This chapter contains sections titled: Introduction Open texture and eye formation Gas formation through propionic fermentation Cheese structure and eye formation Conclusions References

Cheese flavour is the result of a complex mixture of volatile compounds, originating mainly from the enzymatic degradation of curd components by cheese microflora during cheese ripening. Directing cheese flavour development requires knowledge on inter- and intra-species contributions to flavour development, i.e. identification of the volatile (flavour) compounds produced by each strain. The aim of this study was to identify the volatile compounds produced in Swiss cheese by Propionibacterium freudenreichii, one of the species essential for the development of the characteristic flavour of this type of cheese. The volatile profile of compounds obtained from small-scale (1/100) Swiss cheeses, with or without P. freudenreichii, were compared (three strains tested, in association with three thermophilic lactic starters, i.e. twelve cheeses, manufactured in duplicate). Neutral volatile compounds, extracted by dynamic headspace, and free fatty acids were identified using gas chromatography-mass spectrometry. The concentrations of all carboxylic acids and 14 of 58 neutral compounds were significantly higher in the presence of propionibacteria (PAB). The three PAB strains tested produced the same volatile compounds, but observed quantitative differences were strain-dependent. Propionic acid and four propionate esters were detected only in the presence of PAB. Moreover, cheeses with PAB contained two- to three-fold higher levels of free fatty acids derived from lipolysis and five- to fifty-fold higher levels of branched-chain compounds derived from isoleucine catabolism (2-methylbutanal, 2-methylbutanol and 2-methylbutanoic acid) and from leucine catabolism (3-methylbutanoic acid). Lactic starters induced significant variations in the concentrations of some of the compounds produced by PAB, such as methylbutanoic acids and free fatty acids, which varied by 2.0 and 1.4, respectively, as a function of the lactobacilli strains. PAB strains affect the concentration of varied volatile compounds and could therefore have distinct contributions to the formation of Swiss cheese flavour.

Isovaleric acid (3-methylbutyric acid) and 2-methylbutyric acid contribute to Swiss cheese flavour. In order to determine the contribution of propionibacteria (PAB) to the production of methylbutyric acids, mini-Swiss cheeses were manufactured with or without PAB as a secondary starter (25 Propionibacterium freudenreichii strains), associated with different cultures of thermophilic lactic starters. In the presence of PAB, the quantity of methylbutyric acids was three to ten times greater, depending on PAB strain, than in the absence of PAB, regardless of the species and the strain of lactobacilli used (20–63 vs. <6mgkg−1 ripened cheese). PAB produced methylbutyric acids concomitantly with acetic and propionic acids, then kept on producing methylbutyric acids after propionic fermentation. The increase in salt-in-moisture content of cheese from 1.0% to 1.8% induced a strain-dependent inhibition of isovaleric acid production. This study shows that P. freudenreichii is the main contributor to methylbutyric acid production in Swiss cheese.

Isovaleric acid (3-methylbutyric acid) and 2-methylbutyric acid are cheese flavour compounds and are produced in Swiss cheese mainly by propionibacteria (PAB), with large variations in amounts depending on the strain. To evaluate the possibility of screening this property in vitro, the production of both acids (referred to as methylbutyric acids) was compared for eight Propionibacterium freudenreichii strains both in mini-Swiss cheeses and in liquid cultures (salted-yeast extract-peptone-lactate medium, pH 5.4, incubated at 24°C then at 6°C). Similar time courses of methylbutyric acid production were observed in cheeses and in liquid cultures. PAB produced methylbutyric acids during propionic fermentation at 24°C (8–37mgkg−1, and 5–13mgL−1, respectively, in cheeses and cultures), and during further storage at 6°C for 2 months (11–77mgkg−1 and 5–17mgL−1). PAB strains significantly (P<0.05) influenced the amounts produced, and they were similarly classified in cheeses and in liquid cultures. This simple screening method has potential for selection of high- or low-producing strains for cheese manufacture.

Four methods were developed or revised to determine the functionality of melted Emmental-cheese (Swiss-cheese). Flowability was measured using a modified Schreiber test. Stretchability was assessed by a new method involving vertical traction. A butyrometric method was applied to quantify oiling-off. Browning of cheese-gratin was measured objectively with a colour meter (L*, a*, b*). The repeatability and the power of these tests were satisfactory. The functionality of 48 french commercial Emmental-cheeses was determined over one year. Great variations were registered. The flowability index (melted cheese area divided by the initial area) ranged from 1.0 to 2.7. The oiling-off ranged from 7 to 17 g fat per 100 g of cheese (30 to 60 g.100 g(-1) fat). The length of the strings of melted cheese (stretchability) ranged from 80 to 950 mm. Ranges of 13 a.u. L*, 9 a.u.a* and 9 a.u.b* were observed for colour measurement. Meltability, stretchability, oiling-off and browning were strongly correlated. Finally, Principal Component Analysis showed the great diversity of the functionality of melted French Emmental-cheese.

This work aimed to improve the knowledge and the control of interactions between lactic acid bacteria (LAB) and propionibacteria (PAB) in order to control the propionic acid fermentation in European hard cheeses, and therefore their quality. Among more than 50 couples of strains LAB/PAB used in cheese making, some pairs of strains were chosen in function of quality results on cheeses (sensorial analysis). A whey model system and an alternative conductimetric method, with good reproducibility were used to study interactions. The lipolytic activity of PAB strains varied by a factor 4 to 5. The aroma of ripened Emmental cheese was linked to the level and the type of lipolysis in the cheese, correlated with lipolytic activity of PAB. The main pathways of the utilisation of lactate by PAB, studied by 13C RMN, were useful to understand the balance between final products: acetic, propionic acid, CO2.

The effect of salt content on the propionic acid fermentation from 23 strains of Propionibacterium freudenreichii was studied in small-scale Swiss-type cheeses. Two-hundred-and-thirty four cheeses were made with low bacterial count microfiltered milk. Salt in moisture (S/M) of the cheeses (w/w) was 1% (i), 2% (ii) or 3% (iii). During hot room ripening, these values were found respectively in the center of the loaf of Emmental-cheese (i), in an area between 2-8 cm depth (ii), and in the vicinity of the rind (iii). The propionic acid fermentation was followed weekly by measuring cheese volume, volatile fatty acids (VFA) and lactate content during hot room ripening. The increase of S/M reduced dramatically the propionic acid fermentation. In 3% S/M cheeses, VFA contents were 4 to 5 times lower than in the 1% S/M cheeses. Residual lactate content increased in the same extent. Cheese opening was about 12 times lower in 3% S/M cheeses. The effect of increasing S/M was very strain dependent, from low stimulation for some strains to a total inhibition of the propionic acid fermentation for the other ones. The sensitivity to salt was independent from the technological abilities in usual conditions (1% S/M). CO2 production was the most affected by S/M increase. Some salt-sensitive strains showed a molar propionate/acetate ratio close to one. This value corresponds to the third metabolic pathway described for propionibacteria: three lactates --> I succinate + 1 propionate + 1 acetate. A good discrimination of the 21 strains from our own collection was obtained by factorial correspondence analysis (FCA). (C) Inra/Elsevier, Paris.

Eighteen strains of Propionibacterium freudenreichii and one strain of P thoenii were tested for their ability to produce volatile fatty acids and gas in small scale Swiss-type cheese. Microfiltrated milk and standardized propionic strains were used. Sixty-six mini-cheeses were made according to a standardized classical emmental cheesemaking process. During the hot room ripening, weekly determination of cheese volume and bimonthly analysis of volatile fatty acids and lactate contents permitted to distinguish five groups of strains. Propionate content and change in cheese volume were the most discriminant parameters. Four groups of strains showed different technological properties, where propionic fermentation seemed to be achieved after 14 to 28 days. Differences appeared in latent period, fermentation velocity and synchronism of gas and propionate productions. The strains of the last group were unsuitable for Swiss-type cheese technology. Some of them did not grow in the cheeses, some grew very slowly and others produced volatile fatty acids but not CO2 on a great scale. Weekly determinations of volatile fatty acids and lactate contents permitted a better characterization of the more precocious strains. As a general rule, the technological properties also reflected differences in growth and metabolic activities.

L'aptitude technologique de 18 souches de Propionibacterium freudenreichii et d'une souche de P thoenii a été déterminée en technologie emmental.Soixante-six minifabrications de fromages à pâte cuite de type emmental ont été effectuées à partir de lait microfiltré.La préparation des ferments propioniques et la fabrication des fromages étaient standardisées.Pendant l'affinage en cave chaude, des mesures hebdomadaires du volume des fromages et des analyses bimensuelles de teneur en acides gras volatils et en lactates ont permis de classer statistiquement les souches en 5 groupes.L'évolution du volume et de la teneur en acide propionique sont des indices complémentaires, mais aussi les indices les plus discriminants.Les souches des 4 premiers groupes présentent une aptitude technologique certaine, avec des différences notables de temps de latence, de vitesses de fermentation et de synchronisme des productions de propionate et de gaz carbonique.La fermentation de ces souches semble s'achever après 14 à 28 jours.Les souches du dernier groupe apparaissent inadaptées en technologie emmental: absence de croissance, croissance lente ou production d'acides gras volatils sans dégagement gazeux massif. Propionibacterium! emmental! acide gras volatil! CO2! affinageSummary -Technological properties of pure propionibacteria strains: test in small scale Swiss-type cheese.Eighteen strains ofPropionibacterium freudenreichii and one strain ofP thoenii were tested for their ability to produce vo/atile fatty acids and gas in small scale Swiss-type cheese.Microfiltrated milk and standardized propionic strains were used.Sixty-six mini-cheeses were made according to a standardized c/assical emmental cheesemaking process.During the hot room ripening, weekly determination of cheese volume and bimonthly analysis of volatile fatty acids and lactate contents permitted to distinguish five groups of strains.Propionate content and change in cheese volume were the mast discriminant parameters.Four groups of strains showed different technological properties, where propionic fermentation seemed to be achieved after 14 to 28 days.Differences appeared in latent period, fermentation velocity and synchronism of gas and propionate productions.The strains of the last group were unsuitable for Swiss-type cheese technology.Some of them did not grow in the cheeses, some grew very slowly and others produced volatile fatty acids but not C02 on a great scale.Weekly