu/g, respectively. Therefore, taking into account the heterogeneous composition of the tested hybrid burgers, it can be concluded that they met the microbiological requirements relating to the permissible number of aerobic colony-forming units in 1 g of the semifinished product. In turn, the LAB counts, ranging from 9.4 × 10⁶ to 1.8 × 10⁷ cfu/g, were typical (expected) due to the dose of these protective cultures used before the cold storage of raw burgers (Table 2).

3.2. Basic Chemical Composition of Baked Meat, Hybrid, and Plant Burgers

The chemical composition of hybrid burgers depends mainly on the raw materials used. Meat is a rich source of complete proteins, fats (including unsaturated fatty acids), minerals (including zinc, iron, selenium, potassium, magnesium, and sodium), and vitamins (especially B vitamins). Its composition varies depending on the species and method of animal breeding, which causes significant differences in its nutritional and sensory properties. For example, pork contains, on average, about 18% protein and 10% fat, including 5% saturated fatty acids [42]. In turn, the chemical composition of plant raw materials depends, among others, on their species, variety, degree of ripeness, and method of technological processing [43,44]. The analyzed burgers, regardless of the proportion of meat and plant parts and the recipe variant, were characterized by a similar chemical composition (

Table 3. Basic chemical compositions of baked meat, hybrid, and plant burgers.

Recipe Variant	Sample Type
	M100_P0	M75_P25	M50_P50	M25_P75	M0_P100
	Water [%]
I	60.3 ± 0.6 aA	58.3 ± 0.7 aA	58.1 ± 0.8 aA	57.9 ± 0.6 aA	57.3 ± 0.6 aA
II	60.3 ± 0.6 aA	58.2 ± 0.6 aA	58.1 ± 0.7 aA	57.4 ± 0.9 aA	57.9 ± 0.9 aA
	Protein [%]
I	16.9 ± 0.9 aA	14.9 ± 0.3 aA	14.0 ± 0.9 aA	12.1 ± 0.6 aA	11.9 ± 1.1 aA
II	16.9 ± 0.9 aA	15.6 ± 0.6 aA	15.1 ± 0.7 aA	13.6 ± 0.7 aA	12.2 ± 0.9 aA
	Fat [%]
I	15.6 ± 0.6 aA	14.4 ± 0.9 aA	14.1± 0.9 aA	13.5± 1.1 aA	13.1± 0.5 aA
II	15.6 ± 0.6 aA	14.3 ± 1.4 aA	13.9± 1.6 aA	13.8± 0.7 aA	13.6± 0.4 aA
	Fiber [%]
I	0.0 ± 0.1 aA	1.5 ± 0.9 bA	3.0 ± 0.6 cA	4.5 ± 0.7 dA	6.3 ± 0.9 eA
II	0.0 ± 0.1 aA	1.7 ± 0.7 bA	3.3 ± 0.7 cA	5.1 ± 0.9 dA	6.5 ± 0.6 eA
	Salt [%]
I	1.7± 0.3 aA	1.7± 0.4 aA	1.6± 0.5 aA	1.7± 0.7 aA	1.7± 0.4 aA
II	1.7± 0.3 aA	1.7± 0.1 aA	1.6± 0.6 aA	1.8± 0.4 aA	1.8± 0.3 aA

For the denomination of meat, hybrid, and plant burger samples, see Table 1. Different lowercase letters between means values ± standard deviations in the same row indicate significant differences (grouping variable: sample type), p ≤ 0.05. Different uppercase letters between means values ± standard deviations in the same column indicate significant differences (grouping variable: recipe variant), p ≤ 0.05.

). This can be due to the fact that the plant raw materials contain similar amounts of nutrients, especially protein and fat, as pork meat. However, the nutritional value of the tested burgers will also be determined by their qualitative composition, especially the amino acid profile of proteins and the fatty acid composition of lipids. As is known, the origin of these ingredients determines their biological value, bioavailability, and digestibility [45]. Vegetable oils provide more nutritionally beneficial unsaturated fatty acids [46], and animal protein has more essential exogenous amino acids [47].

The only difference between tested burgers was observed in the case of fiber content. As the share of plant parts in burgers increased, the fiber content increased. In plant burgers, regardless of the recipe variant, their content exceeded 6%, which was about four times higher compared to hybrid burgers with the highest meat content (M75_P25_I/II). Therefore, the M50_P50 and M25_P75 hybrid burgers may be labeled with the following nutritional information: “source of dietary fiber” [48]. Increasing the supply of fiber in a diet may bring health benefits, including regulating intestinal function (insoluble fiber fraction) or even reducing cholesterol levels (soluble fiber fraction) [49].

Regarding the salt content, it remained at the level of about 1.7% and did not differ significantly for individual sample types or recipe variants (Table 3). This level was consistent with the technological level of its addition and was typical for raw meat burgers [50]. In contrast, Sadig and Wu [51] reported that plant-based meat alternatives usually have substantially higher salt levels than their meat counterparts. It is worth mentioning that overconsumption of salt and sodium increases the risk of cardiovascular diseases [52].

As Chandler and McSweeney [8] and De Marchi et al. [53] pointed out, hybrid products are the optimal way to introduce plant ingredients rich in amino acids, unsaturated fatty acids, fiber, and minerals into the daily diet. Meanwhile, the meat industry has a chance to diversify its offerings, responding to the growing consumer demand for healthier and more sustainable food products.

3.3. Physicochemical Properties of Baked Meat, Hybrid, and Plant Burgers

Thermal processing aims to ensure the safety and desired sensory characteristics of food. It causes structural changes, generating mass losses (usually water), which determine the yield of the final product. Therefore, heat treatment is one of the key stages of food processing from a technological and economic point of view [54,55].

The cooking yield of the burgers ranged from 81.7 to 90.8%. There were no statistically significant differences in the mean value of this parameter, regardless of the proportion of meat and plant parts and the recipe variant (

Table 4. Cooking yield, water activity, and pH level of baked meat, hybrid, and plant burgers.

Recipe Variant	Sample Type
	M100_P0	M75_P25	M50_P50	M25_P75	M0_P100
	Cooking Yield [%]
I	81.68 ± 2.00 aA	85.86 ± 1.78 aA	86.88 ± 1.86 aA	88.91 ± 1.30 aA	89.41 ± 1.06 aA
II	81.68 ± 2.00 aA	89.10 ± 1.57 aA	90.68 ± 1.43 aA	90.81 ± 1.28 aA	89.74 ± 1.76 aA
	Water Activity [─]
I	0.960 ± 0.006 aA	0.957 ± 0.002 aA	0.954 ± 0.006 aA	0.950 ± 0.003 aA	0.949 ± 0.004 aA
II	0.960 ± 0.006 aA	0.962 ± 0.013 aA	0.956 ± 0.011 aA	0.954 ± 0.003 aA	0.951 ± 0.010 aA
	pH Level [─]
I	5.66 ± 0.31 aA	5.61 ± 0.26 aA	5.60 ± 0.53 aA	5.58 ± 0.55 aA	5.60 ± 0.28 aA
II	5.66 ± 0.31 aA	5.79 ± 0.41 aB	5.84 ± 0.31 aB	5.81 ± 0.64 aB	5.83 ± 0.43 aB

For the denomination of meat, hybrid and plant burger samples, see Table 1. Different lowercase letters between means values ± standard deviations in the same row for indicate significant differences (grouping variable: sample type), p ≤ 0.05. Different uppercase letters between means values ± standard deviations in the same column indicate significant differences (grouping variable: recipe variant), p ≤ 0.05.

). This could be due to the similarity in the basic chemical composition of the meat and plant parts, as well as the comparable ability of the ingredients contained to retain water and fat in the structure. Contrary to this, Vu et al. [28] observed significantly smaller mass losses when baking plant-based burgers (cooking yield of about 88%) compared to meat burgers (cooking yield of about 77%), which they explained by the occurrence of considerable shrinkage of muscle tissue and the release of fluids during heat treatment. Moreover, Argel et al. [21] observed a significant increase in the cooking yield of burgers as a result of replacing the meat with white bean flour. In this case, ingredients with a high water-holding capacity, such as starch and fiber, were introduced and contributed to the structural stabilization of the stuffing [53].

Important food quality indicators that allow for predicting its stability and safety are water activity and pH [56]. The produced burgers did not differ (p > 0.05) in terms of water activity, which ranged from 0.949 to 0.962 (Table 4). Similar results were obtained by Elgadir et al. [57] for full-meat burgers, as well as Botella-Martínez et al. [58] in the case of meat burgers with the addition of mushroom flours. Other sources in the literature [59,60] show that plant-based meat analogs may have slightly higher water activity (up to approx. 0.990).

Regarding pH values, meat, hybrid, and plant burgers did not differ significantly within the same recipe variant. The pH levels of hybrid or plant burgers based on recipe variant II were significantly (p ≤ 0.05) higher compared to those from recipe variant I (Table 4). This difference resulted primarily from the composition of the plant mixtures. The recipe variant I contained dried red tomatoes (constituting 20% of the plant blend—Table 1), which exhibit quite high acidity. As Yusufe et al. [61] reported, the pH values of dried tomatoes typically varied from 4.3 to 4.4. Nevertheless, the pH value of all examined burgers did not exceed 5.85 (Table 4). This was not consistent with the results obtained by Ardila et al. [62], who developed and assessed burger analogs based on seitan, textured soy flour, and insects because the mean pH value of these products was about 6.4. According to Tóth et al. [63], the pH of meat analogs is often close to 7.0; thus, such products are more susceptible to microbiological spoilage than their conventional counterparts. The lower pH of meat–plant and plant burgers tested in this study could be a consequence of the presence of lactic acid bacteria used as part of the bioprotection of these products [64].

It can be concluded that in the case of burger-type products, water activity, and pH depend mainly on the types and amounts of raw materials, additives, and some preservation treatments. Moreover, as products belonging to moist (a_w > 0.9) and low-acid (pH > 4.5) foods, they require appropriate packaging, storage, and distribution conditions [56,60,63].

Color is one of the first food features assessed by consumers when making a purchasing decision. The color of food depends on its chemical composition, including the type and content of pigments, as well as on the changes that occur as a result of its processing [65,66]. The color of the surface and cross-section of meat, hybrid, and plant burgers was determined using an instrumental method and expressed in the CIE L*a*b* color space, as presented in

Table 5. Surface and cross-section color parameters of baked meat, hybrid, and plant burgers.

Recipe Variant	Measurement	Sample Type
		M100_P0	M75_P25	M50_P50	M25_P75	M0_P100
		L* [−]
I	Surface	53.1 ± 2.0 cA	51.2 ± 2.0 bcA	48.9 ± 2.4 abc	47.0 ± 1.5 abA	44.1 ± 1.7 aA
	Cross-section	62.2 ± 1.9 cA	58.6 ± 2.3 bc	56.7 ± 1.0 abA	53.0 ± 2.4 aA	51.8 ± 2.3 aA
II	Surface	53.1 ± 2.0 cA	51.2 ± 1.0 bcA	47.4 ± 1.8 abA	45.2 ± 1.2 aA	45.8 ± 0.6 aA
	Cross-section	62.2 ± 1.9 dA	59.4 ± 1.3 cdA	55.6 ± 1.0 bcA	54.4 ± 1.3 abA	50.4 ± 0.9 aA
		a* [−]
I	Surface	14.2 ± 0.6 bA	10.1 ± 1.6 abA	9.1 ± 1.9 aA	9.1 ± 1.8 aA	8.8 ± 2.5 aA
	Cross-section	14.7 ± 0.8 aA	10.2 ± 2.1 bA	8.0 ± 1.2 abA	5.8 ± 2.1 aA	6.6 ± 1.4 abA
II	Surface	14.2 ± 0.6 bA	11.6 ± 0.9 aA	11.6 ± 0.4 aA	12.5 ± 1.2 abA	13.0 ± 0.7 abA
	Cross-section	14.7 ± 0.8 bA	10.0 ± 1.2 aA	10.4 ± 0.5 aA	9.4 ± 0.9 aA	9.6 ± 0.4 aA
		b* [−]
I	Surface	5.1 ± 0.7 aA	8.0 ± 0.4 abA	10.6 ± 1.4 bcA	12.7 ± 1.8 cA	10.9 ± 0.9 bcA
	Cross-section	4.2 ± 0.3 aA	9.5 ± 1.1 bA	11.2 ± 1.3 bcA	14.0 ± 1.5 cdA	15.0 ± 1.8 dA
II	Surface	5.1 ± 0.7 aA	12.7 ± 2.0 bA	14.3 ± 2.3 bA	14.7 ± 2.2 bA	16.8 ± 3.0 bB
	Cross-section	4.2 ± 0.3 aA	11.9 ± 3.2 bA	15.9 ± 0.9 bcA	18.7 ± 1.8 cB	19.6 ± 1.1 cA
		ΔE1 [−]
I	Surface	-	6.1 ± 1.6 aA	9.0 ± 1.4 abA	11.1 ± 1.3 bcA	12.2 ± 0.4 cA
	Cross-section	-	8.0 ± 2.8 aA	11.3 ± 2.1 aA	16.3 ± 0.9 bA	17.3 ± 0.9 bA
II	Surface	-	8.3 ± 2.2 aA	11.1 ± 1.4 abA	12.6 ± 1.6 abA	14.0 ± 2.4 bA
	Cross-section	-	11.6 ± 1.8 aA	13.6 ± 1.5 aA	17.5 ± 1.1 bA	20.2 ± 0.9 bA

For the denomination of hybrid burger samples, see Table 1. ΔE₁—total color difference in burgers compared to the meat sample (M100_P0). Different lowercase letters between means values ± standard deviations in the row indicate significant differences (grouping variable: sample type), p ≤ 0.05. Different uppercase letters between means values ± standard deviations in the column for a given measurement indicate significant differences (grouping variable: recipe variant), p ≤ 0.05.

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As expected, all tested burgers were characterized by a lighter color (higher values of the L* color parameter) in the cross-section than on their surface, which resulted from the direct impact of various external factors (e.g., oxygen or heat) on the surface of the samples. Overall, the burgers were much darker as more of the multi-ingredient plant mixture was used. However, the observed differences were not due to the recipe variant (p > 0.05). The lighter color of the meat was presumably caused by the thermal denaturation of myofibrillar, sarcoplasmatic, and connective tissue proteins [67,68]. On the other hand, the darker color of plant parts in burgers could result from a relatively high carbohydrate content, which favors Maillard reactions [69,70].

Likewise, reducing the meat content in burgers resulted in a decrease in the a* color parameter, except for the surface color of the burgers, based on the recipe variant II. The redness of the burgers with the highest content of plant-origin parts (M25_P75 and M0_P100) was similar to that of the meat burger (Table 5). The meat is pink and red after thermal treatment due to previous curing. Oxygenated myoglobin (MbO₂), in the presence of nitrites, is transformed into unstable shiny red nitrosomyoglobin (NOMb) and then, under the influence of heat treatment, into stable bright red nitrosomyochromogen [71]. In turn, the reddish color of the surface of hybrid and plant burgers in recipe variant II results from the presence of carrots, which contain a lot of β-carotene, which is responsible for their typical orange color. Therefore, carrots are characterized by a high value of the a* color parameter even after thermal treatment (on average in the range of 10–40, depending on the composition of the raw material, type, temperature, and time of initial/proper processing) [72,73].

Furthermore, the incorporation of plant ingredients into the composition of pork burgers significantly increased the value of the color parameter b* (indicating their yellowness) compared to a 100% meat burger. Even with 25% content of plant parts, the average value of this parameter increased approximately two or three times, regardless of the recipe variant and the measured area (Table 5). However, a statistically significant higher value of the b* parameter was observed for hybrid burgers with the highest content (75 or 100%) of a plant mixture from recipe variant II (with lentils and carrots) compared to their counterparts from recipe variant I (based on a plant mixture with millet groats and red tomatoes). It can therefore be concluded that these differences were primarily the result of the chemical composition of the individual ingredients in the burgers. Both carrot roots (as previously mentioned) and lentil seeds are raw materials rich in carotenoids. However, it should be taken into account that the plant part of recipe variant II of the burgers is dominated by red lentils (about 45%—Table 1), the color parameter b* of which has much higher values (up to 5 times) than the a* color parameter, even after heat treatment [74].

In order to determine the overall differences in color between hybrid or plant burgers, compared to a meat burger, the total color difference ΔE₁ was determined. Its average value exceeded 5.0 for each comparison set (Table 5), which means that the observer may have the impression of two different colors [30]. Moreover, the lower the ratio of meat to plant parts in the recipe, the higher the ΔE₁ level. Similar relationships were observed when measuring the color of both the surface and cross-section of burgers. Nevertheless, the largest total color difference compared to the meat sample (ΔE₁~20.2) was recorded in the case of a cross-section of a plant burger based on lentils and carrots (recipe variant II).

Due to the fact that baked meat products have a characteristic brownish surface, the Browning Index (BI) was determined (based on the basic color coordinates: L*, a*, and b*) of the tested meat, hybrid, and plant burgers, as shown in Figure 1.

The higher the content of plant parts in a burger recipe, the greater the Browning Index of its surface after baking (Figure 1). However, the degree of these changes depended on the recipe variant. Plant burgers based on lentils and carrots (recipe variant II) were characterized by more than twice the BI value of a full-meat burger. In addition, each burger from recipe variant II showed a greater (p ≤ 0.05) degree of surface browning than the samples based on recipe variant I (Figure 1).

The brown surface of baked or roasted food products affects their final acceptance by the consumer. This is mainly the result of non-enzymatic transformations, including Maillard reactions (amine groups with carbohydrate groups), caramelization (degradation of sugars), oxidation of lipids, or structural modifications of proteins. Maillard reactions occur more intensively in products with a high content of carbohydrates (especially reducing sugars) and proteins (especially lysine), at elevated temperatures (>50 °C), and in a non-acidic environment (optimal pH: 6.0–9.0). Highly advanced Maillard reactions contribute to the formation of high-molecular pigments such as melanoidins [75,76,77].

3.4. Textural Properties of Baked Meat, Hybrid, and Plant Burgers

An important distinguishing feature of meat products is their texture, which affects their sensory attractiveness. The texture is defined as a combination of the rheological and structural properties of food that result from the mutual arrangement and interaction of food ingredients, as well as the way in which they are perceived by the human senses [78]. Objective assessment of the textural features of food is possible using instrumental methods. Among instrumental techniques for determining the texture (including tenderness and hardness) of meat and meat analogs are mechanical destructive methods such as the Warner–Bratzler shear test and a penetration test [79,80]. Therefore, in this research, baked burgers were tested using the indicated instrumental methods, and the results are presented in Figure 2a,b.

It was observed that reducing the meat content in the burgers had a significant impact on decreasing the shear force compared to the sample without plant additives, regardless of the recipe variant (Figure 2a). This meant that the hybrid and plant-based burgers were softer and easier to cut into smaller pieces. The results were consistent with the study of Kamani et al. [81], who investigated the effect of partial or total replacement of meat with plant-based proteins on the textural properties of chicken sausages.

Similarly, the penetration force of burgers decreased with an increase in the content of plant parts in the recipe. The difference was that burgers based on recipe variant II were characterized by significantly (p ≤ 0.05) higher values of penetration force at each level of the proportion of meat to plant parts than burgers obtained from recipe variant I (Figure 2b). It can therefore be concluded that the structure of burgers containing a mixture of plant ingredients was more disintegrated under the influence of a point-applied mechanical force at the same level.

The greater tenderness (less hardness) of hybrid and plant burgers, compared to the meat sample, could be caused by the probable competition for water between plant and meat ingredients [82] and a reduction in the content of fibrous structures typical of meat. In hybrid burgers, there is a lack of substances that bind plant and meat ingredients into one integral structure. Thus, the observed structural differences may be reduced by using structured plant proteins (to imitate the structure of meat muscles) [79] and/or adding effective binding agents [83].

4. Conclusions

This work determined the influence of the proportions of meat and plant parts and the recipe variant on the basic chemical composition, physicochemical properties, and texture of baked burgers. Hybrid and plant burgers had similar water, protein, and fat content and cooking yields, compared to the meat burger. Pre-processed multi-ingredient plant mixtures (based simultaneously on vegetables, cereals, legumes, and oil seeds) can help develop a well-balanced flexitarian diet, including an increased supply of dietary fiber.

Moreover, the microbiological safety of raw hybrid and plant burgers was determined as a result of the application of lactic acid bacteria (LAB) cultures. LAB may provide effective protection against the development of undesirable microorganisms in the raw hybrid semifinished product during refrigerated storage and distribution.

One of the possible obstacles to the development of meat and plant burgers can be their tender texture, which determines the perception and final acceptance of the products by consumers. Therefore, research should be continued in the field of extended textural and sensory analyses simultaneously.