Effects of radish powder concentration and incubation time on the physicochemical characteristics of alternatively cured pork products

To manufacture radish powder with high concentrations of nitrate, radishes were purchased from five provinces of Korea (Chungcheong-do, Gangwon-do, Gyeonggi-do, Gyeongsang-do, and Jeolla-do) and randomly selected (approximately 10 kg/region) for the experiment. The selected radish roots were cut into cubes of approximately 2 × 3 × 3 cm³ after washing, peeling, and removing the inedible portions. These radish cubes were blended using a food cutter (C6 VV, Sirman, Marsango, Italy) for 6 min, vacuum-packed in nylon/polyethylene bags (~500 g each), and stored in a freezer (C110AHB, LG Electronics, Changwon, Korea) at −24°C prior to hot air drying. Before drying, the nitrate and nitrite ion contents in the homogenized radish were measured. The frozen radishes were placed on a Teflon sheet (TBS-345, Alphaflon, Seoul, Korea) and dried at 60°C for 12 hours in a food dryer (EN-FO-392S, Enex science, Goyang, Korea). The dried radishes were then pulverized with a blender (51BL30, Waring Commercial, Torrington, CT, USA) for 3 min and filtered through a 30-mesh sieve (Test sieve BS0600, Chunggye Sieve, Gunpo, Korea). The resulting radish powder was vacuum-packed and stored at −24°C until use. The nitrate and nitrite ion content and pH of the radish powder were measured prior to the meat processing.

Fresh pork ham (M. biceps femoris, M. semitendinosus, and M. semimembranosus) and backfat were purchased from a local meat processor (Pukyung Pig Farmers Livestock, Kimhae, Korea) at 24–48 hours postmortem. After removing the intermuscular fat and visible connective tissues from the pork ham, the lean pork meat and backfat were sequentially ground using a chopper (TC-22 Elegant plus, Tre Spade, Torino, Italy) with an 8-mm and a 3-mm plate, respectively. Celery powder (VegStable 502, Florida Food Products, Eustis, FL, USA) with a nitrate ion content of 23,375 ppm (equivalent to 32,020 ppm sodium nitrate), a starter culture (CS 299, CHR Hansen, Milwaukee, WI, USA) of Staphylococcus carnosus, sodium nitrite (S2252, Sigma-Aldrich, St. Louis, MO, USA), and sodium ascorbate (#35268, Acros Organics, Geel, Belgium) were obtained from commercial suppliers.

Ground pork meat and fat (total batches of 30 kg per trial) were randomly assigned to the control group and six treatment groups (Table 1): control group, 0.01% sodium nitrite; treatment 1, 0.15% radish powder and 2 hours incubation; treatment 2, 0.15% radish powder and 4 hours incubation; treatment 3, 0.30% radish powder and 2 hours incubation; treatment 4, 0.30% radish powder and 4 hours incubation; treatment 5, 0.30% celery powder and 2 hours incubation; and treatment 6, 0.30% celery powder and 4 hours incubation. In the alternatively cured treatments, the starter culture was added at a concentration of 10% of the amount of radish powder or celery powder added. The ground pork meat and backfat were mixed with NaCl, sodium tripolyphosphate, vegetable powders or sodium nitrite (depending on treatments), other ingredients, and ice/water using a mixer (5K5SS, Whirlpool, St. Joseph, MI, USA) for 10 min. The mixtures from each batch were stuffed into conical tubes (approximately 50 g each), which were then centrifuged at 2,000 ×g for 10 min (Combi R515, Hanil Science Industrial, Incheon, Korea) to remove air pockets. The controls were placed in a refrigerator at 2°C–3°C for 1 hour and the treatment groups were kept in an incubator (C-IB4, Changshin Science, Pocheon, Korea) at 38°C for 2 hours or 4 hours, depending on the treatment group. The tubes were then cooked to reach an internal temperature of 75°C in a water bath (MaXturdy 45, Daihan Scientific, Wonju, Korea) at 90°C set. The cooked samples were immediately cooled for 20 min on slurry ice and stored at 2°C−3°C in the dark until analysis. All experiments were performed in triplicate.

A pH meter (Accumet AB150, Thermo Fisher Scientific, Waltham, MA, USA) was used to measure the pH on five grams of the fresh radish, the radish powder, and the cooked meat samples homogenized in 45 mL of distilled water.

Each meat sample in the conical tube was weighed before cooking, and then weighed again after cooking and cooling. The cooking yield was determined using the equation:

The Commission Internationale de l’Eclairage (CIE) L* (lightness), a* (redness), and b* (yellowness) values were measured on the freshly cut surfaces of each cooked sample using a colorimeter (Chroma Meter CR-400, illuminant C, 2° standard observer; Konica Minolta Sensing, Osaka, Japan) calibrated with a white plate (L* 94.90, a* –0.39, b* 3.88).

The nitrate (NO₃⁻) and nitrite (NO₂⁻) ion contents in the homogenized radish and prepared radish powder were determined using the zinc reduction method described by Merino [23]. The observed results were converted in terms of the concentration of sodium nitrite and expressed as ppm.

The residual nitrite content in the cooked meat samples was determined by the AOAC method [24]. The results were reported as ppm by a standard curve using sodium nitrite (S2252, Sigma-Aldrich).

Nitrosyl hemochrome and total pigment content in the cooked cured products were determined after extraction in 80% acetone and acidified acetone using a method established by Hornsey [25]. The nitrosyl hemochrome concentration (ppm) was calculated as A₅₄₀ × 290. The total pigment concentration (ppm) was calculated as A₆₄₀ × 680. Finally, curing efficiency (%) was calculated using the equation:

Malondialdehyde (MDA) content was measured based on 2-thiobarbituric acid reactive substances (TBARS) values using a distillation method reported by Tarladgis et al. [26]. The TBARS values were reported as milligrams of MDA per kilogram of the cooked samples (mg MDA/kg).

All experiments were performed in triplicate. The experimental design was a randomized block design with seven groups (one control group and six treatment groups). Data were statistically analyzed using the Proc GLM (general linear model) in the SAS program (Statistical Analysis System 2012, version 9.4, SAS Institute, Cary, NC, USA). If the model revealed a significant difference (p < 0.05), the significance of the means was further separated using Tukey’s multiple comparison test.

The ground raw radish had 2,619 ppm nitrate ion and 0.06 ppm nitrite ion (Table 2). The nitrate and nitrite ion content in radishes varied between regions, and this may be attributed to variations in cultivar type, environmental factors, and fertilizers [12,27]. Chung et al. [17] reported that radish from Korea contained 1,878 ppm nitrate ion and 0.80 ppm nitrite ion, which were lower than the values we obtained. Raczuk et al. [18] found that radishes from Poland had, on average, 2,132 ppm nitrate ion and 1.90 ppm nitrite ion. After hot air drying, the radish powder had 55,935 ppm nitrate ion (equivalent to 76,623 ppm sodium nitrate) and 0.09 ppm nitrite ion (equivalent to 0.14 ppm sodium nitrite) (Table 2). This increase in nitrate and nitrite ion contents could be attributed to the evaporation of moisture due to the drying process [14,22,28].

Commercially available celery powder or juice powder typically contains about 30,000 ppm nitrate [2]. Our results indicated that the radish powder we prepared could have sufficient nitrate content for use in the production of alternatively cured meat products. The radish and radish powder had a pH of 5.92 and 5.09, respectively (Table 2). Similarly, Jeong et al. [14] found that dried powder prepared from radish grown in the Jeolla-do region of Korea had a pH of 5.14.

The cooking yield ranged from 99.43% to 99.53% and was not affected (p > 0.05) by the incubation time or the concentration of radish powder or celery powder added (Table 3). These results were consistent with those of Banerjee et al. [29], who found that nuggets treated with different concentrations of broccoli powder showed no difference in cooking yield. However, our results disagree with those of Choi et al. [6], who reported that naturally cured products treated with white kimchi powder and celery powder had lower cooking yields than sodium nitrite-treated controls. This difference in results could be attributed to the difference in phosphate levels in the products. Unlike in our study, Choi et al. [6] did not include phosphate in their product formulation. Regarding the incubation time, our results are consistent with those of Sindelar et al. [30], who reported that incubation time did not affect the cooking yield in indirectly cured jerkies.

All alternatively cured pork products had significantly higher (p < 0.05) pH than the controls, except for treatment 6 (0.30% celery powder and 4 hours incubation time) (Table 3). Jeong et al. [14] reported similar results; they found that cured pork products treated with Chinese cabbage, radish, or spinach powder showed higher pH than nitrite-added controls. In our study, products treated with 0.15% radish powder (treatments 1 and 2) had higher pH than those treated with 0.30% radish powder (treatments 3 and 4) or 0.30% celery powder (treatments 5 and 6), suggesting that the pH in alternatively cured products decreased as the concentration of vegetable powders increased. However, the incubation time did not affect the pH of the alternatively cured products, irrespective of the concentration of the vegetable powders.

The CIE L* (lightness), a* (redness), and b* (yellowness) values of alternatively cured meat products in this study are shown in Table 3. There were no differences (p > 0.05) in CIE L* values between the naturally cured pork products, suggesting that the types and levels of vegetable powder added and the incubation time did not affect (p > 0.05) CIE L* values. However, cured products incubated for 2 hours (treatments 1, 3, and 5) had CIE L* values higher (p < 0.05) than those of the control, while treatments 2, 4, and 6, which were incubated for 4 hours, had similar CIE L* values as the control. These results are consistent with those of Sindelar et al. [11], who found that the CIE L* values of emulsified cooked sausages were not affected by the concentration of the vegetable juice powder (0.2% or 0.4%) or the incubation time (30 min or 120 min). In this study, CIE a* values ranged from 8.30 to 8.55, with no significant differences (p > 0.05) between the control and the treatments. Interestingly, despite the difference in concentration of added vegetable powder, cured pork products with 0.15% radish powder (treatments 1 and 2) and those with 0.30% radish or celery powder (treatments 3–6) had similar redness. Similar results were reported by Choi et al. [6], who found that using different concentrations (0.2% and 0.4%) of white kimchi powder in combination with a nitrate-reducing starter culture and incubating for 2 hours resulted in similar CIE a* values in alternatively cured meat products. Furthermore, the incubation time (2 hours and 4 hours) did not affect the CIE a* values of the alternatively cured pork products in our study, regardless of the concentration of the vegetable powders. These results indicate that the use of radish powder, along with a short incubation time of 2 hours could produce a redness in alternatively cured meat products comparable to that in traditionally cured products and those treated with commercially available celery powders. CIE b* values of cured products treated with celery powder (treatments 5 and 6) were higher (p < 0.05) than the control, but no differences (p > 0.05) in CIE b* values were observed between the control and other treatments, except treatment 4. These results were consistent with those of Jeong et al. [14], who reported that when cured pork products were treated with different vegetable powders and a starter culture, followed by incubating for 2 hours, the yellowness of the final products were increased after treatment with Chinese cabbage or spinach powder, but those treated with radish powder had similar CIE b* values to the nitrite-added control. Furthermore, Krause et al. [31] reported that uncured sliced ham treated with vegetable juice powder and a starter culture showed higher b* values than the sodium nitrite-treated control, which is consistent with our results for the celery powder treatments. The increased yellowness in the products treated with celery powder may be attributed to the presence of yellow pigments like carotenoids in celery [32]. We also observed that the incubation time did not affect CIE b* values in treatments 1 and 2 (0.15% radish powder). However, in products treated with 0.3% radish powder and 0.3% celery powder, the CIE b* values increased (p < 0.05) with incubation time, which was consistent with the observations of Kim et al. [13] in cured pork loins incubated with spinach and a starter culture.

The products treated with 0.30% radish powder (treatment 4) were found to have higher (p < 0.05) residual nitrite contents than those treated with 0.15% radish powder (treatment 2) (Table 4). Interestingly, treatment 4 (0.30% radish powder) resulted in a higher residual nitrite content than treatment 6 (0.30% celery powder), which may be attributed to the relatively high nitrate content of radish powder prepared for our study. Thus, among the vegetable powder treatments, treatment 4 had the highest (p < 0.05) residual nitrite content, which was not different (p > 0.05) from that of the sodium nitrite-treated control. When alternatively cured products were incubated for 2 hours, the residual nitrite contents in treatments 1, 3, and 5 were lower (p < 0.05) than that in the control. However, there was no difference (p > 0.05) between the residual nitrite contents in treatments 3 and 5, while treatment 1 had a lower (p < 0.05) residual nitrite content than treatment 5. However, when the incubation period was increased (4 hours), residual nitrite content also increased (p < 0.05) in all cases (treatments 2, 4, and 6). These results were consistent with previous studies on cooked sausages and hams, which showed that residual nitrite content increased with incubation time [11,31]. Thus, our results indicate that sufficient nitrate concentrations in the vegetable powder and ample incubation time for converting nitrate to nitrite are critical for achieving high-quality alternatively cured meat products. Radish powder may be a promising natural source of nitrite/nitrate, and its addition to alternatively cured products may result in meat products comparable to conventionally cured products and products treated with commercially available celery powder.

The nitrosyl hemochrome content in the products were significantly changed (p < 0.05) by the concentration of vegetable powders added and the incubation time (Table 4). When incubated for 2 hours, products treated with 0.15% radish powder (treatment 1) had a lower (p < 0.05) nitrosyl hemochrome content than those treated with 0.30% radish powder (treatment 3) or 0.30% celery powder (treatment 6). Jeong et al. [22] reported a study on ground pork products treated with 0.15%, 0.25%, and 0.35% Chinese cabbage powder or 0.4% celery juice powder and incubated for 2 hours prior to cooking. They found that the cooked products treated with 0.15% Chinese cabbage powder had lower nitrosyl hemochrome than the others, which is consistent with our results. Regardless of the type and concentration of vegetable powders used, the nitrosyl hemochrome content of the alternatively cured products (treatments 2, 4, and 6) increased (p < 0.05) with incubation time, and reached a level similar (p > 0.05) to that in the controls. It has been recognized that incubation time is a key parameter in the production of cured meat treated with vegetable-based ingredients [33]. Therefore, it is possible that the increased amounts of nitrite converted from nitrate by the bacteria during the long incubation periods affect the nitrosyl hemochrome content.

In this study, the total pigment content of the products ranged from 45.73 to 46.58 ppm (Table 4). In a previous study [22], different amounts (0.15%–0.35%) of Chinese cabbage powder were added to pork products, which resulted in total pigment content ranging from 47.18 to 49.13 ppm, similar to our results. However, the total pigment content was not different (p > 0.05) between the different vegetable powder treatments and the control, indicating that it was not affected by the types and concentrations of vegetable powders added or the incubation time. These results are also consistent with those of Posthuma et al. [34], who reported that total pigment content did not change in emulsified beef sausages treated with celery juice powder when the incubation time was increased from 5 min to 120 min.

In the cured meat products treated with radish powder, the curing efficiency was significantly increased (p < 0.05) when the concentration of the radish powder added was increased from 0.15% to 0.30% for both incubation times (2 hours, treatments 1 and 3; 4 hours, treatments 2 and 4) (Table 4). These results were consistent with those of Choi et al. [6], who found that pork products treated with 0.4% white kimchi powder showed a higher curing efficiency than those treated with 0.2% white kimchi powder. Regardless of the types and concentration of vegetable powder added, alternatively cured products incubated for 2 hours had a lower (p < 0.05) curing efficiency than the nitrite-treated control. However, the addition of radish powder or celery powder (regardless of the concentration) to the products followed by incubation for 4 hours resulted in an increased curing efficiency (p < 0.05). While treatment 2 had a similar curing efficiency to the control, the highest curing efficiency was observed in treatments 4 and 6. These results suggest that extended incubation times could increase the curing efficiency in alternatively cured meat products. In addition, when radish powder was added at low concentrations (0.15%) to products, sufficient incubation time would be needed to achieve cured meat characteristics as observed for traditionally cured products.

Alternatively cured products treated with 0.15% radish powder (treatments 1 and 2) were found to have higher (p < 0.05) TBARS values than the control, regardless of the incubation time (Table 4). However, as incubation time increased, the TBARS values in the cured products treated with 0.30% radish powder or celery powder (treatments 4–6) decreased (p < 0.05). There were no significant differences (p > 0.05) in the TBARS values of these treatments and the control. These results could be attributed to the high residual nitrite and nitrosyl hemochrome contents in this study. Lipid oxidation can be inhibited by the antioxidant properties of nitrite [35]. Similarly, Kim et al. [13] found that pork loins with high residual nitrite content showed low TBARS values.