Growth and activity of Bulgarian yogurt starter culture in iron-fortified milk

Bulgarian yogurts were manufactured and fortified with 8, 15 and 27 mg of iron kg(-1) of yogurt. The growth and acidifying activity of the starter culture bacteria Streptococcus thermophilus 13a and Lactobacillus delbrueckii subsp. bulgaricus 2-11 were monitored during milk fermentation and over 15 days of yogurt storage at 4 degrees C. Fortifying milk with iron did not affect significantly the growth of the starter culture during manufacture and storage of yogurt. Counts of yogurt bacteria at the end of fermentation of iron-fortified milks were between 2.1 x 10(10) and 4.6 x 10(10) CFU ml(-1), which were not significantly different from numbers in unfortified yogurts. In all batches of yogurt, the viable cell counts of S. thermophilus 13a were approximately three times higher than those of L. delbrueckii subsp. bulgaricus 2-11. Greater decrease in viable cell count over 15 days of storage was observed for S. thermophilus 13a compared to L. delbrueckii subsp. bulgaricus 2-11. Intensive accumulation of lactic acid was observed during incubation of milk and all batches reached pH 4.5 +/- 0.1 after 3.0 h. At the end of fermentation process, lactic acid concentrations in iron-fortified yogurts were between 6.9 +/- 0.4 and 7.3 +/- 0.5 g l(-1). The acidifying activity of starter culture bacteria in the control and iron-fortified milks was similar. There was no increase in oxidized, metallic and bitter off-flavors in iron-fortified yogurts compared to the control. Iron-fortified yogurts did not differ significantly in their sensorial, chemical and microbiological characteristics with unfortified yogurt, suggesting that yogurt is a suitable vehicle for iron fortification and that the ferrous lactate is an appropriate iron source for yogurt fortification.     

Exopolysaccharides produced by lactic acid bacteria of kefir grains

A Lactobacillus delbrueckii subsp. bulgaricus HP1 strain with high exopolysaccharide activity was selected from among 40 strains of lactic acid bacteria, isolated from kefir grains. By associating the Lactobacillus delbrueckii subsp. bulgaricus HP1 strain with Streptococcus thermophilus T15, Lactococcus lactis subsp. lactis C15, Lactobacillus helveticus MP12, and Sacharomyces cerevisiae A13, a kefir starter was formed. The associated cultivation of the lactobacteria and yeast had a positive effect on the exopolysaccharide activity of Lactobacillus delbrueckii subsp. bulgaricus HP1. The maximum exopolysaccharide concentration of the starter culture exceeded the one by the Lactobacillus delbrueckii subsp. bulgaricus HP1 monoculture by approximately 1.7 times, and the time needed to reach the maximum concentration (824.3 mg exopolysacharides/l) was shortened by 6 h. The monomer composition of the exopolysaccharides from the kefir starter culture was represented by glucose and galactose in a 1.0:0.94 ratio, which proves that the polymer synthesized is kefiran.     

Sex chromosome pre-reduction in male meiosis of Lethocerus patruelis (St?l, 1854) (Heteroptera,Belostomatidae) with some notes on the distribution?of?the species

The karyotype and meiosis in males of giant water bug Lethocerus patruelis (Heteroptera: Belostomatidae: Lethocerinae) were studied using standard and fluorochrome (CMA₃ and DAPI) staining of chromosomes. The species was shown to have 2n = 22A + 2m + XY where 2m are a pair of microchromosomes. NORs are located in X and Y chromosomes. Within Belostomatidae, Lethocerus patruelis is unique in showing sex chromosome pre-reduction in male meiosis, with the sex chromosomes undergoing reductional division at anaphase I and equational division at anaphase II. Cytogenetic data on the family Belostomatidae are summarized and compared. In addition, the structure of the male internal reproductive organs of Lethocerus patruelis is presented, the contemporary distribution of Lethocerus patruelis in Bulgaria and in the northern Aegean Islands is discussed, and the first information about the breeding and nymphal development of this species in Bulgaria is provided.     

Growth and Proteolytic Activity of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus in Reduced Sodium Kashkaval Cheese

Summary Reduced sodium Kashkaval cheese was produced from cow’s milk. Mixtures of NaCl, NaCl:KCl (1:1, 2:1) and NaCl:KH₂PO₄ (1:1, 2:1) were used for hot brining and salting of the cheddarized cheese curd. There were no significant differences (P < 0.05) in the count of Lb. delbrueckii ssp. bulgaricus after aging of Kashkaval samples. At the end of the ripening process the counts of Lb. delbrueckii ssp. bulgaricus reached 10⁶ c.f.u./g and the counts of Streptococcus thermophilus varied from 10⁴ to 10⁵ c.f.u./g. Proteolysis during ripening of reduced sodium Kashkaval cheese, initiated by the starter microorganisms Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus, was studied through the changes in the levels of non-casein and non-protein nitrogen. It was observed that non-casein and non-protein nitrogen increased significantly (P < 0.05) during ripening. The amounts of non-casein and non-protein nitrogen accumulated in the studied Kashkaval samples were similar. That indicates that the partial replacement of NaCl with KCl or KH₂PO₄ does not cause significant changes in the course of proteolysis of Kashkaval cheese by Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus.     

First evidence of sex chromosome pre-reduction in male meiosis in the Miridae bugs (Heteroptera)

The karyotype and male meiosis of Macrolophus costalis Fieber (Insecta, Heteroptera, Miridae) were studied using C-banding, AgNOR-banding and DNA sequence specific fluorochrome staining. The chromosome formula of the species is 2n = 28(24+X1X2X3Y). Male meiotic prophase is characterized by a prominent condensation stage. At this stage, two sex chromosomes, "X" and Y are positively heteropycnotic and always appeared together, while in autosomal bivalents homologous chromosomes were aligned side by side along their entire length, that is, meiosis is achiasmatic. At metaphase I, "X" and Y form a pseudobivalent and orient to the opposite poles. At early anaphase I, the "X" chromosome disintegrates into three separate small chromosomes, X1, X2, and X3. Hence both the autosomes and sex chromosomes segregate reductionally in the first anaphase, and separate equationally in the second anaphase. This is the first evidence of sex chromosome pre-reduction in the family Miridae. Data on C-heterochromatin distribution and its composition in the chromosomes of this species are discussed.     

Proteolytic activity of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus in frozen-stored Kashkaval cheese

Proteolytic activity of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus in Kashkaval cheeses of varying aging times, stored at −10 to −12°C for 12 months, was studied. It was established that the proteolysis of Kashkaval cheese induced by the starter culture was significantly delayed by freezing. The noncasein nitrogen (NCN/TN) and nonprotein nitrogen (NPN/TN) as a percentage of total nitrogen increased slightly during frozen storage of Kashkaval. It was found that NCN/TN and NPN/TN values increased to a larger extent in frozen-stored Kashkaval samples with shorter aging time. Enhanced proteolysis was observed during ripening of thawed Kashkaval cheese. There was greater accumulation of noncasein nitrogen in thawed Kashkaval samples compared to the control samples. The enhanced proteolysis during ripening of thawed Kashkaval cheese resulted in larger amounts of high and medium molecular weight peptides and lower amounts of low molecular weight peptides and free amino acids as compared to controls.     

Production of flavour compounds by yogurt starter cultures

The present work studied the production of carbonyl compounds and saturated volatile free fatty acids by pure cultures of Streptococcus thermophilus and Lactobacillus bulgaricus, and by starter cultures for Bulgarian yogurt during cultivation and cooling. The mixed cultures formed volatile aromatic compounds more actively than the pure cultures. A guiding factor in the preparation of the starter cultures was the biochemical activity of Lactobacillus bulgaricus in synthesizing the major carbonyl compounds, acetaldehyde, diacetyl and the volatile fatty acids C₂–C₁₀. The activity of the yogurt cultures in synthesizing carbonyl compounds was at its highest during milk coagulation and cooling, up to 7 h. However, maximum concentration was reached by 22–31 h. In the cooled 22–h starter cultures, acetaldehyde predominated (1415.0–1734.2 μg per 100 g) followed by diacetyl (165.0–202.0 μg per 100 g), acetoin (170.0–221.0 μg per 100 g), acetone (66.0–75.5 μg per 100 g), ethanol (58.0 μg per 100 g), and butanone-2 (3.6–3.8 μg per 100 g). The thermophilic streptococcus and lactobacillus cultures, and the starter cultures contained predominantly acetic, butyric and caproic acids. Received 19 June 1997/ Accepted in revised form 10 January 1998     

Production and monomer composition of exopolysaccharides by yogurt starter cultures

As components of starter cultures for Bulgarian yogurt, Streptococcus salivarius subsp. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus revealed extensive exopolysaccharide (EPS) production activity when cultivated in whole cow's milk. The polymer-forming activity of thermophilic streptococci was lower (230-270 mg EPS/L) than that of the lactobacilli (400-540 mg EPS/L). Mixed cultures stimulated EPS production in yogurt manufacture, and a maximum concentration of 720-860 mg EPS/L was recorded after full coagulation of milk. The monomer structure of the exopolysaccharides formed by the yogurt starter cultures principally consists of galactose and glucose (1:1), with small amounts of xylose, arabinose, and/or mannose.