Introduction
Cooking meat leads to a cascade of reactions involving protein, sugar, and fat. These reciprocal interactions are important for cooked color and flavor development (Dinh et al., 2021; Khan et al., 2015; Suman et al., 2016). In raw meat, myoglobin is the main sarcoplasmic protein that contributes to color. Myoglobin-induced cooked meat color is primarily due to its denaturation. Myoglobin, depending on its redox state, forms ferro- (reduced form; reddish pink) or ferrihemochrome (oxidized form; brown) (King et al., 2023). In addition to the role of protein denaturation, non-enzymatic browning reactions such as the Maillard reaction and caramelization also contribute to cooked color, flavor, and aroma (Sun et al., 2022; Trevisan et al., 2016). The Maillard reaction involves the reaction between a reducing sugar (such as glucose or fructose) and an amine side chain containing an amino acid (such as lysine) when heated at high temperatures (Rufián-Henares and Pastoriza, 2016). Conversely, caramelization requires only sugar, and this reaction occurs when sugar is heated at a higher temperature. Interestingly, both myoglobin denaturation and non-enzymatic browning reactions are influenced by various factors such as pH, packaging, antioxidants, and the primary structure of myoglobin (Joseph et al., 2010; Maheswarappa et al., 2009; Sepe et al., 2005; Seyfert et al., 2004; Suman et al., 2010).
Persistent pinking refers to a pink color in cooked meat (undercooked appearance) after cooking to a United States Department of Agriculture-recommended temperature (Moiseev and Cornforth, 2006). Numerous factors, such as pH, nitrite containment (forms stable nitrosylhemochrome), carboxymyoglobin modified atmospheric packaging (resistant to heat-induced denaturation), carbon monoxide formed from incomplete combustion, microbial growth, or cytochrome c (in the case of poultry), can contribute to persistent pinking (Cornforth et al., 1998; Cornforth and Egbert, 1985; Sammel and Claus, 2003; Smith et al., 2024). However, a greater-than-normal postmortem meat pH is considered the most important contributing factor. A greater-than-normal pH leads to more undenatured myoglobin (Trout, 1989). This phenomenon can cause consumer concerns because the cooked color deviates from the expected brownish hue.
Various approaches, such as acidification of meat, higher cooking temperatures, and the addition of milk powder, have been used to minimize the occurrence of persistent pinking (Denzer et al., 2022; Sammel and Claus, 2003; Sawyer et al., 2009b). However, acidification of meat can influence eating qualities. Enhancing the Maillard reaction and caramelization also provides an alternative approach to minimize cooked pink color. Several other sectors in the food industry, such as baking, brewing, and meat alternatives, use the Maillard reaction and caramelization to improve color and flavor attributes (Sun et al., 2022). Most amino acids and reducing sugars required for the Maillard reaction and caramelization are naturally present in meat. Hence, adding these ingredients has the potential to enhance the brown color. However, limited knowledge is currently available on enhancing the Maillard reaction and caramelization to minimize the undercooked appearance in high-pH beef. This study aims to investigate the effects of lysine and glucose on the cooked color of high-pH beef.
Materials and Methods
Patty preparation
The current research used ground beef to create high-pH meat. This allows easy addition and mixing of ingredients. Fresh course-ground beef chubs (n = 5; 85% lean; 4.5 kg each chub) were purchased on the day of preparation from a local beef purveyor in Stillwater, Oklahoma. In order to have variability, one chub was purchased each day (a total of 5 separate occasions; one chub each occasion). Chub represents cylindrical vacuum-packaged coarse-ground beef. Previous studies also used ground beef chubs as raw material to understand the role of ingredients in raw and cooked ground beef quality (Mancini et al., 2011; Mancini et al., 2022). From each chub, normal-pH control samples were separated without the addition of any ingredients. High-pH was created by slow addition of 30% sodium phosphate solution to increase the pH to 6.4. Following the addition of sodium phosphate, ground beef samples were hand-mixed for 3 min to uniformly distribute phosphate. From the high-pH ground beef, a control high-pH sample was separated. The remaining high-pH meat samples were separated into 3 sections and separately added with 1% glucose, 1% lysine, and 0.5% lysine + 0.5% glucose. The percentages were based on preliminary studies conducted on various levels of glucose and lysine to observe maximum browning. Various treatments included normal-pH control, high-pH control, 1% glucose weight/weight (w/w) added to high-pH ground beef, 1% lysine (w/w) added to high-pH ground beef, and 0.5% glucose + 0.5% lysine (w/w) added to high-pH ground beef. Following the addition of all ingredients, each treatment was reground with a 3 mm stainless steel grinder plate (Biro Model meat grinder, Biro Manufacturing Co., Marblehead, OH, USA). Patties were formed by a half-pound patty maker (100 g patties; KitchenArt Adjust-A-Burger Press, 10.80-cm diameter × 2.75-cm thickness, Delhi, India). Each replication utilized 10 patties (2 patties per treatment × 4 treatments; 50 patties for 5 replications). The first patty was used for pH measurements and raw myoglobin assay, and the second patty was used for cooking. The experiment was replicated 5 times (n = 5).
Cooking
Patties were cooked to an internal temperature of 71°C on a George Foreman Grilling Machine (Salton Inc., Columbia, MO, USA) heated at 170–180°C. Patties were placed on the grill with a closed lid, turned at 2 min (open lid), and then every min thereafter until the desired endpoint temperature was reached (approximately 6 min). Internal temperature was monitored using an Atkins probe thermometer (AcuTuff 340, Gainesville, FL, USA) inserted horizontally into the geometric center of the patty. Cooked patty color measurements are described in section instrumental color analysis section below.
pH
The pH of raw and cooked ground beef was measured using a Hanna pH meter (model HI 99163, Hanna Instruments Inc., Smithfield, RI, USA). Before use, the pH meter was calibrated using buffers at 4 and 7. The pH meter has an inbuilt temperature compensation system. To determine the pH, 5 g of raw or cooked samples that contained surface and interior were combined with 50 mL of deionized water and homogenized for 30 s. The pH values of ground beef homogenates were measured by using a Hanna pH meter. Three measurements were taken each time and averaged for statistical analysis.
Instrumental color analysis (external and internal color of cooked patties)
Instrumental color was measured using a handheld spectrophotometer. L* (indicate lightness), a* (redness indicator), and b* (yellowness indicator) values (CIE, 1976) were recorded from 2 different surface locations on raw patties using a HunterLab MiniScan® EZ 4500L (2.5-cm aperture, illuminant A, and 10°standard observer angle, HunterLab Associates, Reston, VA, USA). a* and b* values were also used to measure chroma value, which indicates the intensity of color (King et al., 2023). The HunterLab MiniScan spectrophotometer was standardized before each use with white and black tiles.
Patties allocated for bloom measurement were kept at 4°C for 30 min in a walk-in cooler. The patties were placed on a white Styrofoam® tray overlapped with polyvinyl chloride (15,500–16,275 cm3 O2/m2/24 h at 23°C, E-Z Wrap Crystal Clear Polyvinyl Chloride Wrapping Film, Koch Supplies, Kansas City, MO, USA). After 30 min of incubation, surface color was measured using a HunterLab MiniScan. Three scans were taken on each patty for statistical analysis.
Cooked patties were removed from the grill and cooled at room temperature for 3 min. After cooling, each patty was bisected parallel to the grilled surface to expose the interior surface, allowing for measurement of the interior color using a HunterLab MiniScan, as described earlier. In addition, spectral readings from 400 to 700 nm were also recorded to measure the browning index (630 nm ÷ 580 nm). A greater ratio indicates red color, and a lower value indicates brown color.
Myoglobin denaturation
For myoglobin denaturation, 10 g of cooked interior sample was blended with 40 mL of 50 mM phosphate buffer (pH 6.8) using an Omni-mixer (Ivan Sorvall Inc., Newtown, CT, USA) for 30 s at 4°C (Hunt et al., 1999). Homogenized samples were centrifuged at 14000 × g for 5 min using an Eppendorf 5418 centrifuge (Eppendorf North America,175 Freshwater Blvd, Enfield, CT, USA). The supernatant was passed through double-layered cheesecloth to remove suspended particles, and for further purification, the supernatant was filtered using 0.22 μm syringe filters (Merck Millipore Ltd., County Cork, Ireland). Extracted filtrate was measured spectrophotometrically by scanning absorption from 400 to 700 nm using a Shimadzu UV-2600 UV-VIS spectrophotometer (Shimadzu Co, Kyoto, Japan). The blank contained only phosphate buffer at pH 6.8. Myoglobin denaturation was determined by the ability of undenatured myoglobin to reduce sodium hydrosulfite. Fifty microliters of freshly prepared 10% sodium dithionite (Sigma, St. Louis, MO, USA) was added to 1 mL of filtrate to estimate undenatured myoglobin. Following the addition of dithionate, the filtrate was again measured spectrophotometrically by scanning absorption from 400 to 700 nm using a Shimadzu UV-2600 UV-VIS spectrophotometer (Shimadzu Co, Kyoto, Japan). The absorbance of the filtrate was measured at 433 nm for both raw and cooked samples. The percentage of myoglobin denaturation was calculated as [(absorbance of raw sample at 433 nm − absorbance 433 of cooked sample)/absorbance of raw sample at 433 nm] × 100. A greater value indicates more denaturation of myoglobin or a cooked brown color.
Absorbance at 420 nm
The brown color formed through Maillard and caramelization reactions was quantified in cooked meat according to the method described by Morales and Van Boekel (1998). A 2 g cooked sample was deproteinized with 2 mL of trichloroacetic acid (TCA, 24% w/v; Sigma Aldrich, St. Louis, MO). The solution was centrifuged at 13,201 g for 10 min using an Eppendorf 5418 centrifuge (Eppendorf North America, Enfield, CT, USA) at room temperature, and the supernatant was analyzed. One milliliter of the supernatant from the sample deproteinized with TCA was taken, and the absorbances at 420 and 550 nm were recorded after a 1:2 dilution. Absorbance at 420 nm was corrected for any turbidity by subtracting the absorbance at 550 nm. A variable wavelength scanning spectrophotometer (Shimadzu UV-2600 UV-VIS spectrophotometer; Shimadzu Co., Kyoto, Japan) was used to measure absorbances. The blank value obtained from an unheated sample was subtracted from the sample reading.
Statistical analysis
The experimental design was a randomized complete block. Each chub served as a block and a random effect. Data were analyzed using the Mixed Procedure of SAS (SAS, 2024). For all parameters except pH, different treatments (normal-pH control, high-pH control, 1% glucose, 1% lysine, and 0.5% glucose–0.5%lysine) were considered as a fixed effect. The random effect was chub and unspecified residual error. For pH analysis, factors included state (raw vs. cooked), treatments (normal-pH control, high-pH control, 1% glucose, 1% lysine, and 0.5% glucose-0.5% lysine), and their interactions. For all parameters, the random effect was chub and unspecified residual error. Least-square means for protected F-tests (P < 0.05) were separated using the pdiff option and were considered significant at P < 0.05.
Results and Discussion
Raw and cooked pH
There was a significant state (raw vs. cooked) × ingredients interaction that resulted in pH (Figure 1). As expected, raw high-pH patties had greater pH (P < 0.05) than raw normal-pH patties. Cooked normal-pH patties had greater (P < 0.05) pH than raw normal-pH patties. Similarly, cooked high-pH control patties had greater (P < 0.05) pH than raw control high-pH patties. There were no differences (P > 0.05) in pHs between raw high-pH control and raw patties containing glucose, lysine, and glucose-lysine combination treatments. However, 1% glucose added in cooked patties had a greater pH (P < 0.05) than cooked lysine or glucose-lysine combination patties.
Effects of ingredient addition and raw/cooked state on pH. P value for ingredient × state (raw/cooked) interaction = < 0.001. Standard error for ingredient × state = 0.005. Least-square means with different letters are different (P < 0.05). High-pH patties were prepared by adding sodium phosphate solution. Lysine and glucose treatments were added to high-pH patties. Normal-pH control represents without any addition of ingredients.
In the current research, a greater-than-normal meat pH (6.4) was created by the addition of sodium phosphate to normal-pH ground beef. Both lysine, glucose, or a lysine-glucose combination, were added to high-pH ground beef. Previous studies also noted greater pH in cooked meat than in raw meat (Fletcher et al., 2000). Cooking can denature protein, exposing the amine group of amino acids (Li et al., 2023). In addition, amine groups can neutralize some of the hydrogen ions present in meat (Li et al., 2023). Further, cooking also leads to the Maillard reaction and caramelization, and some of the non-enzymatic browning reaction products are slightly alkaline in nature. Patties containing 1% glucose had the greatest cooked meat pH than cooked patties containing lysine or lysine-glucose combinations.
Raw patty color
There was a significant treatment effect (P < 0.05) for raw patties a*, chroma, and L* values (Table 1 and Figure 2). In general, greater pH meat leads to lower redness or red intensity (dark red characterized by lower L* and a* values) than normal-pH-bloomed beef products. The addition of glucose or lysine resulted in lower a* (redness) and L* values (P < 0.05) than the high-pH control.
Effects of adding glucose and lysine to high-pH beef patties and their impact on pH and color.
| Treatments | Raw Patties a* Values | Raw Patties Chroma Values | Raw Patties L* Values | Cooked Patties Interior L* Values |
|---|---|---|---|---|
| Normal-pH control | 36.4c | 48.2c | 48.8b | 54.0c |
| High-pH control | 29.4b | 42.6b | 40.2a | 51.1b |
| 1% lysine | 26.4a | 38.2a | 38.9a | 49.4a |
| 1% glucose | 26.5a | 37.1a | 39.4a | 49.2a |
| 0.5% lysine + 0.5% glucose | 25.9a | 38.6a | 40.2a | 48.9a |
| Standard error | 0.2 | 0.3 | 0.2 | 0.4 |
| P value | <0.001 | <0.001 | <0.001 | <0.001 |
High-pH patties were prepared by adding sodium phosphate solution.
Lysine and glucose treatments were added to high-pH patties.
Normal-pH control represents without any addition of ingredients.
Least-square means with different letters are different (P < 0.05).
Pictorial representation of the effects of adding glucose and lysine to high-pH ground beef patties. The top row represents raw patties, and the bottom row represents cooked patties. Treatment names are indicated in the middle. High-pH patties were prepared by adding sodium phosphate solution. Lysine and glucose treatments were added to high-pH patties. Normal-pH control represents without any addition of ingredients.
Several studies noted dark-cutting steaks have lower redness than normal-pH bloomed beef products (English et al., 2016; Hughes et al., 2017). Greater-than-normal pH leads to increased mitochondrial activity and water-holding capacity. Greater oxygen consumption will lead to lower oxymyoglobin levels (Tang et al., 2005). Similarly, greater water holding capacity will lead to greater bound water; hence, less water is available on the surface, increasing absorbance and less reflectance, leading to darker meat color (Ramanathan et al., 2022). Previous studies noted that added ingredients such as lactate or salt can influence the refractive index of sarcoplasm and influence light reflectance properties (Hamm, 1986; Ramanathan et al., 2022). In the current study, high-pH control had lower L* values (darker color) than normal-pH control. The role of glucose and lysine in dark red color via biochemical pathways in this research is not clear.
Effects of glucose or lysine addition on cooked patties’ redness and lightness
The cooked internal color of normal-pH patties was lighter (greater L* values) than that of high-pH controls. Cooked internal color of glucose or lysine-added treatments was darker than that of the high-pH control. The addition of ingredients can change the light reflectance properties of meat. Previous studies also noted that lactate or phosphate added was darker in color (Sawyer et al., 2009).
As expected, cooked high-pH control patties had greater internal redness (a* values), red intensity (chroma), and browning index values (P < 0.05) than cooked normal-pH control samples (Figures 3–6). Increased redness was primarily due to greater levels of ferrohemochrome present in high-pH samples than in normal-pH control samples. Greater muscle pH protects myoglobin against heat-induced denaturation and forms ferrohemochrome (Hunt et al., 1999). In support, high-pH control patties had only 48.3% myoglobin denaturation, while normal-pH patties had 90% denaturation. Most interestingly, the addition of glucose, lysine, or a combination of glucose-lysine reversed the cooked red color, and these treatments had a similar cooked brown color as normal-pH patties. There were no differences (P > 0.05) in cooked redness (a* values) between normal-pH, glucose, lysine, or a combination of lysine and glucose. Cooked normal-pH patties had greater chroma values (P < 0.05) than ingredients added treatments. Chroma values are calculated based on a* and b* values, and Maillard reaction and caramelization products are more brown in color, leading to greater b* values. Myoglobin denaturation of glucose, lysine, or glucose-lysine combination patties was greater (P < 0.05) than high-pH control. The role of the Maillard reaction and caramelization products on myoglobin denaturation is unknown.
Effects of adding glucose, lysine, and glucose-lysine to high-pH patties on cooked patties’ internal redness. A greater a* value indicates red color, and a lower a* value indicates brown color. Standard error bars are indicated; least-square means within different letters (a–b) are different (P < 0.05). Sodium phosphate solution was added to prepare high-pH patties. Lysine, glucose, and lysine-glucose combination treatments were also added to these patties. Normal-pH control represents without any addition of ingredients.
Effects of adding glucose, lysine, and glucose-lysine to high-pH patties on chroma (internal red intensity) of cooked patties. A greater chroma value indicates a more intense color, and a lower chroma value indicates a less intense color. Standard error bars are indicated; least-square means within different letters (a-c) are different (P < 0.05). Sodium phosphate solution was added to prepare high-pH patties. Lysine, glucose, and lysine-glucose combination treatments were also added to these patties. Normal-pH control represents without any addition of ingredients.
Effects of adding glucose, lysine, and glucose-lysine to high-pH patties on myoglobin denaturation. A greater myoglobin denaturation indicates more denatured myoglobin and brown color, while a lower denaturation indicates less denatured myoglobin and red color. Standard error bars are indicated; least-square means within different letters (a–c) are different (P < 0.05). Sodium phosphate solution was added to prepare high-pH patties. Lysine, glucose, and lysine-glucose combination treatments were also added to these patties. Normal-pH control represents without any addition of ingredients.
Effects of adding glucose, lysine, and glucose-lysine to high-pH patties on the cooked patties’ browning index. A greater ratio of reflectance at 630 nm over 580 nm indicates red color, and a lower ratio indicates brown color. Standard error bars are indicated; least-square means within different letters (a–c) are different (P < 0.05). Sodium phosphate solution was added to prepare high-pH patties. Lysine, glucose, and lysine-glucose combination treatments were also added to these patties. Normal-pH control represents without any addition of ingredients.
The Maillard reaction involves an amino acid with an amine side chain and a reducing sugar. This is a complex non-enzymatic glycosidation reaction involving several steps (Rufián-Henares and Pastoriza, 2016). In the current research, treatments containing lysine resulted in greater (P < 0.05) Maillard reaction and caramelization products than the high-pH control treatment (Figure 7). Glucose + lysine had greater (P < 0.05) Maillard and caramelization reaction products than 1% glucose. Meat has inherent lysine as a part of several proteins. Thus, increased browning with glucose addition can be attributed to both the Maillard reaction and other browning reactions such as caramelization. Both normal-pH and high-pH controls had numerically lower Maillard and caramelization reaction products formed compared with other treatments. High-pH beef, such as dark-cutters, has lower glucose and fructose levels due to lower glycogen content (Consolo et al., 2021; Kiyimba et al., 2024; Ramanathan et al., 2020). In the current study, a greater pH was created by adding sodium phosphate to normal-pH meat. Hence, conducting a study using dark-cutters will provide better insight into the role of glucose and lysine in reversing cooked pink color.
Effects of adding glucose, lysine, and glucose-lysine to high-pH patties on the cooked patties’ absorbance at 420 nm. A greater value indicates more brown colored products due to Maillard reaction and caramelization, while a lower value indicates fewer brown colored products are formed. Standard error bars are indicated; least-square means within different letters (a–d) are different (P < 0.05). Sodium phosphate solution was added to prepare high-pH patties. Lysine, glucose, and lysine-glucose combination treatments were also added to these patties. Normal-pH control represents without any addition of ingredients.
Practical significance of this study and limitations
Cooked meat color defects, such as persistent pinking, occur sporadically. Persistent pinking can occur in ground beef, sausages, steaks, and other meat products. Consumers expect cooked meat to be brown, and any deviation from this expectation reduces acceptance and raises food safety concerns related to undercooking. There are several reasons, but pH is the main reason for undercooked or persistent pink color. Avoiding contact with cooking gas (to minimize carbon monoxide contact), testing for nitrite, and acidification of meat products are the most practical approaches to mitigate persistent pinking. However, the addition of acid can affect sensory properties and other functionality of meat. This research explores the enhancement of a naturally occurring chemical reaction to minimize the red color in high-pH ground beef. The Maillard reaction has been positively used to enhance flavor profiles. Recently, this approach has been used to create meat-like flavor profiles in plant-based meat analogs (Sun et al., 2022). Although the flavor profile was not determined, future studies determining the impact of adding lysine or glucose on eating attributes will help understand the benefits of preserving pinkness in cooked meat. Processors often take precautions to avoid a pink or reddish-pink undercooked appearance in cooked meat products that have no added nitrite. In the current study, high-pH conditions were created by adding phosphate. Research using inherently high-pH beef, such as dark-cutting conditions, will help to understand the role of glucose or lysine in minimizing cooked pink color. More specifically, dark-cutting beef has lower glucose levels (Kiyimba et al., 2025); therefore, glucose concentration needs to be optimized to reverse the undercooked pink appearance. This concept can be applied to various meat products, including ground beef, taco meat, pet food, and sausage. Injection enhancement allows the addition of ingredients into intact steaks or loins. Additionally, considerations for labeling and regulations regarding the addition of glucose or lysine must be addressed when using this method to minimize pink color in cooked products.
Conclusion
Myoglobin denaturation, caramelization, and the Maillard reaction are important reactions that contribute to the attributes of cooked meat, including flavor and cooked color. However, greater-than-normal muscle pH protects against heat-induced myoglobin denaturation, leading to the undercooked appearance of fully cooked meat. As expected in this study, high-pH control patties had a pink color after cooking to 71°C. Myoglobin denaturation was less in high-pH control samples than in normal-pH cooked patties. In this research, the Maillard reaction and caramelization are enhanced by the addition of naturally occurring substrates such as glucose and lysine. Addition of glucose or lysine reversed the redness of high-pH ground beef patties, and patties were very similar to cooked normal-pH patties. Therefore, enhancing non-enzymatic browning reactions such as the Maillard reaction and caramelization can address cooked color defects (persistent pinking) in high-pH cooked ground beef or nitrite-containing products.
Author Statement
Meena Goswami conducted the study and wrote the draft manuscript, Rishav Kumar conducted the study and wrote the draft manuscript, Morgan Pfeiffer edited the manuscript, Gretchen Mafi edited the manuscript, Vikas Pathak edited the manuscript, Ranjith Ramanathan supervised, edited the manuscript, and analyzed data.
Declaration of Competing Interest
None.
Data Availability
Data will be made available on request.
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