RESEARCH ARTICLE

Effects of partial substitution of nitrites with purple-fleshed sweet potato powder on physicochemical characteristics of sausages

Sang-Keun Jin1,#https://orcid.org/0000-0002-8983-5607, Teak-Soon Shin2,#https://orcid.org/0000-0001-5362-9206, Dong-Gyun Yim3,*https://orcid.org/0000-0003-0368-2847
Author Information & Copyright
1Department of Animal Resources Technology, Gyeongnam National University of Science and Technology, Jinju 52725, Korea
2Department of Animal Science, Pusan National University, Miryang 50463, Korea
3Department of Agricultural Biotechnology and Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea
*Corresponding author: Dong-Gyun Yim, Department of Agricultural Biotechnology and Research Institute of Agriculture and Life Sciences, Seoul National University, Seoul 08826, Korea. Tel: +82-2-880-4820 E-mail: tousa0994@naver.com

#These authors contributed equally to this work.

© Copyright 2020 Korean Society of Animal Science and Technology. This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Received: Jun 04, 2020; Revised: Jul 21, 2020; Accepted: Jul 27, 2020

Published Online: Sep 30, 2020

Abstract

Synthetic nitrite imparts a reddish-pink color to meat and a distinct flavor to meat products, delays lipid oxidation, and inhibits microbial growth and pathogens. However, excessive intake of nitrite might result in the production of carcinogenic nitrosamine, which might increase the risk of cancer in humans. Therefore, we aimed to find an alternative natural colorant for pork sausages. Pork sausages were mixed with 0.014% sodium nitrite (NaNO2) alone (CON), without either NaNO2 or purple-fleshed sweet potato powder (PP; CON1), 0.5% PP alone (PP1), 1% PP (PP2) alone, 0.011% NaNO2 and 0.5% PP (SP1), and 0.011% NaNO2 and 1% PP (SP2). The sausages were then cooked and stored for physicochemical analysis on days 0, 5, 10, 15, and 20. The a* and W* values were the greatest and lowest in the SP2 and CON1 treatments, respectively (p < 0.05). The concentrations of residual nitrite in the sausages at 20 days decreased in the order of CON > SP1, SP2 > PP2 > PP1, CON1. The fatty acid content was higher, and flavorous amino acids were more in PP2 (p < 0.05). The fatty acid composition was comparable between the SP2 and CON groups, but the contents of glutamic acid and alanine were greater in the SP2 group. In conclusion, SP2 (0.011% NaNO2 with 1% PP) could be added as a natural colorant for pork sausage production, and NaNO2 could be substituted with up to 20% PP without detrimental effects on sausage appearance and/or quality.

Keywords: Purple sweet potato; Sausage; Nitrite; Physicochemical characteristic

INTRODUCTION

Synthetic nitrite imparts a reddish-pink color to meat and a distinct flavor to meat products, delays lipid oxidation, and inhibits microbial growth and pathogens [1,2]. However, the possible generation of carcinogenic nitrosamine due to excessive nitrite intake might be associated with an increased risk of cancer [3], which is an important concern in the meat industry. Consumers now prefer safe foods without synthetic ingredients or additives that might endanger health [4]. The development and utilization of natural additives have recently attracted attention. Most natural additives that are used in sausages are sourced from herbs, spices, fruits, and vegetables [5].

Sweet potato (Ipomea batatas L.) is an abundant source of anthocyanins, vitamins, fiber, and minerals that can inhibit colon cancer, diverticular disease and constipation, decrease blood cholesterol, and help digestion [68]. Purple sweet potato is a source of abundant β-carotene, anthocyanins, and starch and has inherent water retention properties [9]. Anthocyanin and other polyphenols in purple sweet potatoes are powerful antioxidants [8], and anthocyanins can serve as a natural colorant, imparting a red-violet color to food [10]. However, only a few studies have addressed the impact of incorporating purple sweet potato powder (PP) on the physicochemical characteristics of sausages. Therefore, the present study aimed to determine whether PP could minimize the amount of sodium nitrite (NaNO2) that is normally added to pork sausages without compromising quality, appearance and nutritional value.

MATERIALS AND METHODS

Sample preparation

Lean pork (~20 kg) and its fat were taken from a butcher, then the meat was coarsely pulverized in a grinder (Model 200, Lasar, Los Angeles, CA, USA), and categorized into six treatments (3 kg/group).

Preparation of cooked pork sausages

Table 1 shows the amounts of additives mixed with the meat in each group as follows: 0.014% NaNO2 (Sewoo, Seoul, Korea), no NaNO2 or PP, 0.5% PP, 1% PP, 0.011% NaNO2 and 0.5% PP (Jisanfood, Jinju, Korea), and, 0.011% NaNO2 and 1% PP. The meat and additives were mixed for 120 sec to obtain uniform distribution of the additives in an A-20 matrix (Ramon, Oiartzun, Spain).

Table 1. Composition of cooked pork sausage blends1)
Ingredients Composition (%)
CON CON1 PP1 PP2 SP1 SP2
Meat ingredients
 Pork loin meat 72.266 72.28 71.78 71.28 71.769 71.269
 Pork fat 11.18 11.18 11.18 11.18 11.18 11.18
Nonmeat ingredients
 Ice 13.78 13.78 13.78 13.78 13.78 13.78
 Salt2) 1.4 1.4 1.4 1.4 1.4 1.4
 Sodium nitrite (NaNO2)3) 0.014 - - - 0.011 0.011
 Phosphate4) 0.2 0.2 0.2 0.2 0.2 0.2
 Sugar5) 0.5 0.5 0.5 0.5 0.5 0.5
 MSG6) 0.06 0.06 0.06 0.06 0.06 0.06
 Spice7) 0.6 0.6 0.6 0.6 0.6 0.6
 Purple-fleshed sweet potato powder - - 0.5 1 0.5 1
Total 100 100 100 100 100 100

1) Treatments: CON (control), 0.014% sodium nitrite; CON1 (control1); no sodium nitrite and purple-fleshed sweet potato powder combination; PP1, no sodium nitrite and 0.5% purple-fleshed sweet potato powder; PP2, no sodium nitrite and 1% purple-fleshed sweet potato powder; SP1, 0.011% sodium nitrite and 0.5% purple-fleshed sweet potato powder combination; SP2, 0.011% sodium nitrite and 1% purple-fleshed sweet potato powder combination.

2) Refined salt (Woo-Il S&F, Ulsan, Korea).

3) Sodium nitrite (Duksan, Gyeonggi, Korea).

4) FOS/ENR (Taewon Food, Gyeonggi, Korea).

5) Beksul white sugar (CJ cheiljedang, Incheon, Korea).

6) Monosodium L-Glutanate (Shinwon Chemical, Seoul, Korea).

7) Bologna (Taewon Food, Gyeonggi, Korea).

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Meat mixtures (~160 g) were stuffed into E-25 fibrous casings (Sewoo, Seoul, Korea) to result in 120 mm sausages. The sausages were cooked in a water bath (100°C) until the core temperature reached 75°C and then cooled at 5°C for 59 min. Each sausage was vacuum packed with polyvinyl chloride film (Krehalon UK, Beverley, UK). All experimental sausages were stored at 4°C for 0, 5, 10, and 20 days and they were then analyzed on different days. The fatty acid and amino acid composition of the sausages was conducted on day 0. Each treatment was replicated four times.

Physicochemical analysis

The color values for L* (lightness), a* (redness), and b* (yellowness) were measured using colorimeter (CR-310, Minolta Inc., Tokyo Japan). It was standardized using a calibration plate (Y = 93.5, X = 0.3132, y = 0.3198). Color parameters are expressed as: Whiteness (W) was calculated according to the following formula: W = L* − 3b*. Cooked sausages (4 g) were disrupted in distilled water (36 mL) at 3,000×g for 30 s with a homogenizer (Ultra-Turrax T-50, IKA, Staufen, Germany). The pH of the sausages was conducted with a pH meter (Model 440, Mettler-Toledo GmbH, Greifensee, Switzerland). The water-holding capacity (WHC) of samples was measured as described by Joo [11], with modifications. Briefly, samples (3.0 g) of intact meat were placed between two thin plastic films on weighed using Whatman No. 1 filter papers (diameter, 11 cm), then compressed for 5 min between Plexiglas plates with a 2.5 kg load. After removing all of the compressed meat samples, the damp filter paper and the plastic films were immediately weighed, and WHC (%) was determined as:

WHC ( % ) = ( Damp filter paper and plastic film weight ) ( Filter paper and plastic film weight ) Meat sample weight  × 100

Six 1.5 × 1.5 × 1.5 cm samples of cooked pork sausage were sheared with an Instron 3343 (US/MX40, A&D, Norwood, MA, USA) to provide a crosshead speed of 100 mm/min. The shear force values from each sample were obtained and expressed as kg/cm2. Volatile basic nitrogen (VBN) was obtained with the method of Conway micropipette diffusion as modified by Pearson [12] is expressed as mg VBN/100 g of sausage. Residual nitrite was calculated using spectrophotometry as described by AOAC [13].

Analysis of fatty acid content

Total fat was extracted as described by Folch et al. [14]. Samples (5 g) were thawed, and then lipids were extracted in chloroform/methanol (2:1) using BHT as an antioxidant [15]. Fatty acid methyl esters were quantified using an Agilent 7890N gas chromatograph (Agilent Technologies GmbH., Waldbronn, Germany) equipped with a flame ionization detector and fused silica capillary column (40 m, 0.28 mm) with 0.3 mm film thickness (Agilent Technologies GmbH). The carrier gas was helium (5 mL/min), and the injection volume was 1 µL. The temperature of the oven was initially kept at 190°C for 60 sec, then maintained at 280°C for 15 min. Linoleic acid (C18:2) was the internal standard (catalog number H3500, Sigma-Aldrich, St. Louis, MO, USA). Contents of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA) are expressed as ratios (%) of total fatty acids, and PUFA/SFA and n-6/n-3 ratios were counted.

Analysis of protein and free amino acid content

Protein content was obtained with a slight modification of AOAC [13]. For analysis of free amino acids, a modification of the HPLC method described by Bidlingmeyer et al. [16] was given. The samples were delipidized by solvent extraction, then hydrolyzed with 6 N HCl in vacuum-packed tubes for 23 h at 108°C. The hydrolyzed amino acids were sedimented by centrifugation at 5,000×g, and dried under vacuum for at least 1.5 h. The pH was adjusted by adding 20 mL of ethanol: water: triethylamine (2:2:1) and the samples were dried as described above. The samples were derivatized in 20 µL of ethanol:water:triethylamine:phenylisothiocyanate (7:1:1:1) at room temperature (26°C) for 10 min, then dried under vacuum for at least 3 h. The samples were resuspended in 200 µL Pico Tag® diluent (Waters GmbH., Haan, Germany), then 8 µL was injected into a Nova-Pak C18 HPLC column (60 Angstroms, 4 µm, 3.9 × 150 mm; Waters GmbH.) attached to a 1525 HPLC (with binary gradient delivery, 717 auto-sampler and injector, 1500 column heater, and a 2487 dual wavelength UV detector). Amino acids were separated using buffers comprising sodium acetate (pH 6.4), EDTA 5,000 ppm, triethylamine (1:2,000), acetonitrile 6% v/v (buffer A) and acetonitrile 60% v/v, and EDTA 5,000 ppm; buffer B). A control sample was analyzed with the samples before hydrolysis to ensure accuracy and reproducibility. Data were analyzed using Breeze software Z (Waters GmbH).

Sensory color score determination

Seven panelists (age range 20–30 years) participated in sensory evaluations of the sausages at 5-day intervals during the 20 days of storage. All panelists were trained based on procedures published by [17,18]. The sausages were cook at 102°C for 4 min. Samples of the sausages were then cut into 1.5 × 1.5 × 1.5 cm cubes, coded with three-digit numbers and served to the panelists. Color was assessed based on a 9-point scale from 0 (extremely dislike) to 9 (extremely like). Two consecutive tasting sessions of three treated samples and a control sample proceeded with a 10-min break in between.

Statistical analysis

Data were collected in duplicate from the sausage samples (3 replicates × 6 samples × 3 treatments). Treatments and storage days were analyzed using one-way analysis of variance (ANOVA) and Duncan’s multiple range test. All data were statistically analyzed using the general linear model procedure of the SAS statistical package [19] (SAS Institute, Cary, NC, USA). Values with p < 0.05 were considered statistically significant.

RESULTS AND DISCUSSION

Table 2 shows the impacts of PP on the color of sausages during storage (Fig. 1). Lightness and yellowness values were significantly higher in sausages treated with CON1 than with PP plus NaNO2 (SP1, SP2) and PP alone (PP1, PP2) (p < 0.05). Kim and RYu [20] also found lower L* and b* values in sausages containing PP and NaNO2 than in controls. The CIE a* values were positively affected by PP with NaNO2. The CIE a* values were the highest and lowest after storage for 20 days, respectively, in sausages with, not without, added 0.011% NaNO2 and 1% PP (p < 0.05). The higher redness values in the sausages of SP1 and SP2 were due to synergistic effects between NaNO2 pigments derived from the PP and formed nitrosohemochrome, which resulted in a pink-red color during cooking [21]. This difference in meat color might be due to the non-enzymatic browning reaction between sugars in the PP and meat protein [22]. These results of including PP as a NaNO2 replacement in sausages are consistent with those of Lee et al. [23]. Adding both sodium nitrite and PP affected the whiteness values of the sausages, because the SP1 and SP2 samples were whiter than all other treated samples (p < 0.05). These results agreed with those of Jin et al. [24], who showed that adding NaNO2 positively affected sausage whiteness. As shown in Fig. 1, color significantly differed between 0 and 10 days of storage, as panelists favored SP1 and SP2 to CON1, PP1, and PP2 (p < 0.05).

Table 2. Color characteristics of cooked pork sausages containing two different percentages of purple sweet potato combined with nitrite during storage
Items Treatments1) Storage (days)
0 5 10 15 20
L*2) CON 79.20 ± 0.15B 79.04 ± 0.13A 78.69 ± 0.34B 79.01 ± 0.27B 78.70 ± 0.73A
CON1 79.85 ± 0.08A 79.54 ± 0.78A 80.14 ± 0.32A 80.11 ± 0.21A 79.13 ± 1.10A
PP1 75.79 ± 0.24Cb 76.66 ± 0.48Ba 76.38 ± 0.21Cab 76.72 ± 0.19Ca 76.47 ± 0.52Ba
PP2 75.40 ± 0.35C 75.72 ± 0.13C 75.44 ± 0.10D 75.32 ± 0.41D 75.60 ± 0.10B
SP1 75.68 ± 0.15C 76.17 ± 0.05BC 75.80 ± 0.12CD 75.67 ± 0.50D 76.03 ± 0.44B
SP2 72.92 ± 0.38Db 73.87 ± 0.33Da 73.48 ± 0.69Eab 73.15 ± 0.42Eab 73.59 ± 0.18Cab
a* CON 7.08 ± 0.04Cc 7.33 ± 0.12Cb 6.89 ± 0.06Cd 7.65 ± 0.09Ca 7.32 ± 0.07Cb
CON1 2.66 ± 0.15Fa 2.26 ± 0.11Fb 2.02 ± 0.27Fbc 1.69 ± 0.07Fd 1.79 ± 0.19Fcd
PP1 5.14 ± 0.06Ea 4.30 ± 0.29Eb 3.99 ± 0.41Eb 4.26 ± 0.66Eb 3.73 ± 0.25Eb
PP2 5.51 ± 0.10D 5.53 ± 0.22D 4.93 ± 0.31D 5.54 ± 0.68D 5.07 ± 0.55D
SP1 8.49 ± 0.04Bbc 8.59 ± 0.11Bb 8.38 ± 0.13Bc 8.93 ± 0.10Ba 8.47 ± 0.10Bbc
SP2 9.53 ± 0.10A 9.85 ± 0.44A 9.43 ± 0.18A 9.71 ± 0.48A 9.54 ± 0.40A
b* CON 8.19 ± 0.02Bb 8.22 ± 0.05Cb 8.65 ± 0.20Ca 8.07 ± 0.09Cb 8.18 ± 0.04Cb
CON1 10.41 ± 0.10Ac 10.56 ± 0.18Abc 10.83 ± 0.30Ab 11.27 ± 0.04Aa 11.13 ± 0.07Aa
PP1 8.19 ± 0.15Bb 8.94 ± 0.28Bab 9.48 ± 0.44Ba 9.23 ± 0.75Ba 9.58 ± 0.15Ba
PP2 7.82 ± 0.26Cab 7.75 ± 0.38Cb 8.53 ± 0.38Ca 8.22 ± 0.45Cab 8.30 ± 0.48Cab
SP1 6.93 ± 0.11Dab 6.54 ± 0.02Dcd 7.02 ± 0.18Da 6.36 ± 0.15Dd 6.76 ± 0.11Dbc
SP2 5.82 ± 0.32E 5.29 ± 0.42E 5.93 ± 0.10E 5.38 ± 0.23E 5.52 ± 0.51E
W* CON 54.62 ± 0.14Aa 54.38 ± 0.24Ba 52.73 ± 0.47Bb 54.80 ± 0.22Ba 54.16 ± 0.75Ba
CON1 48.63 ± 0.21Ca 47.86 ± 1.31Ea 47.66 ± 0.62Dab 46.29 ± 0.21Dbc 45.74 ± 0.94Ec
PP1 51.22 ± 0.65Ba 49.85 ± 0.43Dab 47.95 ± 1.47Db 49.04 ± 2.21Cab 47.73 ± 0.98Db
PP2 51.93 ± 1.11Bab 52.48 ± 1.22Ca 49.84 ± 1.18Cb 50.67 ± 0.97Cab 50.70 ± 1.42Cab
SP1 54.90 ± 0.48Ac 56.54 ± 0.05Aa 54.75 ± 0.45Ac 56.59 ± 0.10ABa 55.75 ± 0.15ABb
SP2 55.46 ± 0.87Ab 58.01 ± 1.15Aa 55.70 ± 0.83Ab 57.00 ± 1.10Aab 57.03 ± 1.37Aab
Sensory color score3) CON 7.17 ± 0.68ABC 7.14 ± 0.80 7.06 ± 0.62AB 6.75 ± 0.89 6.88 ± 0.99
CON1 6.50 ± 0.32C 6.57 ± 0.53 6.13 ± 0.79C 6.06 ± 0.94 6.13 ± 1.06
PP1 6.67 ± 0.61BC 6.50 ± 0.58 6.38 ± 0.64BC 6.13 ± 0.95 6.06 ± 1.12
PP2 6.58 ± 0.66C 6.50 ± 0.82 6.25 ± 0.93C 6.31 ± 0.75 6.00 ± 1.41
SP1 7.33 ± 0.68AB 7.29 ± 0.49 7.31 ± 0.53A 6.94 ± 0.62 7.13 ± 0.64
SP2 7.67 ± 0.41A 7.07 ± 0.45 7.31 ± 0.70A 7.00 ± 0.65 7.13 ± 0.64

Data represent mean ± SD.

1) Treatments: CON (control), 0.014% sodium nitrite; CON1 (control1),no sodium nitrite and purple-fleshed sweet potato powder combination; PP1, no sodium nitrite and 0.5% purple-fleshed sweet potato powder; PP2, no sodium nitrite and 1% purple-fleshed sweet potato powder; SP1, 0.011% sodium nitrite and 0.5% purple-fleshed sweet potato powder combination; SP2, 0.011% sodium nitrite and 1% purple-fleshed sweet potato powder combination.

2) W = L* − 3b*.

3) Sensory color score were evaluated based on a 9-point scale, 0 = extremely dislike and 9 = extremely like.

A–F Means in the same column within the same aging period with different letters are significantly different (p < 0.05).

a–d Means in the same row within the same aging condition with different letters are significantly different (p < 0.05).

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jast-62-5-702-g1
Fig. 1. Section color of cooked pork sausages containing two different percentages of purple sweet potato combined with nitrite. 1)Treatments: CON (control), 0.014% sodium nitrite; CON1 (control1), no sodium nitrite and purple-fleshed sweet potato powder combination; PP1, no sodium nitrite and 0.5% purple-fleshed sweet potato powder; PP2, no sodium nitrite and 1% purple-fleshed sweet potato powder; SP1, 0.011% sodium nitrite and 0.5% purple-fleshed sweet potato powder combination; SP2, 0.011% sodium nitrite and 1% purple-fleshed sweet potato powder combination.
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Table 3 highlights the effects of adding PP on the physicochemical characteristics and NaNO2 retention in sausages during storage. The pH values were lower in sausages with added PP (p < 0.05), which concurred with the findings of [24,25]. The decrease in the pH value of sausages containing PP might be due to the depolymerization of starch granules caused by cooking, resulting in acidic terminal residues in starch molecules [26]. A lower pH causes an acidic environment in sausages, thus suppressing bacterial growth [27]. In this study, the pH of sausages was affected by the addition of PP. Furthermore, the pH of the PP samples slightly increased over time in storage (p < 0.05). The VBN content was comparable between the SP2 and CON groups, and lower in the PP2 than in the CON group on days 10 and 20 (p < 0.05). These findings agree with those of [23,24]. The low VBN values in the sausages were due to antimicrobial properties of PP. However, dramatic results were not observed in VBN measurement of this study. Our study found that the addition of PP might delay or inhibit spoilage (as indicated by VBN) in sausages. However, further study is needed.

Table 3. Physico-chemical traits and residual nitrite of cooked pork sausages containing two different percentages of purple sweet potato combined with nitrite during storage
Items Treatments1) Storage (days)
0 5 10 15 20
pH CON 6.06±0.01Ad 6.07 ± 0.00Dd 6.16 ± 0.02Ab 6.26 ± 0.02Aa 6.10 ± 0.01Cc
CON1 6.03 ± 0.01ABd 6.12 ± 0.01Bb 6.16 ± 0.01Aa 6.12 ± 0.00Db 6.10 ± 0.01Cc
PP1 6.01 ± 0.01Bc 6.05 ± 0.01Eb 6.08 ± 0.01Ca 6.08 ± 0.01Ea 6.08 ± 0.01Da
PP2 6.01 ± 0.01Bc 6.04 ± 0.00Ec 6.09 ± 0.01Ca 6.09 ± 0.01Da 6.09 ± 0.01Da
SP1 6.04 ± 0.02ABc 6.17 ± 0.01Aa 6.11 ± 0.01Bb 6.11 ± 0.01Db 6.18 ± 0.02Aa
SP2 6.05 ± 0.04Ac 6.10 ± 0.01Cb 6.15 ± 0.00Aa 6.14 ± 0.00Ca 6.12 ± 0.02Bab
WHC (%) CON 79.04 ± 2.39Aa 68.84 ± 0.05Db 70.95 ± 2.10Bb 52.87 ± 0.62Cc 71.68 ± 1.52B
CON1 74.99 ± 0.44BCa 70.48 ± 0.96CDb 71.04 ± 2.20Bb 61.74 ± 1.59Ac 72.58 ± 0.54Ab
PP1 68.90 ± 2.34Db 72.17 ± 0.98BCa 74.48 ± 2.36ABa 56.24 ± 1.34Bc 72.52 ± 0.42A
PP2 72.32 ± 0.88Cb 73.67 ± 0.71ABab 75.78 ± 1.79Aa 57.17 ± 2.06Bc 73.66 ± 1.00Ab
SP1 73.80 ± 0.76BCa 70.55 ± 1.40CDb 71.82 ± 1.74Bab 58.77 ± 1.15Bc 71.78 ± 0.61Ab
SP2 75.75 ± 0.70Ba 75.43 ± 1.40Aa 73.95 ± 0.52ABab 58.24 ± 1.60Bc 72.16 ± 0.45B
Shear force (kg/cm2) CON 1.34 ± 0.04bc 1.27 ± 0.14C 1.25 ± 0.02Dc 1.47 ± 0.04Cab 1.50 ± 0.10Ca
CON1 1.36 ± 0.12C 1.36 ± 0.04C 1.64 ± 0.04Ab 1.43 ± 0.05Cc 1.91 ± 0.08Aa
PP1 1.47 ± 0.06bc 1.40 ± 0.05C 1.40 ± 0.06Cc 1.54 ± 0.11Cb 1.85 ± 0.05ABa
PP2 1.38 ± 0.06C 1.39 ± 0.06C 1.55 ± 0.03Bb 1.58 ± 0.02Bab 1.67 ± 0.08BCa
SP1 1.44 ± 0.09B 1.38 ± 0.15B 1.43 ± 0.02Cb 1.53 ± 0.07BCb 1.96 ± 0.05Aa
SP2 1.40 ± 0.06B 1.38 ± 0.07B 1.37 ± 0.03Cb 1.73 ± 0.03Aa 1.84 ± 0.16Aa
VBN (mg%) CON 4.20 ± 0.28Aa 2.99 ± 0.16C 3.17 ± 0.16ABbc 3.45 ± 0.16B 3.36 ± 0.28Abc
CON1 3.73 ± 0.16ABa 2.89 ± 0.16B 3.08 ± 0.00Bb 3.08 ± 0.28B 2.99 ± 0.16ABb
PP1 3.17 ± 0.16Bab 2.71 ± 0.16C 3.45 ± 0.16Aa 3.36 ± 0.28A 2.89 ± 0.32Bbc
PP2 3.36 ± 0.28Ba 2.80 ± 0.28B 2.61 ± 0.16Cb 3.08 ± 0.28Ab 2.89 ± 0.16Bb
SP1 4.11 ± 0.81Aa 2.99 ± 0.32B 2.99 ± 0.16Bb 3.17 ± 0.16B 3.08 ± 0.00ABb
SP2 3.27 ± 0.16Bab 2.89 ± 0.16B 3.08 ± 0.28Bab 3.36 ± 0.28A 3.17 ± 0.16ABab
Residual nitrite (ppm) CON 13.35 ± 0.62A ND ND ND 11.64 ± 1.40A
CON1 4.31 ± 1.16B ND ND ND 3.72 ± 0.63D
PP1 6.77 ± 2.59B ND ND ND 3.54 ± 0.77D
PP2 5.62 ± 1.34B ND ND ND 6.91 ± 0.25C
SP1 12.85 ± 0.88Aa ND ND ND 9.83 ± 0.17Bb
SP2 15.37 ± 0.42Aa ND ND ND 10.23 ± 0.20Bb

Data represent mean ± SD.

1) Treatments: CON (control), 0.014% sodium nitrite; CON1 (control1), no sodium nitrite and purple-fleshed sweet potato powder combination; PP1, no sodium nitrite and 0.5% purple-fleshed sweet potato powder; PP2, no sodium nitrite and 1% purple-fleshed sweet potato powder; SP1, 0.011% sodium nitrite and 0.5% purple-fleshed sweet potato powder combination; SP2, 0.011% sodium nitrite and 1% purple-fleshed sweet potato powder combination.

A–D Means in the same column within the same aging period with different letters are significantly different (p < 0.05).

a–d Means in the same row within the same aging condition with different letters are significantly different (p < 0.05).

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All samples with added NaNO2 retained the most nitrite by day 20 (CON, SP1, and SP2) (Table 3). The concentration of residual nitrite in sausages at 20 days decreased in the order of CON > SP1 and SP2 > PP2 > PP1 and CON1. More residual nitrite was found in the CON than in the SP1 and SP2 samples on day 20 of storage (p < 0.05). Levels of residual nitrite were significantly lower in samples containing both PP and added NaNO2, which concurred with the results of Lee et al. [23]. Marco et al. [28] found that the amount of nitrite in sausages rapidly decreases, owing to the higher reactivity of added nitrite compared with added nitrate. Thus, a low pH can affect the reactivity of nitrite and decrease the amount of residual nitrite in stored sausages. The present study also found decreased residual nitrite content in sausages with added PP during prolonged storage.

Table 4 describes the influence of PP on the fatty acid composition of sausages. Most of the detected fatty acids were MUFA, but the PUFA oleic acid (C18:1) was the most abundant in all treated samples. Saturated fatty acids were the next most abundant, with palmitic acid as its major component. The most abundant PUFA was linoleic acid. The PP2 sausages had the most PUFA, essential fatty acids (EFA), unsaturated fatty acids (UFA), ratios of UFA to SFA and of EFA to UFA, n-3 and n-6 fatty acids among the treated samples (p < 0.05). However, the composition of major fatty acids did not significantly differ between the CON and SP2 groups. A lower ratio (%) of SFA in the PP2 samples was responsible for the reduced amount of palmitic and stearic acids, which are associated with an increased risk of cardiovascular disease [29]. The most abundant fatty acid in the PP2 samples was linoleic acid (C18:2n-6), which differed from all other samples. Many studies have shown that replacing dietary saturated fat with oils abundant in linoleic acid helps to lower cholesterol [30]. Adding PP to sausages also increased in the amount of linolenic acid in the sausages in the present study. This might be due to the high abundance of linolenic acid in purple sweet potatoes [20].

Table 4. Fat and fatty acid composition of cooked pork sausages containing two different percentages of purple sweet potato combined with nitrite
Treatments1)
CON CON1 PP1 PP2 SP1 SP2
Fat 8.84 ± 0.51 8.77 ± 0.54 8.41 ± 0.37 8.53 ± 0.35 8.23 ± 0.37 8.45 ± 0.27
Capric acid (C10:0) 0.07 ± 0.00 0.07 ± 0.00 0.07 ± 0.00 0.07 ± 0.00 0.07 ± 0.00 0.07 ± 0.00
Lauric acid (C12:0) 0.09 ± 0.01 0.09 ± 0.00 0.09 ± 0.00 0.09 ± 0.01 0.09 ± 0.00 0.10 ± 0.01
Myristic acid (C14:0) 1.69 ± 0.01A 1.62 ± 0.01C 1.57 ± 0.01D 1.59 ± 0.00D 1.65 ± 0.01B 1.59 ± 0.03D
pentadecanoic acid (C15:0) 0.09 ± 0.00A 0.09 ± 0.00A 0.08 ± 0.00C 0.08 ± 0.01BC 0.09 ± 0.01AB 0.08 ± 0.00C
palmitic acid (C16:0) 23.93 ± 0.06A 22.76 ± 0.09D 22.68 ± 0.10D 22.62 ± 0.01D 23.71 ± 0.03B 23.30 ± 0.19C
Palmitoleic acid (C16:1) 2.86 ± 0.01A 2.81 ± 0.01B 2.63 ± 0.02F 2.77 ± 0.01C 2.71 ± 0.01D 2.68 ± 0.02E
Magaric acid (C17:0) 0.43 ± 0.00C 0.43 ± 0.01C 0.45 ± 0.01B 0.42 ± 0.00D 0.47 ± 0.00A 0.47 ± 0.00A
Magaolic acid (C17:1) 0.48 ± 0.00B 0.48 ± 0.01B 0.48 ± 0.01B 0.48 ± 0.01B 0.50 ± 0.01A 0.51 ± 0.01A
Stearic acid (C18:0) 10.96 ± 0.05B 10.40 ± 0.02C 11.00 ± 0.14B 10.34 ± 0.04C 11.45 ± 0.09A 11.38 ± 0.11A
Oleic acid (C18:1) 48.21 ± 0.06B 47.49 ± 0.17D 47.46 ± 0.16D 47.55 ± 0.05D 47.93 ± 0.08C 48.56 ± 0.16A
Linoleic acid (C18:2, n-6) 9.36 ± 0.08C 11.61 ± 0.23A 11.33 ± 0.21B 11.77 ± 0.02A 9.52 ± 0.05C 9.43 ± 0.10C
Linolenic acid (C18:3, n-3) 0.42 ± 0.01C 0.64 ± 0.03A 0.61 ± 0.03B 0.65 ± 0.01A 0.42 ± 0.01C 0.41 ± 0.01C
Arachidic acid (C20:0) 0.16 ± 0.01A 0.15 ± 0.00B 0.16 ± 0.01A 0.15 ± 0.00B 0.16 ± 0.01A 0.17 ± 0.00A
Eicosenoic acid (C20:1) 1.04 ± 0.01B 0.96 ± 0.02E 0.99 ± 0.01D 0.98 ± 0.00D 1.03 ± 0.01C 1.06 ± 0.01A
Eicosatrienoic acid (C20:4, n-6) 0.19 ± 0.01B 0.40 ± 0.05A 0.40 ± 0.05A 0.42 ± 0.01A 0.20 ± 0.01B 0.20 ± 0.01B
SFA 37.43 ± 0.10AB 35.61 ± 0.11D 36.10 ± 0.24C 35.37 ± 0.04D 37.69 ± 0.12A 37.16 ± 0.26B
MUFA 52.60 ± 0.06A 51.75 ± 0.20C 51.56 ± 0.19C 51.78 ± 0.06C 52.17 ± 0.08B 52.81 ± 0.17A
PUFA 9.97 ± 0.09C 12.65 ± 0.30AB 12.34 ± 0.28B 12.84 ± 0.02A 10.14 ± 0.06C 10.03 ± 0.11C
EFA 9.97 ± 0.09C 12.65 ± 0.30AB 12.34 ± 0.28B 12.84 ± 0.02A 10.14 ± 0.06C 10.03 ± 0.11C
UFA 62.57 ± 0.10CD 64.39 ± 0.11A 63.90 ± 0.24B 64.63 ± 0.04A 62.31 ± 0.12D 62.84 ± 0.26C
UFA/SFA 1.67 ± 0.01CD 1.81 ± 0.01A 1.77 ± 0.02B 1.83 ± 0.01A 1.65 ± 0.01D 1.69 ± 0.02C
EFA/UFA 0.16 ± 0.00C 0.20 ± 0.01AB 0.19 ± 0.01B 0.20 ± 0.00A 0.16 ± 0.00C 0.16 ± 0.00C
n-3 0.42 ± 0.01C 0.64 ± 0.03A 0.61 ± 0.03B 0.65 ± 0.01A 0.42 ± 0.01C 0.41 ± 0.01C
n-6 9.55 ± 0.08C 12.01 ± 0.28AB 11.73 ± 0.26B 12.19 ± 0.02A 9.72 ± 0.06C 9.62 ± 0.10C
n-6/n-3 22.61 ± 0.24B 18.89 ± 0.43D 19.46 ± 0.42C 18.76 ± 0.20D 23.38 ± 0.31A 23.74 ± 0.20A

1) Treatments: CON (control),= 0.014% sodium nitrite, CON1 (control1), no sodium nitrite and purple-fleshed sweet potato powder combination; PP1, no sodium nitrite and 0.5% purple-fleshed sweet potato powder; PP2, no sodium nitrite and 1% purple-fleshed sweet potato powder; SP1, 0.011% sodium nitrite and 0.5% purple-fleshed sweet potato powder combination; SP2, 0.011% sodium nitrite and 1% purple-fleshed sweet potato powder combination.

A–F Means in the same column within the same aging period with different letters are significantly different (p < 0.05).

SFA, saturated fatty acids; MUFA, monounsaturated fatty acids; PUFA, polyunsaturated fatty acids; EFA, essential fatty acids; UFA, unsaturated fatty acids.

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Adding PP caused changes in the levels of several amino acids in the pork sausages (Table 5), even though the total protein contents did not significantly differ (p > 0.05). The abundance of free glutamic acid was high. Glutamic acid is associated with the flavor called umami, which has a slightly sweet and sour taste [31]. The samples with purple-fleshed sweet potatoes contained significantly more glutamic acid, alanine, histidine, lysine, and arginine, and lower ratios of aspartic acid and cysteine (p < 0.05). Purple sweet potatoes are abundant in aspartic acid, glutamic acid, serine, alanine, valine, and other amino acids [20,32]. Changes in the levels of glutamic acid in the samples with PP can be attributed to the high amounts of similar amino acids extant in purple sweet potatoes. Levels of glutamic acid and alanine were higher in the SP2 than in the CON group (p < 0.05).

Table 5. Protein and amino acid composition of cooked pork sausages containing two different percentages of purple sweet potato combined with nitrite
Treatments1)
CON CON1 PP1 PP2 SP1 SP2
Protein 17.76 ± 0.58 17.40 ± 0.16 18.09 ± 0.48 17.79 ± 0.38 18.28 ± 1.14 17.99 ± 0.23
Aspartic acid 9.38 ± 0.12A 9.22 ± 0.00A 7.40 ± 0.09B 6.82 ± 0.93B 7.50 ± 0.20B 7.51 ± 0.08B
Threonine 4.65 ± 0.00 4.59 ± 0.00 4.60 ± 0.06 4.65 ± 0.10 4.67 ± 0.02 4.50 ± 0.01
Serine 3.87 ± 0.10 3.75 ± 0.00 4.06 ± 0.18 4.07 ± 0.21 4.07 ± 0.14 4.08 ± 0.18
Glutamic acid 14.91 ± 0.43B 14.77 ± 0.00B 16.00 ± 0.01A 16.11 ± 0.14A 15.67 ± 0.30A 15.59 ± 0.30A
Proline 4.29 ± 0.06 4.40 ± 0.00 4.43 ± 0.26 4.45 ± 0.05 4.42 ± 0.28 4.58 ± 0.16
Glycine 4.89 ± 0.25 4.81 ± 0.00 5.13 ± 0.05 4.95 ± 0.11 4.97 ± 0.02 5.10 ± 0.07
Alanine 5.86 ± 0.09B 5.85 ± 0.00B 6.18 ± 0.06A 6.25 ± 0.15A 6.21 ± 0.11A 6.17 ± 0.03A
Cysteine 1.37 ± 0.11A 1.20 ± 0.00B 0.00 ± 0.00D 0.21 ± 0.00C 0.00 ± 0.00D 0.00 ± 0.00D
Valine 5.48 ± 0.05 5.54 ± 0.00 5.58 ± 0.06 5.67 ± 0.16 5.73 ± 0.01 5.68 ± 0.18
Methionine 2.40 ± 0.74 2.94 ± 0.00 2.71 ± 0.02 2.59 ± 0.23 2.51 ± 0.62 2.76 ± 0.19
Isoleucine 5.08 ± 0.09 5.06 ± 0.00 5.11 ± 0.07 5.24 ± 0.22 5.34 ± 0.13 5.18 ± 0.18
Leucine 8.40 ± 0.02 8.42 ± 0.00 8.50 ± 0.04 8.64 ± 0.17 8.68 ± 0.06 8.61 ± 0.18
Tyrosine 3.32 ± 0.28 3.55 ± 0.00 3.51 ± 0.05 3.42 ± 0.16 3.31 ± 0.23 3.55 ± 0.15
Phenylalanine 4.17 ± 0.01 4.19 ± 0.00 4.27 ± 0.02 4.35 ± 0.10 4.29 ± 0.04 4.31 ± 0.06
Histidine 4.85 ± 0.09B 4.68 ± 0.00C 5.03 ± 0.08A 5.12 ± 0.04A 5.02 ± 0.05A 4.98 ± 0.08AB
Lysine 9.08 ± 0.02BC 9.04 ± 0.00C 9.29 ± 0.04A 9.32 ± 0.06A 9.30 ± 0.13A 9.21 ± 0.01AB
Ammonia 1.53 ± 0.01 1.52 ± 0.00 1.54 ± 0.12 1.58 ± 0.09 1.66 ± 0.06 1.60 ± 0.01
Arginine 6.52 ± 0.11B 6.47 ± 0.00B 6.72 ± 0.07A 6.71 ± 0.10A 6.70 ± 0.00A 6.63 ± 0.02AB
FAA 14.91 ± 0.43B 14.77 ± 0.00B 16.00 ± 0.01A 16.11 ± 0.14A 15.67 ± 0.30A 15.59 ± 0.30A
STAA 19.26 ± 0.45 19.00 ± 0.00 19.96 ± 0.35 19.92 ± 0.15 19.91 ± 0.26 19.85 ± 0.30
SAA 3.78 ± 0.84 4.14 ± 0.00 2.71 ± 0.02 2.63 ± 0.00 2.51 ± 0.62 2.76 ± 0.19
AAA 7.49 ± 0.26 7.73 ± 0.00 7.77 ± 0.06 7.77 ± 0.06 7.60 ± 0.19 7.85 ± 0.21
BAA 31.08 + 0.97 32.43 + 0.00 32.92 + 0.04 33.09 + 0.22 32.88 + 0.71 33.06 + 0.66
EAA 50.60 ± 0.68 50.93 ± 0.00 51.79 ± 0.04 52.29 ± 0.70 52.20 ± 0.27 51.84 ± 0.69
TAA 100 100 100 100 100 100

1) Treatments: CON (control), 0.014% sodium nitrite; CON1 (control1), no sodium nitrite and purple-fleshed sweet potato powder combination; PP1 = no sodium nitrite and 0.5% purple-fleshed sweet potato powder; PP2, no sodium nitrite and 1% purple-fleshed sweet potato powder; SP1, 0.011% sodium nitrite and 0.5% purple-fleshed sweet potato powder combination; SP2, 0.011% sodium nitrite and 1% purple-fleshed sweet potato powder combination.

A–D Means in the same column within the same aging period with different letters are significantly different (p < 0.05).

FAA, flavorous amino acid; STAA, sweet taste amino acid; SAA, sulfur-containing amino acid; AAA, aromatic amino acid; BAA, bitter amino acid; EAA, essential amino acid; TAA, total amino acid.

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CONCLUSION

The inclusion of 1% PP with 0.011% sodium nitrite as a colorant seemed to confer desirable subjective and objective qualities on sausages. The residual nitrite content in sausages with added PP decreased over time in storage. The addition of 1% PP produced major changes in the fatty acid profiles of the sausages, reduced the ratios of SFA and MUFA, and increased PUFA levels. Adding PP also tended to increase the content of flavorous amino acids. The fatty acid composition was comparable between the SP2 and CON groups, but the SP2 groups contained more flavorous and sweet-tasting amino acids than the CON group. Therefore, including up to 1% of PP along with 0.011% NaNO2 could serve as a natural colorant for pork sausages without any detrimental effects on quality. We also found that 20% PP might be able to replace NaNO2. However, further investigation is needed to determine the effects of partially substituting NaNO2 with PP on the oxidative and microbiological stability of meat products.

Competing interests

No potential conflict of interest relevant to this article was reported.

Funding sources

This work was supported in parts by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries (IPET) through High Value-added Food Technology Development Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA, 118039-03-1-HD020) and the 2020th RAIC Gyeongnam National University of Science and Technology in Korea.

Acknowledgements

Not applicable.

Availability of data and material

Upon reasonable request, the datasets of this study can be available from the corresponding author.

Authors’ contributions

Conceptualization: Jin SK.

Data curation: Jin SK.

Formal analysis: Jin SK.

Methodology: Jin SK.

Software: Jin SK.

Validation: Shin TS.

Investigation: Shin TS.

Writing - original draft: Jin SK, Yim DG.

Writing - review & editing: Jin SK, Yim DG.

Ethics approval and consent to participate

This article does not require IRB/IACUC approval because there are no human and animal participants.

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