Effect of Opuntia pulp as a clean label ingredient on techno-functional properties of meat-free burger

Abstract

There is a universal increase from an animal-based diet towards healthy plant-based foods. Opuntia fruit pulp (OFP) as a pigment/hydrocolloid complex provides excellent functional properties. The aim of this study was the reformulation of the meat-free burgers by using OFP (0.5, 1.5 and 2.5%) as a stabilised natural pigment and analysing techno-functional characteristics of uncooked and cooked meat-free burgers. The OFP was a rich source of polyphenols (35.3 ± 2.07 mgGAE/g) and carbohydrates (68.22 ± 0.5%). All uncooked treated burgers exhibited higher water-holding capacity and redness as well as lower thiobarbituric acid reactive substance (TBARS) values. Results confirmed a higher cooking loss in the control sample (22.2 ± 0.63%) compared with the OFP-treated burger at 1.5 and 2.5% content. Incorporation of OFP as a great source of natural pigments and phenolic components had considerably influenced cooking yield, moisture retention, juiciness and oxidative stability of meat-free burgers. The lowest total colour difference value with resemble meat burger was observed in the treated burgers at the highest content of OFP. According to sensory evaluation, the overall acceptability of cooked burgers with 1.5% OFP was more satisfactory than other samples. Using this clean label ingredient provides a sustainable burger, which is beneficial to the public health, environment and animal welfare.

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Introduction

Nowadays, the meat industry faces a rising tide of challenges related to ethical, environmental (greenhouse-gas emissions) and human health issues (chronic diseases, stroke and colorectal cancer (Schreuders et al., 2021). From this point of view, shifting dietary patterns from an animal-based diet towards the plant-based one have been highlighted (Elzerman et al., 2021). Introducing plant-based meat analogues, which closely resemble the meat-based products, is fascinating (Joshi & Kumar, 2015). The appearance of a mimic product is often the first attribute to interest of a customer (Kazir & Livney, 2021). Besides environment and sustainability, health concerns related to artificial ingredients/additives is the most involved for clean labels (Maruyama et al., 2021). In this term, using natural colourant with desirable antioxidant potential had been considered to obtain functional analogue products with reddish tones (Ebeneezar et al., 2020). The prickly pear (Opuntia stricta) is a functional product that offers numerous health benefits (Kossori et al., 1998; Majdoub et al., 2001; Salehi et al., 2019). Opuntia fruit pulp (OFP) is a stabilised natural pigment (betalains, carotenoids and flavonoids) (González-Ponce et al., 2020), which comprise a pigment/hydrocolloid complex. Mucilage is a critical component of opuntia fruit pulp (OFP), which consists of carbohydrate, protein and ash (Salehi et al., 2019). Water-holding capacity, gel-forming ability and thickening properties of OFP are useful for foodstuffs, which are especially linked to the presence of fibres and some other components (Salehi et al., 2019, Dickinson et al., 2009). Opuntia fruit pulp fibre is a rich source of pectin (14.4%) (Kossori et al., 1998). The beneficial effect of fibres to improve textural properties of meat analogue products had been previously reported (Kazir and Livney, 2021). From this point of view, various technological applications of Opuntia-derived products, including beef burgers (Parafati et al., 2021), sliced beef (Palmeri et al., 2018), burger patties (Parafati et al., 2019), cereal-based products (Moussa-Ayoub et al., 2015), gluten-free crackers (Dick et al., 2020) and yogurt (Bernardino-Nicanor et al., 2021) had been introduced to the food industry.

In terms of providing better nutritional values in vegetarian diets, the meat analogue is required to be enriched. On the other hand, growing demand for clean label food reflects consumer opposed synthetic additives, so considering the chemical component and nutritional information of OFP in designing resemble meat products could be valuable. Despite the incredible growth in the meat industry during the past decade, there are no published data on the utilisation of Opuntia as a clean label ingredient in the meat-free formulated products. In this regard, the aim of this study was to evaluate the effect of the OFP consent on the techno-functional properties of veggie burgers.

Materials and methods

Materials

All solvents used in this study were analytical grade (Merck Company, Germany). Folin–Ciocalteu and sodium carbonate were provided from Sigma-Aldrich (St Louis, MO, USA). Texturize soy protein (carbohydrate 29%, protein 50% and fat 3.1%) was obtained from the Sobhan company (Behshar, Iran). The mature prickly pear fruits (Opuntia stricta) were collected in February from Neka (Mazandaran Province, Iran).

Preparation of Opuntia fruit powder

The OFP powder was prepared according to Kharrat et al. (2018). Briefly, the fruits were washed several times and partially hand-peeled. The obtained pulp was dried at temperature 45 by force oven dryer and powdered using a coffee grinder (Braun, Germany) and stored at 4°C until use.

Proximate analysis of OFP powder

Physicochemical parameters of the OFP (moisture, pH, ash, protein and lipid) were determined using standard AOAC methods (AOAC, 1998). Total phenolic content was determined as described by Ndhlala et al., (2007).

Meat-free burger preparation and cooking process

For producing a meat-free burger, the textured soy protein (TSP) as a main nonmeat ingredient (20%) was weighed and mixed with water (33.34%). The NaCl (1%w/w) was added with the soaked TSP and other batches in the following percentages: bread crumbs (11%), whey protein powder (4.6%), gluten (3.1%), onion (15.8%), sunflower oil (5.5%), garlic (2.8%), species (1.86%) and guar (0.3%). According to the preliminary test, the meat-like structure of the meat-free burger drastically changed by replacement of the OFP with textured soy protein. For this reason, the OFP was substituted with bread crumbs at different contents (0.5, 1.5 and 2.5%). Each formulation was homogenised in a meat blender for 5 min. The pieces (50 g each) were formed using a burger press maker to give the diameter of 10 cm and a thickness of 1 cm and stored at −18°C.

For the cooking process, the frozen burger was firstly thawed and then grilled at 180°C for 6 min. The cooked samples naturally cooled down in the air (Parafati et al., 2021).

Analysis of uncooked meat-free burger

Proximate analysis of uncooked meat-free burger

Physicochemical parameters of uncooked meat-free burgers (moisture, pH, ash, protein and lipid) were measured using standard AOAC methods (AOAC, 1998).

Colour measurement

The surface colour of uncooked meat-free burgers was determined as previously described by Shahiri Tabarestani & Mazaheri Tehrani, (2014). Total colour difference value was taken into the difference between colour parameters of meat-free burger sample and target plate, which is considered as a meat resemble burger (L^*_Target = +41.6, a^*_Target = +30.2, b^*_Target = +10.6). Chroma (C) and total colour of difference (ΔE) were calculated using the following equations.

Water-holding capacity (WHC)

Uncooked meat burgers (10 g) mixed with 40 ml of distilled water and then centrifuged (at 3000 rpm for 30 min). After removing excess water, the samples were weighed. The WHC of the samples was determined using the following equation (Ngadi et al., 2001).

Lipid oxidation of frozen meat-free burger

Lipid oxidation of frozen raw meat-free burgers was evaluated at 6 months of storage in a freezing temperature (−18°C) through thiobarbituric acid reactive substances (TBARS) as previously described by Shahiri Tabarestani & Mazaheri Tehrani, (2014). In brief, an uncooked sample (25 g) was mixed with 50 ml trichloroacetic acid (20%) in a Moulinex® blender for about 2 min. After washing the blender with deionized distilled water (50 ml), the mixture was filtered, and then, the separated upper liquid layer was heated in a water bath at 100°C for 1 h. The absorbance of the pink colour solution was read at 532 nm using a UV/vis spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Oxidation products were expressed as mg malonaldehyde/kg of a burger.

Analysis of cooked meat-free burger

Cooking weight loss (CWL)

The CWL of the meat-free burger process is related to the total amount of fluid released undercooking, which is determined as described by Parafati et al., (2021). Each burger was weighed before and after cooking. All cooked meat-free burgers were weighed after cooling at room temperature (23 ± 1°C). The CWL was calculated with the following equation:

Texture Profile Analysis (TPA)

The textural parameters of meat-free burgers, including hardness (N), springiness (cm), cohesiveness (ratio gumminess to hardness) and chewiness (N.cm), were determined using a Texture Analyzer (TA-XT, England) equipped with a rectangular probe (5 cm × 4 cm) and a 1 kg load cell as previously described by Parafati et al. (2021). Briefly, the sample was compressed twice with a cell load of 50 N using a crosshead speed constant of 20 mm/min. All experiments were performed in three replicates, and the results were shown as the ± standard deviation.

Moisture retention

The moisture retention values represent the kept water content in the cooked product per 100 g of raw sample. The moisture retention of samples was measured according to the standard methods (AOAC, 1998) as described by the following equation:

Shrinkage

The shrinkage percentages of uncooked burgers were measured using an electronic digital caliper ultraprecision (Germany) as described by AOAC (1998).

where T₁=thickness of uncooked burger; T₂=thickness of cooked burger; D₁=diameter of uncooked burger; D₂=diameter of cooked burger.

Juiciness

A sample was taken from the centre of the burger (1 g) and placed between previously weighed filter paper. After covering with aluminium foil, the selected sample was put under a 25 kg force using the texture analyser (TA-XT, England) for 1 min. Finally, the filter paper was weighed, and the per cent of juiciness was determined by the following equation:

Sensory evaluations

Sensory evaluations of cooked burgers were carried out by 20 trained students of Gorgan University of Technology (Gorgan, Iran). Each panelist has received four samples in a randomised order to evaluate the preference of the texture, flavour, colour and overall acceptability based on a 9-point hedonic scale (9 = like extremely; 5 = neither like nor dislike; 1 = dislike extremely). Unsalted crackers and mineral water were used between samples to neutralise the mouth of residual taste by panelists (Pietrasik et al., 2020).

Statistical analysis

The experiment was performed in a complete randomised design using version 8.2 of Statistical Analysis System (SAS Institute Inc., Cary, NC, USA) at three replications. The significant differences among treatments of meat-free burgers were statistically analysed using the Duncan test at the 5% probability level.

Results and discussion

Chemical composition of OFP

The chemical composition of additives is effective to obtaining desirable structure and sensory quality of analogue products (Kazir & Livney, 2020). The main components present in the OFP are carbohydrate (68.22 ± 0.05%), moisture (10.45 ± 0.08%), protein (5.2 ± 0.07%), fat (3.66 ± 0.09%) and ash (12.47 ± 0.14%). Comparing our results with findings of El Kossori et al. (1998) confirmed the lower total fat and ash values and also the higher carbohydrate contents for Opuntia ficus-indica. The dependency of chemical composition of opuntia pulps to cultivation site, climate and fruit variety had been previously reported (Salehi et al., 2019). The protein content of prickly pear regarding leguminous plants was low and also comparable with sweet potatoes, cassava and yam. The total polyphenols content of opuntia pulp is beneficial in terms of nutrition and health properties as well as retarding lipid oxidation (Salehi et al., 2019). The TPC content of opuntia pulp in the present study (35.3 ± 2.07 mg GAE/g) was higher than reported value by Parafati et al. (2021).

Effect of OFP on properties of uncooked meat-free burger

The physicochemical properties of uncooked meat-free burgers containing different contents of OFP are shown in Table 1. As can be seen, the moisture, protein, fat and ash values of uncooked burgers were significantly increased by adding OFP (p > 0.05). Generally, minerals play a significant role in improving emulsification ability, viscosifying features and gel-forming ability (Espino-Díaz et al., 2010). Surprisingly, calcium ion, a significant element in the ash content of opuntia fruit (Kossori et al., 1998), tends to interact with a carboxyl side chain of carbohydrates, which could promote the formation of a firmer product (Espino-Díaz et al., 2010; Kazir & Livney, 2020).

According to the obtained results, the pH value of treated uncooked free-meat burgers varied from 5.76 to 6.32, which significantly was lower than the control burger (P < 0.05). It is likely due to organic acids in the opuntia fruit (Kharrat et al., 2018). Since the pH value of plant-based products has been greater than meat ones, using a combined effect of adding OFP on the growth rate of microorganisms as a hurdle technology could be desirable.

Colour is an economically relevant quality of burgers that affect sensory perception and consumers’ acceptance (De Marchi et al., 2021). As can be seen in Table 1., the redness index (a^*) increased significantly with the increase of OFP content in the burger formulation (P < 0.05), which is related to the presence of beta-cyanin with red-purple colour. The yellowness index of the control (+22.48 ± 0.58) was more significant than the meat-based burger (+14.46), which was reported by De Marchi et al., 2021. In addition, the yellow index was significantly decreased by increasing the OFP content in burger formulation (P < 0.05), which could be commercially attractive for food processors. The total colour difference of the sample without OFP (control) was 22.29 ± 0.30. This value indicated that the control sample was visually far apart from the reddish colour. As can be seen in Table 1, the lowest total colour difference value with resembled meat burger belonged to the treated burgers at the highest content of OFP. In fact, increasing the OFP content led to obtaining more resembling beef burger products.

Water-holding capacity is defined as the ability to prevent/retain water from a product during food processing. As can be seen in Fig. 1, the WHC of the burger was significantly increased by adding the OFP (P > 0.05). These results are consistent with the previous work of Kazir & Livney, 2020, who reported the function of dietary fibre on the improvement of water-holding capacity of meat analogue. Opuntia fruit pulp is well known for dietary fibre such as pectin (14.4%) (Kossori et al., 1998), which could be providing gel-forming properties with considering the water absorption/retention absorption capacity (Dickinson et al., 2009; Majdoub et al., 2001; Salehi et al., 2019).

Effect of OFP on lipid oxidation of the uncooked meat-free burger

TBARS (mg malondialdehyde/kg of a sample) is a suitable indicator of lipid oxidation in several food products (Sobral et al., 2020). The lipid oxidation of uncooked burgers was determined at 6 months of storage in a freezer (−18°C). As shown in Fig. 2, the TBAR value in burgers containing 0.5, 1.5 and 2.5% OFP was 1.48, 1.36 and 1.26 mg/kg respectively. These values were significantly lower than the sample without OFP (1.67 mg/kg), which could be linked to the protective effects of phenolic compounds in lipid oxidation due to scavenging of free radicals, chelating pro-oxidant metals (Nikmaram et al., 2018). The lipid oxidation of meat-free burgers significantly decreased by increasing OFP content from 0.5% to 2.5% (P < 0.05). These results were consistent with previous studies that observed the oxidation stability of soy-based burger (Trujillo-Mayol et al., 2021) and soy protein-based emulsion (Djuardi et al., 2020) improved in the presence of polyphenols.

Evaluation of cooked meat-free burgers

Physical and textural properties of cooked meat-free burgers

Cooking loss is a technological parameter to define the weight decrease before and after cooking. Cooking loss value of the control burger was 22.2 ± 0.63%. This value was obviously lower than the observed value in beef burgers (Soltanizadeh & Ghiasi-Esfahani, 2015; Trujillo-Mayol et al., 2021). As can be seen in Table 2, there is nonsignificant difference in cooking loss between the control burger and the treated sample with 0.5% OFP. This result was in good agreement with previous studies based on the incorporation of avocado peel extract in soy-based burger up to 1.5 (Trujillo-Mayol et al., 2021). Even though, a significant reduction in cooking loss value was observed by increasing the OFP content (2.5%) in meat-free burgers (P < 0.05). This can be attributed to phenolic compounds and nonstarch polysaccharide components of Opuntia fruit that could provide excellent capacity to water retention (Kharrat et al., 2018; Parafati et al., 2021). Our findings were consistent with previous studies based on adding blueberry puree in soy-based burgers (Small, 2007), fruit albedo in meat-based burgers (López-Vargas et al., 2014), complex carbohydrates in soy-based burgers (Basati & Hosseini, 2018) and lemon albedo in meat-based burgers (López-Vargas et al., 2014).

Shrinkage of burgers commonly occurs in the cooking process by denaturation of protein and releasing of trapped water and fat (Pietrasik et al., 2020). As shown in Table 2, the cooking shrinkage value of the control sample was 2.11 ± 0.39%. There were no significant differences between the cooking shrinkage value of the burger sample with 0.5% OFP and the control one (P ˃ 0.05). However, a considerable reduction in the aforementioned value was observed by increasing OFP concentration in the burger formulations (P < 0.05). The same trend was observed by adding the complex carbohydrates and fibres (Soltanizadeh & Ghiasi-Esfahani, 2015; Basati & Hosseini, 2018).

Retain moisture capacity has a positive effect on preventing excessive shrinkage (Pietrasik et al., 2020). The moisture retention value of all burger treatments containing the OFP was more than the control one (P < 0.05). It may be linked to the binding and the stabilising ability of the OMP. As shown in Table 2, the moisture retention of burgers did not improve by increasing OFP concentration (P > 0.05).

The TPA tests (containing chewiness, springiness, adhesiveness, etc.) are widely applied on meat analogue (Schreuders et al., 2021). Springiness refers to returning of food products to the primary situation after elimination of chewing force without any deformation. As shown in Table 2, the springiness value of the sample with 2.5% OFP was more significant than the control one (P > 0.05). These could be related to the reduction of aggregation of protein’s network, which led to a decrease in elasticity.

The juiciness of cooked burgers is an important contributor to eating quality. As shown in Table 2, juiciness of burgers significantly increased (P < 0.05) by increasing the OFP concentration. Our results were in line with Kharrat et al. (2018), who confirmed the capacity of prickly pear extract to prevent the loss of water and keep juiciness.

Chewiness intimates require energy for chewing food products to be ready for swallowing. The chewiness of cooked burgers was not significantly changed by adding different content of OFP (P > 0.05).

Sensory analyses of cooked meat-free burgers

The sensory analyses of meat-free burgers formulated by incorporation OFP are shown in Table 3. Results showed that OFP content had a significant influence on final burgers (P < 0.05). The control burger was the least liked in appearance. The overall acceptance scores (6–8) of the meat-free burgers with the OFP were successfully sufficient to suggest into the marketplace. Although, the sensory evaluation for meat-free burgers with 1.5% Opuntia ranked the highest for texture, taste and overall appearance. Moreover, there was no significant difference in the colour of cooked samples (P > 0.05).

Conclusion

Nowadays, health and environmental issues are the driving force behind the popularity of meat-free burgers. Opuntia fruit pulp is an innovative clean label ingredient in the food industry due to nutrient components, polysaccharides and dietary fibres. The lipid oxidation of meat-free burgers was related to the total polyphenol content of the OFP in meat-free burgers during freezing. The preference of texture and overall acceptance of the formulated meat-free burger was positively correlated with incorporating the OFP content. By increasing the OFP content in burger formulations, the relevant sensory properties were decreased due to receiving astringent taste. The results of an ongoing study that deals with the challenge of bioaccessibility of some minerals in veggie burgers by using this clean label ingredient will be presented in a forthcoming paper.

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

Ethics approval was not required for this research.

Author Contribution

Parisa Shahiri Tabarestani: Data curation (lead). Mahboobeh kashiri: Conceptualization (lead); Supervision (lead); Writing – original draft (equal); Writing – review & editing (equal). Yahya Maghsoudlou: Investigation (supporting). Hoda Shahiri Tabaestani: Formal analysis (lead). Mohammad Ghorbani: Methodology (supporting).

Peer review

The peer review history for this article is available at https://publons.com/publon/10.1111/ijfs.15657.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

References