Fermented meat products

Food Research International 27 (1994) 299-307

Fermented meat products Friedrich-Karl Liicke Microbiology Laboratory, FB Haushalt & Erniihrung, Fachhochschule Fulda, PO Box 1269, D-36012 Fulda, Germany

This paper provides general information on the fermentation of meat, the types and manufacture of the resulting products, the microorganisms involved and the factors affecting microbial activity. Subsequently, recent developments in the following three main research fields are reviewed: (i) Instrumental control of meat fermentation: Sensors for continuous measurement of fermentation parameters such as pH, water activity and weight loss of fermenting meats have been developed, making the on-line control of the fermentation climate feasible. This could lead to a marked reduction in fermentation time and costs without affecting product quality. (ii) Selection of antagonistic lactic starter cultures: Such cultures, ideally, would kill rather than inhibit pathogens, not only in fermented meats sensu strict0 but possibly also in non-fermented products such as sliced perishable meats. (iii) Role of microorganisms in flavour development: A better understanding of the effect of microorganisms (in particular Micrococcaceae and their ability to scavenge oxygen, destroy peroxides and hydrolyze lipids and proteins), meat enzymes and non-enzymic reactions on aroma and flavour of fermented meat is required to maintain a large diversity of fermented meats and to improve their sensory quality and shelf life. Keywords:

fermented

meat, microbial

activity,

instrumental

control,

starter

cultures.

INTRODUCTION: WHAT ARE FERMENTED MEAT PRODUCTS?

Historically, it appears that the first fermented sausages were made in certain parts of the Mediterranean. This may be due to the fact that a proper climate (temperature and relative humidity) is crucial for the drying process, and fairly stable wet and cool conditions prevail there during the winter. Nowadays, more than 700000 t are produced and consumed within the EEC and EFTA countries each year, particularly in Germany, Italy, Spain and France (Liicke et al., 1990). Production figures in the New World are much lower: in the United States, about 153 000 t dry and semi-dry sausages were produced in USDA-certified plants in 1981 (Adams, 1986). There are few traditional fermented meat products in Asian and tropical countries, ‘nham’ (a meat product popular in south-east Asia) being an exception (Campbell-Platt, 1987). The Chinese sausage of the ‘Lup Cheong’ type is a dried, but not fermented meat product (Leistner, 1986a). Dry cured, unground raw meats are mainly preserved by salting and drying, and excellent raw hams can be prepared without significant microbial activity. Rather, the activity of meat enzymes

According to the definition given by CampbellPlatt (1987), a food is termed ‘fermented’ if it ‘has been subjected to the action of microorganisms or enzymes so that desirable biochemical changes cause significant modification of the food’. This applies to most types of raw sausages and raw hams. Fermented sausages are defined as ground meat mixed with salt and curing agents, stuffed into casings and subjected to a fermentation process in which microorganisms play a crucial role. Most fermented sausages are dried and can be stored with little or no refrigeration. In Europe, the manufacturing pracess does not include a final heat treatment although some varieties (especially fresh raw sausages) are consumed after cooking. In the United States, fermentation of sausages made from ‘uncertified pork’ is followed by heating to 58~3°C to kill any trichinella possibly present. Food Research International 0963-9969/94/$07.00 0 1994 Canadian Institute of Food Science and Technology 299

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is important for the development of the aroma and tenderness of these products. Bacteria are needed mainly for the reduction of nitrate which is still frequently used as a curing agent, and bacteria have also been reported to improve the flavour. In addition, injection of lactic acid bacteria along with sugar has been suggested in order to lower the pH of hams and facilitate water removal. Accordingly, some bat terial strains are available or have been suggested as starter cultures (see Hammes, 1986; Leistner & Lticke, 1989; Lticke et aZ., 1990, for reviews). Some perishable meat products may also be preserved by addition of selected strains of lactic acid bacteria antagonistic to pathogens and spoilage flora (see Lticke & Earnshaw, 1991, for a review). Ideally, however, the added bacteria should not change the flavour and appearance of the product. In particular, a large drop in pH is not desired in meat or most meat products. Such foods are, by definition, not fermented, and the cultures used are more aptly called ‘protective cultures’ rather than ‘starter cultures’. Reviews on fermented meats have been provided by Liepe (1983), Bacus (1984), Liicke (1985), Adams (1986), Leistner (1986a) and Leistner & Liicke (1989). The present paper will focus on the microbiology and technology of fermented sausages.

From a regulatory point of view, fermented sausages are usually subdivided according to their raw material (meat type, fat content) and their degree of drying. These factors make up most of the production costs. Criteria are, for example, the water/protein ratio or, as in Germany, the content of collagen-free protein. Table 1 indicates the parameters that the processor may vary in order to produce different products. There is a large variety of fermented sausages, and consumer preference varies considerably between countries, even between different regions within Germany. For example, ‘sour’, smoked, semi-dry sausages are well accepted in countries like the US or The Netherlands while consumers in many other countries, particularly in France and the Mediterranean, do not readily accept smoked sausages, particularly those with an unbalanced sour taste. In addition, such sausages are more prone to fat deterioration and less suitable for longterm storage. Spreadable varieties comprise about 30% of the raw sausages produced in Germany but are uncommon in all other countries.

SAUSAGE FERMENTATION Meat as a substrate for microorganisms

MANUFACTURE AND TYPES OF FERMENTED SAUSAGES For the manufacture of typical dry fermented sausages, lean meat (60-70%) and fatty tissue (3040%) are comminuted, mixed with about 2.43% salt, curing agents, some sugar, spices and, in many cases, starter cultures. The mix is placed, with inclusion of as little oxygen as possible, into vapour-permeable casings and subjected to a fermentation and drying process. Control of temperature and relative humidity is essential for the production of dry sausages. Ripening chambers with precisely adjustable temperature and humidity are expensive, particularly if they are equipped with smoke generators and smoke combustion devices. This is the main reason why today only a small proportion of fermented dry sausages are manufactured on an artisanal scale, even in the Federal Republic of Germany and France with their many small charcuteries. The chambers also require a high energy input, even though this can be reduced considerably by using ambient air to control the relative humidity (Stiebing et al., 1982).

In contrast to milk, lean meat contains considerable amounts of peptides and amino acids but only small amounts of glucose and glucose-6phosphate (up to 0.15% of each; Dainty, 1986). The content of these fermentable sugars, as well as the content of lactic acid and the pH, depend on the glycogen content of the muscle at slaughter and may vary considerably. As a rule, meat with a pH above 59 contains too little lactate and sugar for a safe fermentation; it binds water tightly and provides better conditions for growth of acidlabile bacteria. Such muscles should be sorted out and used for other purposes. The same is true for various muscles of poultry and limits the use of poultry as raw material for fermented sausages: according to Holley et al. (1988a), not more than 15% of mechanically deboned poultry meat should be added. Furthermore, poultry meat is frequently contaminated with salmonellae. Accordingly, German regulations require a minimal ripening time of 4 weeks for raw sausages containing poultry meat. Obviously, meat fermentation is a solid-substrate fermentation with bacteria growing in microcolonies (Katsaras & Leistner, 1988). In contrast

Fermented

meat products

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Table 1. Parameters influencing tbe qoality of femmted sausages Parameter

Variables (examples)

Guidelines”

Animal species (beef/pork/poultry) Age at slaughter Fats/oils in pig feed; Type of fatty tissue (back/belly) Formulation (fat content)

pH I 5.8; good microbial quality; no antibiotics

Additives

Sodium chloride Curing agent (nitrite/nitrate) Sugar amount Sugar type (glucose/sucrose/lactose/dextrins) Lactic acid bacteria Acidulants Micrococcaceae Ascorbate Spices

Initial a, 0~9554965 Addition of 100 mg NaNO,/kgb 0.247% 0.24.5% of rapidly fermentable sugar pH reduction to I 5.3 during fermentationC

Comminution

Method (grinder/cutter) Degree (coarse/fine)

Low temperature (to avoid melting of fat)

Filling

Filling equipment Casing material (natural/collagen-based/ cellulose-based) Casing diameter

No air inclusions Permeability high for vapour and smoke, low for oxygen; shrinkable, peelable

Ripening

Fermentation climate - temperature - time - humidity (% ERH)

Raw material

Ageing/drying climate - temperature - humidity (% ERH) - air movement - time Surface treatment

Smoke Mould starter

No soft or rancid fat

2 25°C Until pH I 5.3 No vapour condensation; ERH in chamber 5-10 units below ERH of product

I 15°C until a, < 0.90 ERH in chamber 10-15 units below ERH of product Uniform drying

No growth of undesired moulds

‘Certain deviations are possible if proper precautions are made (e.g. low ripening temperatures). bLower amou nt s re q uir’f~lower fermentation temperatures or faster acid formation; use of nitrate instead of nitrite requires lower fermentation temperature. Necessary rate depends on fermentation temperature.

to many other fermentation substrates, it cannot be pasteurized without changing its appearance completely. Hence, any starter culture must be able to compete with the microflora of the raw material. The interior of the meat - be it cornminuted or not - rapidly becomes anaerobic, which restricts growth of obligate aerobes to the surface. Development of the lmicroflora

The conditions prevailing in sausage fermentation (listed in Table 1) ,strongly favour the lactic acid bacteria. Particularly in rapidly fermenting,

smoked products, there is little if any growth of other groups of microorganisms: they are inhibited by pH in combination with anaerobic conditions in the interior and by smoke constituents on the surface, respectively. When acid production is slow or delayed, when nitrate rather than nitrite is used and/or when the products are not smoked, non-pathogenic catalase-positive cocci may attain levels in excess of 107/g at the surface. This is characteristic for sausage varieties common in the Mediterranean and France. The increased risk of growth of undesired acid-sensitive bacteria is compensated for by low fermentation temperatures.

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Table 2. Microorganisms as starter cultures for sausage fermentation (Uicke et al., 1990) Microbial group

Species used as starters”

Useful metabolic activity

Benefits to sausage fermentation

Lactic acid bacteria

L. plantarum, L. pentosus, L. sake, L. curvatus, P. pentosaceus, P. acidilactici

Formation

Catalase-positive

S. carnosus, S. xylosus, M. varians

Nitrate reduction and oxygen consumption Peroxide destruction Lipolysis? Nitrate reduction

Delay of rancidity Aroma formation Removal of excess nitrate

cocci

of lactic acid

Inhibition of pathogenic and spoilage bacteria Acceleration of colour formation and drying Colour formation and stabilization

Yeasts

Debaryomyces hansenii

Oxygen consumption Lipolysis

Delay of rancidity Aroma formation

Moulds

Penicillium nalgiovense

Oxygen consumption Peroxide destruction Lactate oxidation Proteolysis Lipolysis?

Colour stability Delay of rancidity Aroma formation Aroma formation Aroma formation

biotypes 2, 3, 6

“Abbreviations:

L., Luctobacillus; P., Pediococcus; S., Staphylococcus; hf., Micrococcus.

Species composition and the role of desired microorganisms Table 2 lists microbial species available as starters for sausage fermentation. Many of these, in particular the psychrotrophic, salt-tolerant LuctobaciZZus species L. sake and L. curvatus (see Hammes, 1986, for review), the non-pathogenic Staphylococcus species S. xylosus (Fischer & Schleifer, 1980; Delarras & Laban, 198 1; Seager et al., 1986; Selgas et al., 1988), the yeast Debaryomyces hensenii (Leistner & Bern, 1970; Comi & Cantoni, 1980) and the mould Penicillium nalgiovense (Leistner, 19863) also dominate in the ‘spontaneous’ fermentation flora. The role of microorganisms in sausage fermentation is also shown in Table 2. Sausages may be fermented by lactic acid bacteria with little or no participation of catalase-positive cocci. This may still result in a product of acceptable colour and sufficient shelf-life for largescale production and distribution provided that nitrite is used as the curing agent, peroxide formation is avoided and the pH is rapidly lowered to 5-O or below. Such fermented sausages are common in the United States, but are also produced in Sweden, The Netherlands and Belgium. However, consumers in many European countries, particularly in France and the Mediterranean, do not readily accept sausages with a plain sour taste. In addition, such sausages are more prone to fat deterioration and less suitable for long-term storage. By careful

selection of formulations (particularly amount and type of fermentable carbohydrate added) and ripening conditions, manufacturers restrict the activity of the lactic acid bacteria to a level which is sufficient to eliminate microbial hazards but does not interfere too much with the desired activities of the acid-sensitive flora. For the same reasons, catalase-positive cocci are even more frequently used as starters than lactic acid bacteria, and many manufacturers prefer Lactobacillus plantayum and pediococci. At 20-25”C, these form acid more slowly than Lactobacillus sake and L. curvatus (Hechelmann et al., 1988; Landvogt & Fischer, 1990). The stoichiometry and kinetics of fermentation of Belgian-type (smoked) sausage have been thoroughly studied by D. Demeyer and co-workers (reviewed by Demeyer et al., 1986). They found that degradation of amino acids contributes to about lOoh of the metabolized organic compounds, and that only 60% of these (expressed as pyruvate equivalent) are transformed to lactate; 5% end up as acetic acid, and 35% are further oxidized to CO,. These figures are subject to major variations, the trend being that the absence of oxygen and the addition of defined cultures of lactobacilli shifts the stoichiometry towards a more ‘homolactic’ fermentation. Kinetic studies indicate that this could be due to a much shorter lag until initiation of acid formation and, consequently, earlier inhibition of respiratory and/or acetateforming bacteria.

Fermented meat products

Factors affecting microbial processes As already mentioned, the onset, rate and extent of acid formation are critical in the manufacture of fermented sausages. They must be adjusted carefully to achieve both favourable sensory properties and safety from pathogens. The length of the apparent lag phase before measurable acid formation depends on the initial number of lactic acid bacteria adapted to the prevailing conditions. Addition of lactic acid bacteria (usually about 106/g) may shorten the lag considerably, provided they are sufhciently competitive in the (initially cold) sausage mix and they are not damaged during harvesting, freezing or lyophilization. The initial water activity also affects the onset and rate of acid formation (Landvogt & Fischer, 1990). At a, values below 0.955, acid formation may be too slow to inhibit Staphylococcus aureus (Marcy et al., 1985; Hechelmann et al., 1988). However, at a, values above 0.965, salt-labile pathogens compete much better, and additional precautions are needed to suppress these organisms. At initial a, values above 0.97, the risk of faulty fermentation becomes too great (Wirth, 1988). The rate of acid formation is further determined by the type of added carbohydrate, the dominating species of lactic acid bacteria, and the temperature. Generally, glucose, sucrose and maltose are fermented rapidly, and lactose slowly, but the different sugar fermentation pattern of various lactic acid bacteria must be taken into account (Hammes et al., 1990). At 2&25”C, Luctobacillw sake and L. curvatus grow fastest (Hechelmann et al., 1988; Landvogt & Fischer, 1990) while at 40°C (a temperature sometimes used in summer sausage manufacture in the United States), Pediococcus acidilactici is most active. However, at high fermentation temperatures, mesophilic pathogens compete better, and temperatures above 25°C sbould be avoided unless a very rapid pH decline is guaranteed. Because of their high manganese content, natural spices tend to increase the rate of lactic acid formation (Nes & Skjelkvale, 1982; Zaika & Kissinger, 1984). The extent of acid formation depends on the amount of sugar added, on the rate of drying and on the development of an acid-consuming surface microflora (particularly on unsmoked products). If the initial pH is correct (5.8 or below), a pH decline to below 5.3, as normally required for microbiological safety and for optimal drying, can be achieved by adding as little as 0.2 % of a rapidly fermentable

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sugar. Addition of sugar in excess of about 0.7% may lead to undesired secondary fermentation during ageing and storage because only at a, values below 0.92 does the rate of acid formation become negligibly small. Finely ground products of large diameter attain lower pH values than coarsely ground products of small diameter (large surface/volume ratio; Rodel, 1986) because they dry slower, and the better access of oxygen causes some shift from lactic acid to CO, and volatile fatty acid formation (van Hoye & Demeyer, 1987). Use of starter cultures In meat fermentations, the effect of adding starter cultures is smaller than in dairy fermentations because in most cases, sufficient numbers of the microorganisms necessary are present in the raw material. The first defined starter cultures for meat fermentation were introduced in the early 1960s and the use of starter cultures increased in parallel with the need for standardization of the ripening process, the product quality, and the trend towards shorter ripening times. In the United States, a major trigger for the widespread use of starters were outbreaks of staphylococcal food poisoning due to improperly fermented sausages. Lticke et al. (1990) provided data on the current use of starter cultures for meat fermentation in Europe. In the former Federal Republic of Germany, for example, virtually all dry smoked sausages with total ripening times of the order of 3 weeks are produced with the addition of starters, predominantly a combination of a lactic acid bacterium with a Staphylococcus or Micrococcus strain. There is also a trend towards adding such cultures to undried, spreadable raw sausages. Use of defined starters in Mediterranean countries is increasing, but is less common; this reflects the fact that traditional slow processes benefit only slightly from the addition of cultures.

CURRENT RESEARCH TOPICS AND FUTURE DEVELOPMENTS Improved control of sausage fermentation and ageing In commercial practice, both a standard formulation and a time schedule for the ripening of a sausage variety is (more or less empirically) developed. The ripening schedule specifies how long the product remains in which climate. The climate is then adjusted by hand or by timers and/or the

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sausages are transferred from one ripening room to another. In view of the unavoidable variations in the raw material and its microbial load, this does not always exclude batch-to-batch variation in the product. An on-line control of the fermentation and ageing climate could lead to a marked reduction in ripening time and costs without affecting the quality of the products. For example, once the pH has been lowered to 5.3, fermentation can be slowed down by lowering the temperature, and water removal can be speeded up by lowering the relative humidity. Sausage fermentation and ageing can be followed by measuring the pH, the water activity, the weight loss and the firmness. Methods for on-line measurement of the former three parameters have been developed (Rode1 & Stiebing, 1987). An elegant method of controlling the ripening climate is by measuring the water activity of the sausage surface by means of psychrometry (Stiebing & Riidel, 1989). The surface a, value, in conjunction with the pH value, can subsequently be used to adjust the ripening climate so that the difference between the relative humidity in the chamber and the relative humidity at the sausage surface is optimal for the drying process. Improved control of pathogens Despite ail efforts by food hygienists, food-borne diseases are increasing rather than decreasing even in countries with a large supply and variety of cheap foods. Meat regularly contains pathogenic bacteria, and ingestion of fermented sausages has occasionally resulted in outbreaks of illness caused by S. aureus and salmonellae. The control of these pathogens in rapidly fermenting, smoked sausages is well known (see Table 1 and the reviews cited in the Introduction). Later research (reviewed by Lticke & Earnshaw, 1991) has shown that formulations and ripening parameters controlling these bacteria are also inhibitory to other bacterial pathogens. On the other hand, there is an increasing demand for more ‘natural’, less severely processed meat products with no ‘chemical’ additives. A crucial part in the development of new fermented meat products and in the optimization of the manufacture of traditional products is the evaluation of the fate of pathogens. Research is focusing on the selection of lactic starter cultures which, ideally, would kill rather than inhibit pathogens, and/or which could also be used as ‘biological preservatives’ for non-fermented meats.

Much research has been carried out on the fate of S. aureus during sausage fermentation. It turned out that at initial ripening temperatures above 20°C a lactic acid fermentation lowering the pH to about 5.3 is important for the control of this bacterium (see Genigeorgis, 1989, for a review). Consequently, the American Meat Institute in 1982 specified the maximum time allowed in ‘good manufacturing practice for fermented dry or semi-dry sausage’ to reach pH 5.3 (e.g., 80 h at 24°C). If nitrate was used instead of nitrite, the addition of nitrate reducing starters (e.g. Micrococcus variuns, Meisel et al., 1989) contributed to the suppression of S. aureus. In unsmoked sausages, the risk of growth of S. aureus may be greater because the moulds colonizing the surface raise the pH, and antimicrobial smoke constituents are absent. However, if the fermentation temperature is low enough or the rate of acid production fast enough, S. aureusis reliably suppressed (Metaxopoulos et al., 1981; Holley et al., 1988b). However, genuine salamis are made with little sugar, allow only a moderate pH reduction and are therefore traditionally ripened at lower temperatures. When such sausages were fermented at 23”C, addition of lactic acid bacteria (Lactobacillus plan tarum, Lactobacillus sake, Lactobacillus curvatus or Pediococcus pentosaceus) markedly contributed to the inhibition of S. aureus (Hechelmann et al., 1988). This was due to a rapid de-

crease of the pH to values below 5.3, and no evidence for involvement of any other mechanism was found. At lower fermentation temperatures, acid formation became less important. Initial a, values below 0.955 even favoured S. aureus because it slowed down acid formation. Nitrite had only a minor inhibitory effect. Salt and nitrite play an important role in sup pression of salmonellae early in the fermentation when the pH is still above 5.3. Schillinger and Lticke (1989) found that sausages with only 2% NaCl and 40-70 mg/kg sodium nitrite added (about half the usual amount) can be safely produced if the pH is lowered to 5.3 within l-2 days at about 20°C. This, however, was only feasible with very rapid acid producers such as Lactobacillus sake or by the use of acidulants such as glucone-delta-lactone. Strains of lactic acid bacteria differed somewhat in their effect on salmonellae; these differences, however, were found to be largely, if not entirely due to the formation of lactic acids from fermentable sugars (glucose, glucose(j-phosphate, ribose) originally present in the meat.

Fermented

Table 3. Components of aroma and tlavour of fermented sausages Compounds added as such: - Salt

-

Spices Smoke constituents

Products of microbial degradation of carbohydrates: - Lactic acid

-

Acetic acid

Products of protein degradation by microbial or meat enzymes: - Amino acids

-

Peptides Volatile fatty acids Carbonyl compounds

Products of lipid degradation - Medium- and long-chain fatty acids (formed by microbial

-

or meat lipases) Carbonyl compounds Volatile fatty acids Hydrocarbons

(from hydroperoxides)

Transformation products from additives (e.g. smoke or spice

constituents)

Growth potential of Listeria monocytogenes during sausage fermentation is small, and conditions controlling salmonellae and S. aureus will also control this pathogen. However, L. monocytogenes may survive quite well in dry sausages, and it would be desirable to have starter cultures killing rather than merely inhibiting this bacterium, L. monocytogenes is sensitive to various bacteriocins formed by lactic acid bacteria, including some strains competitive in sausage fermentation. Schillinger et al. (1991) tested the effect of a bacteriocin-producing Lactobacilh sake strain on L. monocytogenes in raw sausage mixture. Compared with its bacteriocin-negative variant, this strain gave reproducibly about one log unit lower Listeriu counts at ‘normal initial pH (55-5.7) and a delay of growth of L. monocytogenes at high initial pH (6.36.4). Similar results were obtained by Berry et al. (1990) studying the effect of bacteriocin-producing pediococci in summer sausage fermentation. However, the benefit of bacteriocinogenic strains or bacteriocins in meat fermentations or meat preservation is limited: bacteriocins may act against desired lactic acid bacteria, they are slowly inactivated by binding to meat phospholipids, and resistant mutants of target bacteria can easily be isolated. Suppression of amiple formation

The concentration of biogenic amines in fermented sausages tends to increase with ripening time and the activity of the proteolytic surface

meat products

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microflora. The concentrations vary considerably 1977; Wortberg & Woller, (Vandekerckhove, 1982). Recent work by Tschabrun et al. (1990) indicated that the main cause of variation in the histamine content was the age and microbiological quality of the raw material. However, further research is needed to minimize the amine content in fermented sausages. Better flavour in less time Lticke et al. (1990) collected the opinions of various experts from industry and research on desirable improvements of sausage fermentation, with special reference to starter cultures. Faster acid formation ranked very low, and excessive acid formation, often associated with colour defects, sometimes also with gas formation, appears to be the main defect of fermented sausages in virtually all countries. Priority was given to the acceleration of the formation of the curing colour, aroma and flavour, and to the extension of the shelf-life of the product by delaying oxidative rancidity. Accordingly, research towards a better understanding of aroma and flavour formation, particularly on the role of microorganisms, was advocated in various comments both from meat scientists and from manufacturers of starter cultures. However, the picture is extremely complex because microbial enzymes, meat enzymes, and non-enzymatic reactions all contribute to the development of a ‘balanced’ aroma and flavour (Table 3). In addition, different consumers prefer different flavours. Practical experience shows that extensive ageing of slowly fermented sausages prepared with highquality, firm fatty tissue leads to products with superior aroma and flavour containing higher amounts of products of lipid and protein degradation. It is also commonly observed that some metabolic activities of catalase-positive cocci are beneficial to the sensory quality of fermented sausages, at least for the German, French or Mediterranean consumer. There is some evidence that Micrococcaceue, in addition to reducing nitrate, protect the product from deleterious effects of oxygen by means of their peroxide and hydroperoxide degrading enzymes (Table 2; see Lticke, 1985, for a review). However, they are unable to replace the antioxidative effect of nitrite or to delay the development of oxidative rancidity in fermented sausages prepared without nitrite and nitrate (Riidel, W. & Lticke, F.-K., unpublished). It is still doubtful if and to what extent catalase-

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positive cocci contribute to aroma formation by lipolytic activity. The strains available as starter cultures show little if any lipolytic action on pork fat, and addition of a strongly lipolytic strain has little effect on the aroma and flavour of a mouldripened or chorizo-type fermented sausage (Rode1 et al., 1989; Arboles & Julia, 1991). Because few details are known on the biochemical activities related to aroma and flavour development during sausage ripening, it is very tedious to optimize the ripening process and very difficult to accelerate it without affecting the sensory quality. Likewise, it is very tedious to screen large numbers of potential starter strains for producing a desired aroma or delaying rancidity, let alone to ‘engineer’ strains to produce good-tasting products within a minimum of time.

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