. Nov 26;11(23):. doi: 10./foods

Toughness Variations among Natural Casings: An Exploration on Their Biochemical and Histological Characteristics

Wenjun Liu

Wenjun Liu

1Graduate School of Science and Technology, Niigata University, Niigata 950-, Japan Conceptualization, Data curation, Formal analysis, Software, Validation, Writing ' original draft Find articles by Wenjun Liu 1, Xing Chen

Xing Chen

2State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi , China Writing ' review & editing Find articles by Xing Chen 2, Satomi Tsutsuura

Satomi Tsutsuura

1Graduate School of Science and Technology, Niigata University, Niigata 950-, Japan Writing ' review & editing Find articles by Satomi Tsutsuura 1, Tadayuki Nishiumi

Tadayuki Nishiumi

1Graduate School of Science and Technology, Niigata University, Niigata 950-, Japan Conceptualization, Funding acquisition, Methodology, Supervision, Writing ' review & editing Find articles by Tadayuki Nishiumi 1,* Editors: Peng Wang, Baohua Kong, Xiufang Xia

1Graduate School of Science and Technology, Niigata University, Niigata 950-, Japan 2State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, Wuxi , China

Roles

Wenjun Liu: Conceptualization, Data curation, Formal analysis, Software, Validation, Writing ' original draft Xing Chen: Writing ' review & editing Satomi Tsutsuura: Writing ' review & editing Tadayuki Nishiumi: Conceptualization, Funding acquisition, Methodology, Supervision, Writing ' review & editing Peng Wang: Academic Editor Baohua Kong: Academic Editor Xiufang Xia: Academic Editor

Received Sep 10; Accepted Nov 24; Collection date Dec.

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© by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Abstract

We investigated the mechanical, biochemical, and histological properties of hog and sheep casings produced in different countries to elucidate the responsible factors for the toughness quality of natural casings. The toughness and collagen characteristics of sheep and lamb casings were also investigated to elucidate the effect of animal slaughter age on the relationships between connective tissue and the mechanical properties of natural casings. The results showed that the main component of hog and sheep casings was collagen with many layers of sheets. The contents of collagen, elastin, and proteoglycan in hog and sheep casings were similar. The toughest Chinese casings (p < 0.01) possessed a significantly lower heat solubility of collagen (p < 0.01), and a different size and arrangement of collagen fibers. Sheep casings were significantly tougher than lamb casings (p < 0.01). Compared with lamb casings, sheep casings had a significantly low heat-labile collagen content, a low heat solubility of collagen, a large size of collagen fibers, and a high pyridinoline concentration (p < 0.01). Therefore, the high thermal and structural stability of collagen in aged animals may contribute to the enhanced mechanical properties of casings.

Keywords: hog and sheep casings, toughness, biochemical characteristic, histological characteristic, animal age

1. Introduction

Sausage casing, also known as sausage skin or simply casing, is the material that encloses the filling of a sausage. Nowadays, many types of sausage casings are used, including natural, manufactured collagen, cellulose, and plastic casings, as well as co-extruded casings made of collagen and alginate, and alginate-collagen hybrid casings [1]. Among them, natural casings are considered as the golden standard in sausage production owing to their desirable tenderness and high permeability to both moisture and smoke [2]. Natural casings are usually produced from the intestines of pigs, sheep, goats, and cattle (and horses, in certain cases). Among them, hog and sheep casings are mainly used in the sausage industry. The part of the small intestine often used in the sausage industry has a structure of tunica mucosa, submucosa, tunica muscularis, and tunica serosa from the inside. In hog and sheep casings, their submucosa, which is the remaining layer of the intestine after processing, forms the natural sausage casing [3].

The barrier properties and mechanical strength, which are two essential casing physical properties, strongly affect consumers' perception of bite/snap and flavor [4]. There are still many quality defects in natural casings for sausage manufacturers, and their physical properties are required to be improved to meet the needs of consumers. Some hog and sheep casings with large diameters show high toughness. Therefore, when eating sausages, only the natural casing part remains in the mouth. There are even some hog sausage casings that need to be removed before eating. To suit consumers' taste for soft sausages, the study performed by Sakata, Segawa, Morita, and Nagata [5] proposes the tenderization of hog casings with lactic acid and pepsin. On the other hand, excessive tenderization of natural casings is not acceptable. The strength and elasticity of a casing are also crucial during the processing of the sausage, as the casing must be strong enough to hold the meat but also able to expand during stuffing and cooking [6,7]. It is necessary to improve the quality of natural casings to an appropriate toughness by controlling its mechanical properties. However, little research has been performed on the mechanical, chemical, and histological properties and their relationships in natural casings.

Rupturing during stuffing is caused by gliding defects in natural casings, which is another problem for the industrial production of sausages. Houben, Bakker, and Keizer [8] reported that hog and sheep casings treated with trisodium phosphate facilitated the gliding of the casings over the test pipes and had a lower shear force (non-significant) than the control casings after being used as skins for cooked and smoked sausages. Hog casing modified with surfactant solutions combined with lactic acid has the best resistance to burst pressure or rupture force in sausage processing [9]. Although some studies have improved the gliding properties of natural casings, the structural configuration of the surfaces of natural casings has not been discussed.

Natural casings are by-products of livestock often obtained from a large number of animals from different countries and/or producers. This may lead to the disadvantage of non-uniform strength quality in natural casing products and causes the risk of rupturing during automatic filling in sausage production. However, few previous studies have been performed on toughness variations among natural casings and their influencing factors.

Although studies have been carried out to improve the mechanical properties of natural casings, there is still less research elucidating the fundamental structure'toughness relationship in natural casings. In particular, the reasons for the distinction in toughness has not been clarified from the perspectives of the histological structure, quantitative determination of the components, and comparative discussion among various kinds of natural casings. In this regard, this study investigated the toughness, and biochemical and histological characteristics of hog and sheep casings originated in different production areas (countries) or considering different slaughter ages; the aim was to probe the basic factors influencing the mechanical properties of casings, which might allow the sausage casing quality to be controlled in the natural casing retailer and sausage industry.

2. Materials and Methods

2.1. Materials

Salted natural hog casings (32'34 mm in diameter; from China, Japan, and USA), sheep casings (20'22 mm; from China, Australia, Egypt, and New Zealand), and lamb casings (20'22 mm; from New Zealand) were used. All materials were obtained from a natural casing retailer, New Asia Trading Co., Ltd., in Japan. The salted casing samples stored at 4 °C were washed and desalted in running water for 1 h at room temperature and then were used for a series of analyses. The sheep and lamb casings used in this study were obtained from sheep older than 2 years and lambs from 1 to 2 years old, respectively.

2.2. Toughness Measurement

The desalted casings were cut into one layered sheet; then, the toughness of each casing sheet was measured using a rheometer (NM-J; Rheotech) on the middle part. The toughness was indicated by the maximum load value (breaking stress, in N) when a columnar plunger with a diameter of 3 mm was loaded perpendicularly and the plunger broke through the casing. A total of 240 different locations were tested for the toughness evaluation of each sample, which was composed of 12 different animal casings randomly selected from 3 hanks. Each casing for toughness determination was divided by 1 m for histological analyses before being cut into one layered sheet.

2.3. Biochemical Analysis

The casings used for measuring toughness were also used to make dried'defatted matter (DDM) for biochemical analyses. The DDM was made as follows: The casings were frozen and crushed with liquid nitrogen; then, the samples were degreased and dried with a mixed solution of chloroform'ethanol (2:1). The biochemical characteristics of the DDM were evaluated (n = 12) by measuring the collagen, elastin, and proteoglycan contents and the heat solubility of collagen according to the methods previously reported by Nishiumi, Kunishima, Nishimura, and Yoshida [10].

2.3.1. Collagen Measurement

According to Hill [11], the powdered DDM was heated in distilled water at 77 °C for 70 min and then separated into insoluble and heat-soluble fractions with two centrifugations. The samples were then hydrolyzed with 6 mol/L HCl at 110 °C for 24 h, and the hydroxyproline content in the hydrolysates was determined [12]. Hydroxyproline values were converted to insoluble and soluble collagen using the coefficients of 7.25 [13] and 7.52 [14], respectively. The total collagen concentration of the samples was determined by adding the amounts of heat-soluble and insoluble collagen. Then, we calculated the percentage of soluble collagen in each sample.

2.3.2. Elastin Determination

The powdered DDM was hydrolyzed in 0.1 mol/L NaOH for 50 min at 98 °C, and the remaining residue was hydrolyzed in 6 mol/L HCI; then, hydroxyproline was quantitated. The concentration of DDM elastin was determined by multiplying the hydroxyproline content by 66.225 [15].

2.3.3. Uronic Acid Content

The DDM was extracted with activated papain (by incubating papain (type III; Sigma) with 0.1 mol/L EDTA) solution for 24 h at 65'70 °C, and the supernatant was collected using centrifugation [16]. The residue washed with 0.1 mol/L phosphate buffer (pH 6.4; containing 0.3 mol/L NaCl) was centrifuged again to collect the supernatant [16]. The uronic acid content in both supernatant fractions was estimated using glucuronolactone as a standard [17].

2.4. Histological Analysis

A histological analysis was performed on each sample using the casing left at the time of toughness measurement. As specimens for histochemistry, casings were fixed with 10% formalin-PBS and then stained with Verhoff's Van Gieson (Elastin Stain Kit; Sigma). Elastin fibers and collagen fibers in each casing were observed under an optical microscope.

Furthermore, to observe the structure of collagen fibers, the casing sample was fixed with a 2% paraformaldehyde'2.5% glutaraldehyde solution (0.1 mol/L phosphate buffer, pH 7.4). According to the cell maceration/ scanning electron microscope (SEM) method of Ohtani, Ushiki, Taguchi, and Kikuta [18], the fixed samples were treated with a 10% NaOH aqueous solution for 5 days and then washed with distilled water for 3 days. The samples were treated with the tannin'osmium method [19] and dehydrated with an alcohol system, and the solution was replaced with t-butyl alcohol for freeze-drying [20]. The dried sample adhered to the sample holder, and gold and palladium were vapor-deposited; then, the sample was observed at an acceleration voltage of 15 kV using a Hitachi S-N or Hitachi S-430 scanning electron microscope.

2.5. Determination of Pyridinoline Content

The quantification of pyridinoline was performed according to the method of Arakawa, Kim, and Otsuka [21]. Casings were hydrolyzed with hydrochloric acid, and we sufficiently removed the hydrochloric acid using a rotary evaporator. Then, the samples were dissolved in 3 mL of distilled water and filtered through a chromatodisc (0.45 µm pore size). A part of these samples were subjected to hydroxyproline quantification, and the rest were analyzed using a high-performance liquid chromatographic (HPLC) assay. For the HPLC analysis, a Shimadzu LC-10AD HPLC instrument connected with an Inertsil ODS-2 column (4.6 mm I.D. × 25 cm) was used. In the mobile phase, a mixture of 0.1 mol/L sodium phosphate (pH 3.5) and acetonitrile (75:25, v/v) containing 0.1% SDS and 0.% EDTA was used at a flow rate of 1 mL/min. The eluate was monitored with a spectroscopic fluorescence detector (RF-10A; Shimadzu Corporation)'fluorescence excitation at 295 nm and emission at 395 nm. The pyridinoline concentration in the casings was calculated using commercially available pyridinoline (Wako Pure Chemical Industries) as a standard solution and expressed as mole per mole of collagen.

2.6. Statistical Analysis

Means and standard errors were calculated in each group. The one-way analysis of variance (ANOVA) was used for statistical analyses using BellCurve for Excel (Social Survey Research Information Co., Ltd.). Significant differences between means were determined with Tukey's multiple test method at a significance level of p < 0.01.

3. Results and Discussion

3.1. Biochemical and Histological Characteristics of Hog and Sheep Casing Toughness Difference

3.1.1. Hog and Sheep Casing Toughness Properties

To compare and explore the physical properties of natural casings, toughness measurements (breaking stress, in N) were carried out on hog and sheep casings of different origins. As shown in Table 1, the breaking stress was significantly (p < 0.01) higher in hog casings than in sheep casings overall. Gzik-Zroska et al. [22] also reported that hog casings have higher breaking stresses than sheep casings because of the individual factors of each species and the various geometry. Feng, Otani, Ogawa, and García-Martín () reported that in their study, the thicknesses of hog and sheep casings were about 0.03 and 0.019 mm, respectively [23]. Moreover, the toughness difference in casings from different animal was reported to be caused by their thicknesses [24]. In both hog and sheep casings from each country, the Chinese ones showed significantly (p < 0.01) higher breaking stress. This result was also consistent with the report by Sakata et al. [5] indicating that the Chinese hog casing was harder than the American one. Additionally, the breaking stress of Chinese casings fluctuated greatly, and some of them showed the same value as other domestic casings, but some of them showed even higher breaking stress; as a result, there was a large variation in toughness. On the one hand, there were no significant differences in toughness between hog casings from Japan and the USA and sheep casings from Australia and Egypt.

Table 1.

Origin Breaking Strength (N) Hog casing China 7.91 ± 1.99 A USA 6.51 ± 1.06 B Japan 6.11 ± 1.15 B Sheep casing China 4.89 ± 1.41 C Egypt 3.23 ± 1.01 D Australia 3.11 ± 0.74 D

3.1.2. Biochemical Characteristics of Hog and Sheep Casings and Their Relationship with Toughness

Table 2 shows the total collagen content, the heat-labile collagen content, the heat solubility of collagen, the elastin content, and the uronic acid content of the natural casings.

Table 2.

Origin Total Collagen
Content
(mg/g DDM) Heat-Labile
Collagen Content
(mg/g DDM) Heat Solubility
of Collagen
(%, w/w) Elastin
Content
(mg/g DDM) Uronic Acid
Content
(mg/g DDM) Hog casing China 921 ± 42 A 14.4 ± 5.0 A 1.56 ± 0.26 A 22.0 ± 1.6 A 1.72 ± 0.34 A USA 830 ± 101 A,B 19.1 ± 6.3 A 2.30 ± 0.45 B N.D. 1.82 ± 0.24 A Japan 837 ± 82 A,B 18.5 ± 4.4 A 2.12 ± 0.36 B 16.3 ± 0.7 B 1.99 ± 0.27 A Sheep casing China 868 ± 92 A,B 19.7 ± 8.1 A 2.27 ± 0.61 B 23.9 ± 1.2 A 1.86 ± 0.15 A Egypt 844 ± 86 A,B 30.1 ± 6.5 B 3.57 ± 0.58 C N.D. 1.72 ± 0.17 A Australia 803 ± 64 B 29.0 ± 6.2 B 3.61 ± 0.37 C 22.8 ± 2.1 A 1.79 ± 0.37 A

Both hog and sheep casings (submucosa) contained high contents of collagen (80'92%). No differences in the total collagen content was observed between hog/sheep casings in all samples, even though they had different toughness. A study on total collagen content and distribution in the human colon showed similar results indicating that the percentage mean intensity of total collagen staining in the submucosa was 74 (3.2)% in the adult ascending colon [25].

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The elastin content, which mainly constitutes blood vessels, was about 20 mg/g DDM, and this value was as low as the elastin ratio in the intramuscular connective tissue [10]. Since there were almost no differences among all the casing samples, it was considered that elastin is not the major inducing factor of toughness difference among natural casings. In collagen membranes, elastic fibers run parallel to the collagen fibers and the functional interaction of these two fibrous components imparts the tissue with both tensile strength and elasticity [26]. Therefore, elastin may affect toughness in veiny casings that are visibly vascularized.

The uronic acid content, as an index of proteoglycan, did not differ among all casing samples. It has been suggested that certain proteoglycans interact with collagen and contribute to the structural stability of collagen [27]. However, in this study, there were no relationships between the uronic acid content and the toughness variation in the natural casings.

In terms of composition, the natural casing is the connective tissue in the small intestine of livestock and is composed of a large amount of collagen, a small amount of elastin, and proteoglycan. Among all the samples, although there were significant differences in the toughness properties, there were no significant differences in the contents of collagen, elastin, and proteoglycan. From this, we infer that the difference in the toughness property of natural casings has no correlations with the content of its components.

On the other hand, the heat solubility of collagen was significantly (p < 0.01) lower in both Chinese hog casings and sheep casings than that in other domestic casings, which corresponded well to the difference in toughness. The heat solubility of collagen indicates what percentage of collagen is solubilized as gelatin by heating at 77 °C for 70 min and is an index indicating the thermal stability of collagen. The thermal stability of collagen is thought to be affected by the formation of collagen intermolecular cross-links and the size and orientation of collagen fibers [28]. Therefore, the heat solubility of collagen is considered to reflect not only the thermal stability of the tissue mainly composed of collagen but also the toughness of the tissue. In other words, it is conjectured that the toughness of the very thin natural casing is caused by the structure of collagen fibers and the formation of intermolecular cross-links.

3.1.3. Histological Characteristics of Hog and Sheep Casings and Their Relationship with the Mechanical Property

The distributions of elastin fibers and collagen fibers in each casing were observed with a light microscope after staining using Elastin Stain Kit. Elastin Stain Kit stains elastin in black and collagen in yellow. It was observed that blood vessels mainly composed of elastin were buried in collagen fibers at low magnification (Figure 1a: green arrow, arteriole; blue arrow, venule). The densities of elastin fibers in arterioles (A) and venules (V) varied (Figure 1c) when dilating the fig of blood vessels. As shown in Figure 1b, a casing is composed of sheets consisting of multiple layers of collagen fibers, where each sheet includes a woven structure of wavy collagen fibers with thin elastic fibers running into it (Figure 1d). The microstructures were similar among all casings (either hog casings and sheep casings or casings from the various production areas). Gunn, Sizeland, Wells, and Haverkamp [24] also reported that sausage casings from different species (cattle, hog, and sheep) are similar in collagen structure. Notably, this characteristic of woven structure with wavy collagen fibers can contribute to the advantages of moisture permeability, smoky ingress, and good elasticity.

Furthermore, a comparative study of the influence of collagen fiber structures in natural casings on different toughness values was performed using a scanning electron microscope. Figure 2 shows the collagen fiber structures of hog casings from Japan and China that showed a significant difference in toughness property. The structure of the collagen fibers on the outside of the casing was similar in the hog casings from both Japan (Figure 2a) and China (Figure 2b) and was composed of a thin sheet layer, in which the collagen fibers ran in parallel. The collagen fibers running on the outside of the hog casings were thicker than those on the inside. On the other hand, the inside of the casing from Japan (Figure 2c) had a spongy-like structure that was formed by collagen fibers aligned in a network, but that was hardly observed in the casing from China (Figure 2d). Collagen fiber distribution on the inner surface of natural casings might affect its slipperiness in the sausage filling processing. A previous study showed that the network-like structure with collagen fibers became visible on the inside surface (mucosal surface) of the porcine small intestinal submucosa and that the inside surface was noticeably smoother than the outside surface (serosal surface) [29].

Figure 3 shows the collagen fiber structure of Australian and Chinese sheep casings that had a significant disparity in terms of toughness. The outside of the Australian sheep casing (Figure 3a) was very finely woven and consisted of tiny collagen fibers. Moreover, in the Chinese sheep casing (Figure 3b), the collagen fibers were relatively thick and had a rough woven shape, which was unlike the orientation of the collagen fibers on the outside of the Australian sheep casing, resembling the appearance of the hog casing. In both Australian and Chinese sheep casings, the inside structure of collagen fibers ran to surround a large hole. However, the Australian sheep intestine (Figure 3c) had a fine, lace-like appearance, while the Chinese sheep intestine (Figure 3d) had thick collagen fibers and a rough appearance. Thimo Maurer et al. reported that the average diameter of collagen fibrils is positively correlated with the tensile strength of the connective tissue [26]. Overall, we surmise that it is the thickness and orientation of collagen fibers that differentiate the mechanical properties of casings. In particular, the Chinese casing was relatively thick with different orientations, thus leading to high toughness and rough appearance.

and A; veins, and V; elastin fibers, . ( ), and veins ( ) are embedded in the predominant collagen fibers. ( ) are scattered sparsely in collagen fibers.

3.2. Toughness and Collagen Characteristics of Sheep and Lamb Casings

3.2.1. Toughness of Sheep and Lamb Casings

As shown in Table 3, the breaking stress of older sheep casings was significantly higher than that of young lamb casings (p < 0.01) indicating that casings become tougher with animal age progression. Furthermore, Parry () reported that the general pattern of collagen fibril growth can be established as a function of age [30]. Considering the results of measuring the toughness of national sheep casings, New Zealand lamb, Australian, and Egyptian sheep casings were relatively tender, New Zealand sheep were slightly tougher, and Chinese sheep casings were especially tough. Natural casings are a by-product of livestock, it is not clear at what age they are slaughtered in the natural casing industry. It is believed that in the natural casing industry, animals with different ages show variations in the casing toughness.

Table 3.

New Zealand Casing Breaking Strength
(N) Total Collagen Content (mg/g DDM) Heat-Labile Collagen Content (mg/g DDM) Heat Solubility of Collagen (%, w/w) Pyridinoline Concentration (Mole/Mole of Collagen) Lamb 2.93 ± 0.83 A 865 ± 27 A 94.4 ± 7.8 A 10.91 ± 0.42 A 0.046 ± 0.003 A Sheep 3.99 ± 1.18 B 884 ± 51 A 28.4 ± 4.0 B 3.32 ± 0.22 B 0.128 ± 0.009 B

3.2.2. Collagen Heat Solubility and Pyridinoline Content in Sheep and Lamb Casings

As shown in Table 3, there were no significant differences in the total collagen contents in lamb and sheep casings, while the amount of heat-labile collagen content in sheep casings was significantly (p < 0.01) lower, and the pyridinoline concentration was significantly (p < 0.01) higher than in lamb casings. The differences in pyridinoline content correspond to the toughness disparity between the lamb and sheep casings, indicating that collagen has higher thermal stability in tougher sheep casings. A natural casing is mostly composed of collagen, and the collagen molecules are cross-linked to provide stable strength for the collagen connective tissue. With age progression, the unstable cross-links reduce and are converted to heat-stable mature cross-links (e.g., pyridinoline), which stabilizes the collagen fiber structure and reduces the heat solubility of collagen [31]. Therefore, this study suggests that as the livestock grows up, the casing collagen fiber network becomes stronger, and the casing becomes tougher due to the accumulation of mature cross-links among collagen molecules.

3.2.3. Comparison of Collagen Fiber Structures in Sheep and Lamb Casings

The collagen fiber structures of lamb and sheep casings were observed using a scanning electron microscope (Figure 4). The orientation of collagen fibers on the casing surface was almost the same for lamb (Figure 4a) and sheep (Figure 4b), in that collagen fibers were assembled similar to knitted fabric. However, each collagen fiber in the sheep casing was slightly thicker than in that from lamb. Previous studies reported similar observation on the collagen fiber structure of the submucosa of the small intestine (which is the material of natural casings), where collagen fibers run in a grid pattern similar to woven fabric in all animals [32,33]. In addition, the results support the argument that the lattice orientation of collagen fibers does not change with animal aging, but that the collagen fibers become thicker upon aging [34]. In this study, we also observed the cross-section of lamb and sheep casings. In the cross-sectional scanning electron microscopy study, both the lamb casing (Figure 4c) and the sheep casing (Figure 4d) consisted of several layers of thin collagen fiber sheets. However, sheet layers were large in quantity in the sheep casing, and the thickness of the casing was 0.10 to 0.13 mm for lamb and 0.15 to 0.20 mm for sheep, which was about twice as thick as the lamb casing. This result was also consistent with the report by Feng et al. () indicating that the thickness of sheep casings was about 0.019 mm [23]. These histological features of natural casings are believed to be directly related to the toughness of the casing.

4. Conclusions

The present study showed that the main component of hog and sheep casings was collagen with many layers of sheets. The contents of collagen, elastin, and proteoglycan in hog and sheep casings were similar. The variety of the toughness properties of natural casings was due to collagen stability. The toughness of sheep casings was increased by animal age. The collagen stability of sheep casings was enhanced by animal age. Overall, this work shed a light on the structural fundamentals responsible for the toughness of casings, which may benefit the control of casing mechanical properties in the natural casing retailer and sausage industry.

Acknowledgments

The authors would like to acknowledge natural casing retailer New Asia Trading Co., Ltd. for providing the samples. We gratefully acknowledge the offered assistance of members of our laboratory for the research progress.

Author Contributions

Conceptualization, Software, Validation, Formal analysis, Data curation, and Writing'original draft, W.L.; Writing'review and editing, X.C.; Writing'review and editing, S.T.; Conceptualization, Methodology, Funding acquisition, Supervision, and Writing'review and editing, T.N. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

This research received no external funding.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

Funding Statement

This research received no external funding.

Footnotes

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So I got my grinder and love it. Have a freezer full of deer and a cow elk hunt in a few weeks. I am expecting to get a cow and with the amount of meat we will get it is a lot for my wife and I. I want to try and make other things, mainly brats and some of those sausage sticks. I am not a breakfast sausage person.

I would like as much info as possible on how to make brats or sausage, down to the small details. As you can imagine I would hate to waste any meat so I am reacing out to learn and not throw away meat from an animal that I killed. Any online resources like videos or web pages would be great.

Some questions I have are how to get the pork fat, what to look for? Do I need a mixer? Can I use previously ground meat that is frozen? I may not be able to make it all right so should I freeze ground meat or whole meat to thaw and grind later and then use in sausage products? Best casing? Any good all in one kits that have spice blends ect.? To make summer sausage or those snack sticks do I need a smoker?

Please school me and dont leave anything out. The art of making sausage is too much for me to go into here.

But to answer some of your questions: I've never had an issue with using previously frozen ground meat. It's not preferred, but it won't be terrible. I'd freeze the meat and then thaw and grind as needed. Which leads to the next thing: keep your meat, grinder, work bowl, and utensils as cold as possible. I freeze til it just stiffens. Also freeze the grinder parts and work bowl until use. Cold fat and meat mix better and stay together. Mixing warm (45f+) meat results in a crumbly dry sausage. You can get pork fat from most butcher departments. I also mix 60-70% venison with pork shoulder, which is ~30% fat. I have 25lb capacity lug that I mix by hand in.

You can get some good supplies from sausagemaker dot com. I use a book from a chef/food writer Dennis Ruhlman called Charcuterie. He discusses a lot of the background and history of good sausage and how to make it at home.

I also get a lot of information from a couple of forums: wedlinydomowe.pl/en/index.php and Smokingmeatforums.

I haven't made them but to make true snack sticks, you do need a smoker, I think. But you can make a basic one pretty simply. True summer sausage is fermented, but you can make it without fermenting.

The flip side is that while it's tricky to make great sausage like true fermented and dry cured meats, it's not hard to make good sausage.

Good luck! You can estimate. A pork shoulder is about 30% fat and a belly is a bit more (45-55%). An 8lb shoulder x 30% = ~2.5lbs fat. A few friends and I butcher hogs in the fall and I collect as much fat as I can and freeze for later use. Pork fat freezes well.

As for how much, it also depends on what I'm making. The final fat content (~15-30%) for brats or polish is what you want to aim for. If it's not exact it doesn't matter much. You want to pay more attention to your seasoning (especially salt) amounts. Do everything by weight if possible. Also fry up a bit of sausage before you stuff casings. If it's too lean, add some bacon or fat, too fatty add more venison.

I don't stress about fat. A good sausage has to have some, but just not too much. And if you're using elk or venison you don't have to worry about the too much fat side of the equation.

I like using pork shoulders mostly because they're cheap and I don't like overly fatty sausage. I'll chime in as well, I've spent the last 4 years getting it a little more right each year. I think I finally found my formula last year, so hopefully I can speed up your learning curve.

I've never made snack sticks, but I've made summer sausage. I've found the 3" diameter mahogany casings to not be very permeable, so I'm not sure how much my smoker even helps flavor those. Bottom line, I think you'd be okay just baking those in the oven. I'd set it at 200 deg and pull as soon as it reaches 155 deg IT. Brats and links definitely need to go in a smoker.

Personally, I would make some bulk sausage and summer sausage to start out. It gets a little more complicated with link sausage and you need more gear. In particular, I found the smoking part to be the most finnicky. (If you buy a smoker, get a good one that can smoke at lower temperatures!!) But if you want to dive in, my recommendation is definitely get a meat mixer and sausage stuffer. The stuffer attachments for grinders are terrible and will take forever. I use natural hog/sheep casings (intestines). I order the pre-tubed ones from sausage maker, they are on amazon too. I might try collagen casings this year, I've heard some good things about those except they break apart more easily.

I like to keep my meat as lean as possible (otherwise what's the point of killing a lean game animal). I use 6% fat in burger and 15% in sausage links. It's definitely on the leaner side so you have to be careful not to overcook as it will dry out. If you're going to smoke anything let me know, I can share a few tips on that to help you avoid overcooking. Smoking meat forums is a great site on this as well, lots of veterans over there.

For seasonings - I've used the mixes from sausagemaker and they're pretty good, but a touch bland. I'm going homemade from now on. I used a few recipes from this book last year and they've all been great. http://www.amazon.com/Home-Sausage-Making-Techniques-Enjoying/dp/X I'm sure there are other recipes out there as well. The maple breakfast sausage was incredible.

Hope that helps. I have more tips I can share, but that's a decent start. Sausage making is definitely an art form, lots you can learn!! When I butcher my deer and elk I set aside the meat that will go to the grinder and cut it into 1 inch cubes then wrap them up in 5 lb packages. This does a couple things. First I can easily do small batches of grinding into either burger, suasage or brats; also the meat is already cut small enough that all I have to do it pull it from the freezer and let thaw for a few hours (mostly frozen but workable, as has been said frozen meat is easier to grind and mix). I use pork fat that I either buy from grocery stores, local butchers or local farmers.

Burger is the easiest: I mix 10% by weight fat into the meat (ex: 18 lbs meat +2 lbs fat) Grind the meat, then the fat, then mix together and run through the grinder one more time.

Sausage: Just like the burger but with 20% fat. Mix the seasonings when mixing the meat and fat.

Brats: I just started doing brats last year and so far have had nothing but success. I start out just like I'm doing burger or sausage by grinding the meat and fat through my larger cutting plate. Then I mix the meat and fat (20%) and seasonings together and grind it together this time through the small cutting plate. Then if I have anything else to add I mix it in at this point (usually chopped jalepenos or cheese).

I use natural hog casings to stuff the bratwurst. Again using the grinder with the sausage stuffing attachment I take the meat mixture and run it into the casings. Don't stuff the casing as tight as it will go because later when you twist it into links it will fill out the remaining capacity of the casing without breaking. If the casing is too full it could break when you try to form links. At this point you can either smoke them or just freeze them. I froze mine last year but this year I will smoke some to see how it turns out.

I watched youtube videos and searched many websites before attemtping my first batch. I didn't follow any one process exactly as written, instead I cross referenced 4 or 5 and pieced together my process from what seemed like the most common steps from each source. You can always make fresh brats and then grill them later, I prefer to smoke all of mine so they are pre-cooked and I can just warm them up on the grill when I want to eat them. I was just trying to clarify that I found the summer sausage could easily be baked and taste about the same, but I wouldn't try that with brats as the smoke adds a lot of flavor to them.

What I had found was the 3" mahogany casings for summer sausage don't allow much smoke in anyways, so I just smoke them for a couple of hours and then bring up to temp in the oven. I'm not sure the smoke adds any flavor when using less porous casings. Just noticed I never answered your question about the smokers from before. I'm no expert on smokers, I used to have the "can" style electric smoker from Char Broil, but it wouldn't produce smoke unless the temp was really high and that dried out my brats. I got a Bradley digital for Christmas a couple years ago and it works great. I think the trick is having two separate heating elements in there. One that smokes the wood pucks, and one that controls the oven temp. I can produce nice smoke at really low temps which is great.

"Budget friendly" is a relative term, but my Bradley was around $500 which is getting up there. Head over to www.smokingmeatforums.com and you can get any and all questions answered about smoking. it's a great site with smoker reviews as well. Overall, I think you'll have a tough time finding a cheap smoker that will do links real well. I'll also toss in a few notes from my processing this year, just completed my doe this weekend.

1. I tried a few more recipes out of the book I mentioned above. Sicilian style sweet italian sausage, bratwurst and southwest pork sausage. All were excellent, I really love that book.
2. I went with collagen casings and had mixed results. A few batches came out okay, but I had some where the casings were wet and falling apart at the end. Looks like they need higher temps in the smoking process. I'll go back to natural casings next year.
3. Last year my sweet italian sausage links were a little bitter after smoking. This year I went without smoking and just put them in 170 deg water until they hit 155 degrees. They came out really nice, and the italian flavoring came through real well instead of just tasting smoke. That was a new method for me and worked pretty well. I'll bet you could probably put them in a roasting pan, covered and with water in the bottom and it would work well too.
4. I just baked my summer sausage (in 3" mahogany casings) in the oven instead of smoking. First time I tried that and they came out great, so again I'm not sure how much smoke actually gets through those casings anyways.

Overall, happy to have another year with a stocked freezer even if I didn't get an elk this fall. Thanks again Vids. I made up 3 kinds over the holiday out of the Home Sausage Making book. I did the moose sausage and the venison sausage from the game section using elk. Both turned out good but I will cut back on the lemon zest if I make the venison recipe again. I also did the summer sausage and it also turned out good. Easy to just stuff the casing and stick in the oven. I learned that I like more garlic than whats called for so I will up that next time too. So overall I am loving that book.

I also learned I really need a smoker. I worked a shift at the fire station Friday and took some elk meat to share for dinner. There is a smoker there so we smoked the meat then finished in the oven. It was so good. I will be getting one soon. Sausage making was one of my favorite parts of school. Fill your cooler with ice and water and drop your grinder parts in it for 30 minutes or so between batches. I like the idea of pre cubing, measuring and freezing meat. If you have the freezer space freeze the cubes of meat individually on cookie sheets. When I am making meat products I try to make them in winter months, and use cardboard boxes on the porch overnight to freeze things. An aluminum pan put on the cold concrete of the porch with a card board box o ver it to keep neighborhood cats out will work when temps drop to the mid-low 20s. For fat, none of my local grocery stores will sell it to me, since they really don't package/cut meat much, and the ones that do put thier trim fat into thier ground meats and sausages. Find a butcher shop that processes carcasses or primals. The local organic meat market sells me beautiful, clean suet for $1.99/lb, $2.99/lb finely ground. As far as smokers go, my budget has not allowed the purchase of one, but I have built two of the four smokers that Alton Brown has built on his TV show, been very satisfied with the results, and not been upset when playful, mischievious kids damaged or destroyed part of one. Gimmie a couple of days and I can probably find youtube links for building all four designs. Bon Appitie Magazine in July of this year had a smoker built from a filing cabnet. Feed your inner McGuiver in all senses of the word.

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