The effect of refrigerated storage of raw milk on the physicochemical and microbiological quality of Tunisian semihard Gouda‐type cheese during ripening

A Tunisian semihard Gouda‐type cheese made from milk kept at 4 °C for 24, 48, 72 and 96 h was monitored during 45 days of ripening. The effect of milk refrigeration on the evolution of physicochemical parameters in relation to the quantitative variation of the microbial population during ripening of Gouda‐type cheese was investigated. Microbiological and physicochemical analyses were performed on raw milk and cheese samples after curding, 2, 9, 16, 23, 30, 37 and 45 days of ripening time. The raw milk kept under refrigeration at 4 °C for 96 h showed the highest microbial count and proteolysis level. The duration of storage significantly reduced the cheese yield as a result of important solubilisation casein in proteoses‐peptones. Results of different nitrogenous fractions by Kjeldahl method showed enzymatic hydrolysis products of casein whose intensity depended on the maturing stage as well as the refrigeration time. Besides the evident action of the plasmin, original milk protease, on the hydrolysis of casein in soluble fractions, the proteolysis of cheese caseins is also initiated by proteolytic action of the chymosin and extracellular heat‐resistant proteases notably produced by the same psychrotrophic microflora. Lactic acid bacteria starters that constitute the dominant microflora of this type of cheese are also considered as aroma precursors.     

Storage stability of traditional Tunisian butter oil produced from spontaneous fermentation of cow's milk

Physicochemical characteristics of traditional Tunisian butter (TTB) and thermal stability of the butter oil were studied. Thermal stability of samples was estimated by using the accelerated shelf life testing method. Effect of heating on some quality characteristics of traditional Tunisian butter oil (TTBO) was investigated and compared at different temperatures (60°C, 100°C and 130°C). Induction period of sample heated at 60°C was important compared to that at 100°C and 130°C. This result may indicate the sensitivity of TTBO to elevated temperature. Absorption at 232 and 270 nm, acidity and PV increased rapidly after reaching the oxidation induction time. The temperature had a significant effect on the formation of oxidation products in traditional butter oil (TTBO). Fatty acid composition of TTBO was also changed after heating. This may explain the observed modification on some quality characteristics of TTBO such as viscosity, hardness, cohesiveness and colour. As a consequence, heat treatment produced alterations in the oxidative status of the butter oil that could affect the shelf life.     

INDUCTION, PROPERTIES AND APPLICATION OF XYLANASE ACTIVITY FROM SCLEROTINIA SCLEROTIORUM S2 FUNGUS

Extracellular xylanase activity was produced in a submerged culture by Sclerotinia sclerotiorum S2 fungus, on wheat bran as an inducing substrate. The enzyme was partially purified and biochemically characterized. The optimum pH and temperature for the activity were 5.5 and 65C, respectively. The xylanase was stable over a pH range of 4.0–9.0 and retained more than 75% of its activity after incubation for 24 h without substrate. The enzyme was stable at 50C and 60C for 80 min of incubation at pH 5.0, while the half‐life was 20 min at 70C. The novel xylanase activity was useful as an aid in orange juice clarification. The clarification was observed with concomitant production of reducing sugars (0.350 mg/mL from juice) after 24 h of incubation of juice with the xylanase. Xylanase aided juice clarification, which resulted in a 27% decrease in insoluble materials.     

VARIATIONS IN ESSENTIAL OIL COMPOSITION DURING MATURATION OF CORIANDER (CORIANDRUM SATIVUM L.) FRUITS

The composition of volatile components of the essential oils extracted from fruits of coriander ( Coriandrum sativum L.) at four stages of maturity was studied by gas chromatography–flame ionization detector and gas chromatography-mass spectrometry. Essential oil yields showed marked increase during maturation process and 41 compounds were identified. Linalool was the main compound in all ripening stage amounting from 36.69 to 72.35%. Geranyl acetate (35.17%), β-caryophyllene (3.47%) and borneol (3.26%) were the main compounds at the first stage of maturity (immature fruits). At the second stage, geranyl acetate (8.21%), camphor (4.01%) and menthol (2.96%) were reported as the main constituents. In the third stage, geranyl acetate (20.66%), α-humulene (5.44%) and limonene (1.68%) were the main compounds. At the final stage of maturity (mature fruits), essential oil consist mainly on geranyl acetate (1.49%) and borneol (0.97%) in addition to linalool (72.35%). Additionally, accumulation of monoterpene alcohols was observed during maturation process of coriander fruit.

PRACTICAL APPLICATIONS

The chemical composition of the essential oil of coriander ( Coriandrum sativum L.) fruits is characterized by the occurrence of numerous components with great economical values. The oxygenated monoterpene and linalool were the most used ingredient in the cosmetic, perfume and food industries. On the other hand, the increasing market demands of the oxygenated components, widely present in this oil make it in the first rank of natural product. The formers were only obtained from the mature fruits. In this context, knowledge of their evolution during maturation process of the fruits seems to be a helpful tool for a better use of this economical potential.     

A comparison of the physicochemical, microbiological and aromatic composition of Traditional and Industrial Leben in Tunisia

Traditional Tunisian Leben (TL) was produced according to the traditional method. Physicochemical and microbiological characteristics and major aromatic compounds were studied and compared to industrial Leben (IL) and experimental Leben (EL). The results show a decrease in lactose content and pH value and an increase in lactic acid during spontaneous fermentation. TL and EL were characterised by higher protein, lactose and ash contents but were less fatty and acidic than IL. Lactic acid bacteria (LAB) and yeasts present in TL were responsible for lactic acid fermentation and aroma development. The LAB and yeast counts in TL were higher than that in EL and IL. Dynamic headspace extraction procedure shows the existence of four major volatile compounds: acetaldehyde, ethanol, diacetyl and acetoin in TL, IL and EL. However, TL has the most important quantity of aroma.