Fourteen boneless pork loins were obtained from a commercial pork harvest facility on the day after slaughter. Selected loins varied in weight from 4.1–4.5 kg and in pH from 5.50–5.80. The loins were vacuum packaged and transported to the Iowa State Meat Laboratory within 90 min of packaging and stored at 2–3°C until used. Ingredients for this study included sodium tripolyphosphate (Innophos Canada, Inc., Lowbanks, Ontario, Canada), sodium bicarbonate (Church & Dwight, Ewing Township, NJ, USA), potassium chloride (NuTek Natural Ingredients, Omaha, NE, USA), yeast extract (OHLY^® STTF, Ohly, Hamburg, Germany), potassium lactate (PURASAL^® Hi Pure P), dried vinegar powder (Verdad Powder N6, Corbion, Lenexa, KS, USA), acerola cherry powder (VegStable^® 515), rice bran extract (VegStable^® Plus 452, Florida Food Products, Inc., Eustis, FL, USA), plum concentrate (Sunsweet Growers Inc., Yuba City, CA, USA), and Proteus^® functional meat protein extract (Kemin Industries, Inc., Des Moines, IA, USA).

The experiment consisted of 2 controls and 5 different treatments of enhanced raw pork loins, shown in Table 1. The control group consisted of loins injected with a combination of water, salt, sodium tripolyphosphate, and potassium lactate (SPH) to represent a common industry enhancement level and uninjected loins (UNIN) to serve as a negative control. The treatment group consisted of loins injected with water, sodium bicarbonate, potassium chloride, yeast extract, dried vinegar powder, and acerola cherry powder (INCL) to represent a base clean label formula and 4 treatments based on the INCL treatment but with an additional clean label ingredient. The RICE treatment included 1% rice bran extract, the PLUM treatment contained 0.35% plum concentrate, and PR 13 and PR18 brines both contained 0.85% Proteus^®.

All samples were manufactured in the Iowa State University Meat Laboratory in Ames, Iowa, and in the following order: UNIN, SPH, INCL, RICE, PLUM, PR13, PR18. Brines were prepared using a 53-cm Big Stik immersion blender (Waring Commercial, Torrington, CT, USA) to aid dispersion of ingredients into a solution, with the least soluble ingredients being added first. Once the brines were prepared, they were injected into pork loins using a multineedle injector (IMAX 300SL, Schröder Maschinenbau GmbH & Co. KG, Werther, Germany). The manufacturing sequence was chosen to separate the uninjected treatment and the phosphate treatment from the subsequent treatments by injecting the uninjected and phosphate treatments first. The injector was thoroughly rinsed between each of the treatment groups by running water through it several times between each of the batches. For the injected treatments, the target injection level was 13% of meat weight with the exception of PR18, which was injected to 18%, as recommended by the supplier. The PR13 treatment allowed comparison of the Proteus^® ingredient at the same use level as all the others, while the PR18 treatment allowed evaluation of this ingredient according to the supplier’s recommendation of an 18% injection level. Side port needles (4 mm) were used for all treatments except PR13 and PR18, which used hypodermic needles (4 mm), due to the viscosity of the brine. The injection target amounts for all treatments were verified by reweighing each of the loins following the injection and calculating the percentage of brine injected. Two loins per treatment were injected, transferred to separate tumblers (DVTS R2-50, Daniels Food Equipment Inc., Parkers Prairie, MN, USA) with a dedicated tumbler used for each treatment, and tumbled under vacuum at approximately 16 rpm for 2 h. After tumbling, loins were sliced into ten 25.4-mm thick chops and two 101.6-mm thick roasts, as shown in Figure 1. Chops and roasts were then placed into vacuum bags (oxygen transmission rate of 3–6 cm³/m²/24 h at 23°C, 0% RH [relative humidity]; water vapor transmission rate of 7.8–9.3 g/m²/24 h at 38°C, 100% RH; Cryovac, Sealed Air Corporation, Duncan, SC, USA), sealed and stored in boxes at 2°C for 0, 7, 14, and 21 d. Chops and roasts from the first loin were used for day 0 and day 7 analyses, and those from the second loin were used for day 14 and day 21 analyses. Sections 1–6 and 7–12 from 1 loin were used for day 0 and day 7 analyses, respectively, and the same sections from the second loin were used for day 14 and day 21 analyses, respectively. Chops 1 and 2 were used for compositional assays, and chops 3, 4, and 5 were used for color, pH, cooking yield, and Warner-Bratzler shear force (WBSF) assays. For sensory analysis, section 7 from the first loin was used for day 7, and sections 6 and 7 from the second loin were used for days 14 and 21, respectively. Chops 8, 9, and 10 were used for color, pH, cooking yield, and WBSF assays on day 7. Chops 11 and 12 were used for composition, similar to chops 1 and 2. A second loin for each treatment was subdivided in the same way for analysis on days 14 and 21.

Proximate composition was determined for each of the treatments to verify product formulation and ensure raw material consistency. Chops 1 and 2 were first homogenized in a food chopper (Express Chop, SharkNinja Operating LLC, Needham, MA, USA), and protein content was analyzed using the CEM Sprint Rapid Protein Analyzer (CEM Corporation Matthews, North Carolina, USA; AOAC method 2011.04, AOAC International, 2019). Moisture and fat content were measured in tandem according to AOAC method 2008.06 (AOAC International, 2019) using the SMART 6 system for moisture and the ORACLE system (CEM Corporation Matthews, North Carolina, USA) for fat content. All measurements were done in duplicate and averaged.

The pH of the chops was measured using a Mettler Toledo SevenMulti pH meter with a InLab solids Pro-ISM electrode (Mettler Toledo, Columbus, OH, USA). Each chop was measured by inserting the electrode into 3 different locations.

Color was evaluated on days 0, 7, 14, and 21 postpackaging. The raw chops were removed from the package and the color was measured by a HunterLab MiniScan EZ 4500L colorimeter (Hunter Associates Laboratory, Inc., Reston, VA, USA) using illuminant D65 (daylight at 6500 K), 10° observer angle, and a 2.54-cm aperture (King et. al., 2023). The Commission Internationale de l’Eclairage (CIE) L*a*b* color space was used to represent color. Color scans were taken in 3 different locations on each chop.

Texture was analyzed on days 0, 7, 14, and 21 postpackaging. Each chop was weighed before and after cooking to calculate cooking yield (cooked weight divided by raw weight). The chops were cooked in a Ninja Foodi Grill (model AG301, SharkNinja Operating LLC, Needham, MA, USA) on the grill grate with the “grill” function set to 260°C. They were turned over after 5 min, cooked to an internal temperature of 71°C, and allowed to cool to room temperature. Three separate 13-mm-diameter cores were cut and removed from each chop and sheared with a Warner-Bratzler knife with guillotine block fixture (TA-7, Stable Micro Systems, Surrey, UK) attached to a TA-XT2i texture analyzer equipped with a 30-kg load cell (Stable Micro Systems, Surrey, UK) at a test speed of 3.5 mm/s⁻¹. Results of the 3 measurements were averaged.

Sensory analysis was conducted on days 7, 14, and 21 postpackaging by a 5-member highly trained sensory panel. The panel was comprised of students, faculty, and staff of Iowa State University, all of whom had participated on multiple occasions in other trained panels involving similar products and were intimately familiar with the sensory protocol and vocabulary used in this study. For this particular study, one 60-min training session was held 1 wk before sample evaluation began, using untreated and enhanced pork chops. To prepare the sensory analysis samples, the roasts designated for this analysis were first cut into four 25.4-mm chops. The 2 middle chops were then placed in a Ninja Foodi Grill (model AG301, SharkNinja Operating LLC, Needham, MA, USA) on the grill grate with the “grill” function selected and set to 260°C and cooked an internal temperature of 63°C, turning them over 5 min into the cooking process. The chops were then cut into 25-mm cubes and mixed in a large bowl to randomize the sampling units. Cubes were then placed inside expanded polystyrene cups (1 cube per cup) labeled with 3-digit random numbers. The cups were covered with plastic lids, and the samples were served to the panel within 2 min of the end of cooking. The panelists were also provided with water and unsalted crackers as palate-cleansing agents. The panelists evaluated samples in separate examination booths under fluorescent lighting using a 15-cm unstructured line scale. The following sensory attributes were evaluated (scale anchor descriptors in parentheses): “juiciness” (dry/not juicy, juicy/not dry), “tenderness” (tough/not tender, tender/not tough), “chewiness” (not chewy, chewy), “pork flavor” (none, intense), and “other flavor” (none, intense). “Other flavor” was defined as any flavor that would be unusual for the product. The panelists were asked to describe the other flavor if detected. Data were collected using Compusense 5 sensory evaluation software (release 5.6, Compusense Inc., Guelph, ON, Canada). The sensory analysis protocol was reviewed and approved by the Iowa State University Institutional Review Board (IRB ID 21-458), and informed consent was obtained from all panelists prior to initiation of the study.

The experiment was designed as a randomized complete block and was replicated 3 times with separate manufacturing dates for each replication. The meat raw materials were from production days as described, and nonmeat ingredients were each obtained from a single production lot. For each replication, loins were assigned to treatments randomly. Data were analyzed statistically using JMP Pro 16 (SAS Institute, Cary, NC, USA) with treatment and day treated as fixed factors and replication, sensory evaluation sessions, and sensory panelists treated as random factors. Differences among treatments were determined using the Tukey-Kramer pairwise comparison method, with significance established at P < .05. P values for fixed main factor (treatment and storage time) effects and the interaction between them are shown in Table 2.

The proximate composition data are shown in Table 3. Fat content of UNIN chops was greater than in RICE chops (P = .047) due to no additional water, while INCL, RICE, PR13, and PR18 chops were higher in moisture than UNIN as a result of added brine. The SPH chops were also lower in moisture (P < .001) than the other treatments, which may reflect the somewhat lower pH in this treatment. The UNIN treatment, which was not injected, might be expected to have greater fat and protein and lower moisture content due to the lack of additional ingredients, especially water, present in the injection brines of the other treatments. The RICE and PR13 treatments resulted in higher pH (P < .05) than that of SPH and UNIN chops. Cooking yield was higher in RICE chops than in UNIN and INCL chops (P < .001). The elevated pH of the RICE treatment would have contributed to a greater cooking yield.

Summarized instrumental color data over the 21-d shelf-life are shown in Table 4. There were no storage time or treatment × storage time differences for L*, a*, or b* values (P > .050). UNIN chops were lighter than RICE, PLUM, PR13, and PR18 chops (P = .002) most likely due to its lower pH (Table 3). However, RICE chops were darker than all others, which may be attributed to the darker color of the rice bran extract. In a study by Min et al. (2009), researchers noted that treatment of catfish patties with rice bran also resulted in darker color than the controls. INCL, RICE, PLUM, PR13, and PR18 chops were redder (higher a*) than SPH chops (P = .002), which may be attributed to the ascorbic acid content of the acerola cherry powder. The b* values were higher in chops from the INCL, PR13, and PR18 treatments than in those from UNIN, SPH, and RICE treatments (P < .001), which is consistent with the yellow hue of the Proteus^® protein extract.

WBSF results are shown in Figure 2. There were no treatment (P = .189) or treatment × day (P = .884) differences. The only significant difference observed was at days 14 and 21, when chops had lower shear force values than at day 7 (P = .005). This might be expected as a result of proteolysis during aging and storage (Warner, 2021; Johnson et al., 2023). During the postmortem period, proteins such as titin, nebulin, and troponin-T, as well as Z line-associated proteins such as desmin, filamin, dystrophin, and talin, are disrupted and degraded, leading to myofibrillar fragmentation, loss of muscle cell integrity, and, ultimately, a softer and more tender product (Huff-Lonergan et al., 2010). It has been reported that proteolysis can affect major changes and lead to increased tenderness within the first 7–14 d postmortem (Miller, 2002). Furthermore, Davis et al. (2004) demonstrated that protein hydrolysis in pork loins was not affected by injection brines, and protein degradation continued in the higher ionic strength environment in the loins following brine injection, an observation supported by Maddock et al. (2005) who showed that, while high ionic strength decreased activity of μ- and m-calpain, it did not stop proteolysis completely.

Sensory results are shown in Table 5. The main effect of treatment was significant for all attributes as was storage time for tenderness. Juiciness was greater in SPH, RICE, PLUM, and PR18 than in UNIN chops (P = .003), which is consistent with our observation that moisture content and cooking yield for the SPH, RICE, PLUM, and PR18 treatments were greater than for UNIN (P < .001; Table 3). Tenderness was higher in RICE and PR18 chops than in UNIN (P = .005) and increased over time, being greater on day 21 (P = .009) than on days 7 and 14. This time dependency was similar as that observed for instrumental texture, as discussed previously. UNIN chops were also higher in chewiness than RICE, PR13, and PR18 chops (P = .006). RICE chops had greater pork flavor than those from all other treatments, while RICE, PLUM, and PR18 chops were all more flavorful than UNIN chops (P < .001). Because UNIN chops did not have any added ingredients, less flavor may be expected. These results confirm the expected improvement of these sensory properties in pork chops with the use of enhancement brines for pork loins. However, the sensory panel noted that SPH was higher in “other flavor” than all other treatments (P = .001). These flavors were described by the panelists predominantly as “sour,” “salty,” “metallic,” and may be due to the phosphates, which have been noted for contributing soapy, salty, sour, and metallic taste and flavor notes to food products (Chun et al., 2020; Ecarma and Nolden, 2021). The phosphate concentration (0.3%) used in this study is typical for commercial enhancement brines and is not normally expected to contribute to undesirable flavors. However, it is likely that the highly trained and experienced sensory panel members in this study were more perceptive of the “other flavors” noted.

This study has some potential limitations that future studies should consider. The sample size was limited, the loins were injected in a nonrandom order, and 2 different types of injection needles were used. The sample size was smaller than it could be due to the availability of a single multineedle injector. To help minimize sample size effects, loins were hand selected directly from the processing line at the pork harvest facility based on weight (4.1–4.5 kg). They were processed and injected in a nonrandom order to prevent potential contamination of the no-phosphate treatments with phosphate of the no-phosphate treatments due to the availability of a single multineedle injector. The use of hypodermic needles to inject the PR13 and PR18 treatments was unavoidable due to the viscosity of the brine. Any follow-up or confirmatory studies should consider the use of hypodermic needles for all treatments.