During sausage fermentation, drying and ripening a number of microbial, biochemical and physico- chemical reactions take place in the sausage mince converting the raw meat mixture into a firm, sliceable product.
The sensory quality of the fermented dried sausage is determined by its appearance, texture and flavor and those criteria may be further divided into specific parameters that are commonly sought when discussing sausage quality in the industry.
Sausage appearance is largely covered by its color, but descriptors such as sausage structure, particle size, glistening of fat, appearance of tendons etc. are also of major part of the appearance.
However, those parameters are primarily determined by the mince production procedure and not so much by the changes that go on during the conversion of mince into dried sausage.
Color formation
The overall color of a fermented sausage is determined by the color tone and intensity of the meat and fat particles.
The color of the meat particles is partly determined by the meat type (chicken is lighter than pork and beef, and horse is very dark) and partly by the color forming reactions going on in the meat during the production process.
The color of the fat is primarily a result of the quality of the raw materials.
The color of fresh meat is caused by the content of myoglobin and oxymyoglobin that have purple and bright red color tones, but are not very stable.
During sausage production myoglobin and oxymyoglobin are transformed via a number of reactions, including nitrite, into the more stable nitrosylmyoglobin that is dark red and gives the sausage the typical reddish-brown appearance.
During preparation of the sausage mince the added nitrite acts as a very reactive oxidant and is rapidly reduced into nitric oxide (NO) parallel to the oxidative formation of metmyoglobin (the iron atom in the heme group of the molecule is oxidized from the ferrous (Fe2+) to the ferric state (Fe3+)).
This results in an immediate grayish discoloration of the mince. Later in the process NO reacts with metmyoglobin and myoglobin to form nitrosylmyoglobin, simultaneously converting the grayish color to reddish.
This reaction is promoted at reducing conditions as the iron atom in metmyoglobin must be reduced to Fe2+.
Figure 2 shows a simplified reaction scheme.
Apart from being produced during metmyoglobin formation, NO is also formed by microbial reduction of nitrite or chemically from nitrous acid, in particular if the sausages are added ascorbate.
Thus ascorbate speed up color formation (Figure 3). It is not certain which reactions predominate since the color formation mechanisms are not fully elucidated.
However, as mentioned above low redox potential will in general promote and stabilize color.
That is, lack of oxygen and other oxidative compounds in the mince and the presence of anti- oxidative components such as sodium ascorbate, alpha-tocopherols (vitamin E), phenolic compounds from added spices etc.
When nitrate is used instead of nitrite as the color forming agent, the nitrate molecule must be reduced to nitrite before the color forming reactions can take place (Figure 3).
This conversion is performed by Micrococcaceae species producing nitrate reductases during growth in the mince.
Inevitably, this means that the color forming process will be more dependent on the activity of the Micrococcaceae species and that color formation will take longer than in sausages with added nitrite.
Since Micrococcaceae species are inhibited at low pH, sausages relying on nitrate reduction must be fermented by a traditional process unless the Micrococceae are specifically selected to perform at low pH.
Nitrate is still used by many sausage manufactures because nitrate serves as a long time reservoir of nitrite, but it has also been reported that sausages cured with nitrate have a better flavor than when cured with nitrite.
Color stability
During storage of the finished dried sausage, in particular if sliced, the sausage color is prone to fading and becoming grayish.
This is caused by oxidation of the hem group of the nitrosylmyoglobin molecule as the ferrous iron is oxidized to the ferric state.
In general, the susceptibility of nitrosylmyoglobin to oxidize is tightly linked to lipid oxidation and redox potential and increases with decreasing pH.
Parameters such as atmospheric oxygen, oxidized (rancid) fat containing large amounts of peroxides and free radicals, and hydrogen peroxide producing microorganisms growing in the sausage or on the surface of the slices will have a negative impact.
In order to avoid pigment oxidation to take place, anti-oxidative compounds are added to the sausage mince as mentioned above, and the sausages are packed in air-tight or modified atmosphere packages.
Growth of Micrococcaceae species in the sausage and their ability to produce catalase will reduce the redox potential and peroxide accumulation, respectively.
Texture formation
Formation of the correct texture is an important part of the overall quality of fermented dried sausages and commonly texture is described by the attributes hardness, firmness, fattiness, juiciness, stickiness, tenderness, softness, granularity etc.
In general, Southern European style sausages are much firmer than the Northern European and US style sausages that tend to be more soft, elastic and rubbery.
Sausage texture is a result of the physico-chemical reactions taking place in the sausage mince during the fermentation and drying cycles and is influenced by the mince ingredients as well as the processing procedure.
Simplified, the texture formation process can be divided into three steps: extraction of proteins during and after meat mincing, formation of a protein gel during fermentation and release of water during drying.
During mincing the added salt solubilizes and extracts proteins (primarily myosin) from the meat myofibrils forming a sticky protein film around the mince particles.
In the succeeding fermentation process, pH decreases, coagulating the solubilized proteins, forming a firm gel that embeds and binds the fat and meat particles closely together.
Coagulation by acidulation is associated with the release of water and this water is removed continuously in the beginning of the drying process.
As the drying process continues the more tightly bound water will be released as well, but at a slower rate.
Depending on the processing parameters and the drying time the resulting texture will exhibit different properties.
The protein extraction during the mincing procedure is directly influenced by the intensity of chopping and salt concentration.
High protein extraction will result in a sausage with a more elastic texture, on the other hand high protein extraction could induce too high-water binding capacity of the mince impeding the drying process.
Additionally, salt interacts with the myofibrillar proteins, decreasing their iso-electric point from approximately pH 5.3 down to as low as pH 4.3 depending on the salt concentration as shown in Figure 4.
This has a drastic effect on the water-binding capacity of the proteins since the intermolecular spaces for retaining water is at a minimum at the iso-electric point.
Thus as the pH value approaches the iso-electric point during the fermentation cycle, the release of water is increased.
However, since pH-lowering will also induce coagulation of the meat proteins and this process begins around pH 5.3, the gellification process and the partly entrapment of water will start at pH below 5.3, opposing the water release that otherwise could have taken place.
In fact, practical experience shows that sausage recipes with normal salt quantities show an optimal iso-electrical range from 4.8 to 5.3; in general acidification to pH below 4.8 will not increase the rate of water loss.
As described above, the fermentation process is of utmost importance for the texture formation in fermented dried sausages.
In fact, texture development during fermentation is determined by the drop in pH, primarily, whereas the further texture development during drying is determined by the loss of water.
Hardness increases sharply when sausage pH reaches 5.3 and is further increased until pH 4.8. If pH is not lowered below 5.3, it is necessary to reduce the aw to below 0.90 in the drying process to ensure formation of sausage texture, but the texture may still not become optimal.
In order to control texture development, it is therefore essential to control the fermentation process.
Flavour formation
Flavor is a sensorially perceived quality involving complex interactions between taste and aroma, and is also influenced by texture and other sensory sensations arising during the eating process.
With regard to dried sausage, this involves as an example the burning and stinging sensations from spices such as cayenne pepper or hot peppers that are added to the mince.
However, in the following, the description of flavor formation will be focused on the characteristic taste and aroma formation that takes place during the sausage production and which is determined by the processing procedure.
In fermented dried sausages the formation of aroma plays a more important role than the formation of taste for obtaining the characteristic sausage flavor.
This is due to the high sensitivity of the nasal receptors for the volatile aroma components released during chewing of the sausage and due to the very complex volatile profile.
It has been shown that the taste fraction of dried sausages are just composed of broth-like, sour, salty and bitter tastes of no similarity to the sensory experience when eating dried sausage, whereas the aroma fraction consists of an immense number of aroma notes with different characteristics that when mixed together create the dried sausage odor (table 1).
The volatile compounds responsible for the odor notes in table 1 cover a wide variety of compounds such as aldehydes, acids, ketones, esters, sulfides, thiols, O-heterocycles and more.
Many of those compounds are formed in the product by enzymatic and chemical reactions during production, whereas others arise from added spices, surface smoking (in North European style sausages), and from the raw materials of the meat and fat (e.g. boar and mutton taints).
However, the compounds formed during the production process are by far the most important compounds for creating the typical dried sausage flavor.
Simplified, proteins in the mince are hydrolyzed into smaller proteins and peptides by endogenous enzymes of the meat, and those peptides are further hydrolyzed into amino acids by microbial enzymes.
Hereafter, the amino acids are degraded into a wide variety of volatile compounds such as aldehydes, acids, thiols and sulfides with very low sensory threshold values.
Lipids are hydrolyzed both by adipose and microbial enzymes, releasing unsaturated and saturated free fatty acids that are oxidized into aldehydes and ketones by both chemical (autoxidative) and microbial oxidation. Sugars are degraded into primarily lactic acid, which is a taste compound, but also into side products such as acetic acid and diacetyl, contributing with vinegar and buttery aroma notes.
Regarding taste compounds formed during production, primarily lactic and acetic acids contribute to the sour taste, and free amino acids and small peptides to the broth-like and bitter taste.
NaCl added to the mince will of course contribute to saltiness whereas nucleotides are not considered being of importance for the taste.
The microorganisms responsible for producing the aroma compounds are primarily Micrococcaceae species of the genera Staphylococcus and Kocuria, but also lactic acid bacteria, yeast and, in molded sausages, the fungi growing on the surface.
When comparing the flavor profiles of typical South European sausages versus North European types it has been shown that except for relatively few compounds, the volatile compounds are much the same but the relative proportions different.
South European sausages contain typically higher levels of lipid oxidations products, fruity esters and specific aldehydes from amino acid degradation, whereas North European sausages in general contain higher amounts of lactic and acetic acids. Regarding specific differences, molded sausages contain a very characteristic popcorn-smelling compound with an extremely low sensory threshold value.
Those differences are readily reflected in the sensory perceived aroma: South European sausages are rated as having the most complex flavor being more rancid, fatty, oily, porky, nutty, cheesy, flowery and ammonia-like than the North European style sausages, which on the other hand are more acidic tasting.
The differences between US, North and South European sausages are not so much a result of differences in the mince ingredients as to the starter culture and the processing procedure.
The major cause for differences in the aroma profiles is the milder acidification in the South European technology that enables better growth of the Micrococcaceae within the starter culture and also the longer ripening period and mold coverage.
However, it has been shown that sausages with added nitrate instead of nitrite contain higher levels of many essential aroma compounds from amino acid degradation and that the softer fat often used in South European technologies is more prone to chemical oxidation and rancidity.
Source: Chr. Hansen