[Biological value of the unicellular protein of Kluyveromyces marxianus var. lactis]. / Estudio del valor biológico de la proteína unicelular de. Kluyveromyces marxianus var. lactis.

The ever increasing problem of environmental pollution and protein scarcity led us to begin producing protein biomass from cheese whey, having selected for this aim, Kluyveromyces marxianus va. lactis strain, which represents the right relationship between its protein content (53.3% d.w.) and that of the RNA (4.63% d.w.). On the other hand, the distribution of its essential amino acids is balanced, although it shows a deficiency in methionine, its value being 1.5 g/16 gN. The purpose of this work was to assess the biological quality of this protein concentrate, so as to use it in animal feeding. Protein quality was determined by the NPU method (net protein utilization), according to Miller and Bender's technique. Three balanced diets were prepared (10% protein), the test sample with casein, and the rest with protein biomass, one of then supplemented with methionine (0.5 g/100 g food). Based on the results it can be concluded that the biomass analyzed has an adequate biological value (55.37%) for use in animal feeding, especially when methionine as added (60.23%).

[Chemical composition of the cellular biomass of yeasts]. / Estudio de la composición química de biomasas celulares de levaduras.

This work is aimed at analyzing yeast strains, possibly used in animal feeding, obtained by batch cultivation from cheese whey as main carbohydrated substrate. For that purpose 10 yeast strains selected for its biomass production capacity were chemically analyzed. From the results, it can be observed that the chemical composition of the strains is quiet variable, showing in all cases high protein content, good solubility and enzymatic digestibility. In all of them, the RNA content is low, being this important if the biomass is utilized in human feeding. On the other hand, they have an adequate content of essential amino acids, although the sulphur amino acids content is deficient. Among the tested yeasts, Kluyveromyces marxianus var. lactis (No. 10) stands out for the good relationship it has between protein and RNA content, as well as for the detection of methionine among its essential amino acids.

[Optimizing conditions for the discontinuous production of unicellular protein using whey]. / Optimización de condiciones para la producción discontínua de proteína unicelular utilizando suero de leche.

The cheese whey is one of the most important effluents which is being disposed off in our area. That is why the study for optimizing conditions in the process of recovering whey to produce protein biomass in a batch fermenter was undertaken. A yeast strain (Kluyveromyces marxianus var. lactis) was propagated on a medium based on whey from cheese making plants, the following conditions for cell development being optimized: temperature, pH and initial concentration of lactose. A methodology based on performing several tests ordered according to a "Latin Squares" structure was proposed; this enables the simultaneous study of three variables with a small number of experiences. Such experiences were performed in a cylindrical (air lift type) glass fermenter, obtaining a maximum yield (4.78 g/l by dry weight) when working with an initial lactose concentration equal to 4.8%. For all temperatures (27, 30 and 35 degrees C) and pH (3, 4 and 5) employed, an increase in the cell number occurred with the initial lactose concentration increasing from 2% to 4.8% (Table 2). It is preferable to work at 27 degrees C and pH 4 (since these conditions minimize the bacterial contamination) and with a lactose concentration equal to 4.8%, i.e. the concentration in the residual cheese whey. Thus, an optimum yield in protein biomass is obtained, enabling a good utilization of this effluent, and also diminishing its initial BOD from 60,000 to 15,000 ppm.

[Trials of cheese-making using concentrated bacterial starters]. / Ensayos de elaboración de quesos con fermentos bacterianos concentrados.

Studies about cheese preparation with frozen and concentrated bacterial starters have been carried out. The Pategras cheeses were obtained from raw milk. The starters were prepared with a selected strain of Streptococcus lactis, concentrated until reaching a value of 3.10(9) colony forming units/ml and resuspended in milk previously supplemented with 8% of yeast extract. These concentrates were frozen at -40 degrees C and kept at -20 degrees C for 60 days. Three kinds of starters were tested: one thawed by placing the flask in a 40 degrees C water bath, another added to the cheese vat without previous thawing and a control sample prepared in steamed reconstituted milk. In order to evaluate the convenience of each technique several chemical and microbiological analysis were performed during the preparation (Table 1 and 2) and the ripening of the cheese (Table 3). The results have showed that the direct use of thawed frozen concentrates in the cheese vat allows the obtaining of high quality cheese. On the other hand, the technique based on thawing through a water bath did not lead to good results.

Ensayos de elaboración de quesos con fermentos bacterianos concentrados / Assays of cheese preparation with concentrated bacterial starters

En el presente trabajo se llevaron a cabo estudios de elaboración de quesos con fermentos bacterianos concentrados y congelados. Los citados quesos, de pasta semicocida, se obtuvieron a partir de leche pasteurizada. Los fermentos se preparaon con una cepa seleccionada de Streptococcus lactis, concentrada hasta alcanzar un valor de 3.10**9 U.F.C./ml y resuspendida en lecha adicionada con el 8% de extracto de levadura. Dichos concentrados se congelaron a -40- y se almacenaron a -20-C durante 60 días. Se ensayaron tres varianes de fermento: uno descongelado en baño María a -40-C, otro agregado a la tina quesera sin previo descongelamiento y un testigo preparado en leche reconstituída estéril. Con el objeto de evaluar la conveniencia del empleo de dichas variantes, se realizaron análisis químicos y microbiológicos durante el proceso de elaboración y maduración de los quesos. Los resultados obtenidos han demostrado que el empleo de concentrados congelados descongelados directamente en la tina, permite obtener quesos de muy buena calidad. No sucedió lo mismo con los descongelados en baño María, lo que torna desaconsejable esta tecnología