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A tecnologia de alta-pressão (AP) é reconhecida como uma das mais promissoras tecnologias
de processamento a baixas temperaturas, podendo ser aplicada numa vasta gama de trinómios
de condições de pressão/tempo/temperatura. O processamento de leite cru pela tecnologia de
AP pode causar alterações nas suas propriedades microbiológicas, físico-químicas e
tecnológicas. Assim, o objetivo deste trabalho de investigação foi avaliar o impacto do
processamento não térmico por AP no leite, como alternativa ao processamento térmico
convencional, no sentido de produzir leite seguro em termos microbiológicos, mas com maior
aptidão tecnológica para o fabrico de queijo, expresso sob a forma de maior rendimento. Para
tal foram testados diferentes binómios de pressão/tempo de processamento (P/t) de leite com o
intuito de estabelecer qual a melhor combinação P/t para permitir uma melhor qualidade
microbiológica, e consequentemente um tempo de vida útil mais prolongado, assim como um
maior rendimento de coalhada. Numa primeira fase testaram-se 3 pressões (300, 450 e 600
MPa) durante 6 minutos, para numa segunda fase fixar a pressão a 600 MPa e testar tempos de
processamento variáveis (6, 13 e 20 minutos). Em todos os ensaios o leite, antes e após
tratamento, e ao longo do tempo de armazenamento sob refrigeração até 15 dias, foi analisado
em termos de parâmetros microbiológicos (aeróbios mesófilos, aeróbios psicrófilos, bactérias
lácticas, enterococos, estafilococos, enterobactérias e bolores e leveduras), físico-químicos
(cor, pH e acidez titulável) e aptidão tecnológica (rendimento de coagulação).
Da primeira fase observou-se que pressões mais elevadas do que 300 MPa permitiram uma
maior inibição do crescimento microbiano. Os grupos microbianos mais sensíveis a 300 MPa
foram as enterobactérias e enterococos mas a aplicação de pressões a 450 e 600 MPa resultou
na inibição de todos os grupos microbianos-alvo. No que respeita a parâmetros físico-químicos
estes foram afetados positivamente com o aumento da pressão. Por outro lado, aplicação de AP
no processamento de leite teve um impacto variável ao longo do tempo de armazenamento;
após 7 dias observaram-se 24 a 46% de perdas de rendimento de coagulação.
Independentemente da ordem de magnitude da AP, só se observaram ligeiras alterações na
luminosidade, e nos valores de a* e b*.
Da segunda fase, constatou-se que o aumento do tempo de processamento de 6 para 20 minutos
a 600 MPa permitiu manter um número de células viáveis de bactérias lácticas, enterococos,
estafilococos e enterobactérias inferior aos limites de deteção ao longo de 7 dias e de
estafilococos e enterobactérias até 15 dias de armazenamento a 4 ºC. Ligeiro decréscimo dos
valores pH acompanhado por um aumento de acidez titulável foi observado ao longo de 15 dias
de armazenamento. As amostras de leite processadas por AP a 600 MPa e por 13 e 20 minutos
permitiram valores de rendimento de coagulação mais elevados e estáveis ao longo de todo o
tempo de armazenamento, novamente sem impacto significativo na cor.
High pressure is recognized as one of the most promising low temperature processing technologies and can be applied across a wide range of pressure/time/temperature conditions trinomials. The processing of raw milk by high pressure technology can cause changes in its microbiological, physicochemical and technological properties. Thus, the objective of this research was to evaluate the impact of non-thermal processing by high-pressure technology on milk, as an alternative to conventional thermal processing, in order to produce microbiological safe milk, but with greater technological aptitude for the manufacture of cheese, expressed in the form of higher yield. Thus, different pressure/processing time (p/t) of milk were tested in order to establish the best p/t combination, to allow a better microbiological quality, and consequently a longer shelf life, as well as curd yield. In a first phase, 3 pressures (300, 450 and 600 MPa) were tested for 6 minutes. In a second phase the pressure was set at 600 MPa and tested different processing times (6, 13 and 20 minutes). In all trials the milk, before and after treatment, and during storage under refrigeration up to 15 days, was analyzed in terms of microbiological parameters (aerobic mesophiles, aerobic psychrophils, lactic bacteria, enterococci, staphylococci, enterobacteria and molds and yeasts), physical-chemical (color, pH and titratable acidity) and technological aptitude (coagulation yield). From the first phase it was observed that pressures higher than 300 MPa allowed a greater inhibition of microbial growth. The most sensitive microbial groups at 300 MPa were enterobacteria and enterococci, but the application of pressures at 450 and 600 MPa resulted in inhibition of all target microbial groups. Regarding physical-chemical parameters, these were positively affected by the increase of pressure. On the other hand, high pressure application in milk processing had a variable impact throughout the storage time; after 7 days, 24 to 46% of coagulation yield losses were observed. Regardless of the order of magnitude of the High Pressure, only slight changes in luminosity were observed, and the values of a * and b * were observed. From the second phase, it was found that the increasing of the processing time from 6 to 20 minutes at 600 MPa allowed maintaining a viable number of lactic acid bacteria, enterococci, staphylococci and enterobacteria less than detection limits over 7 days and staphylococci and enterobacteria up to 15 days of storage at 4 ° C. Slight decrease in pH values accompanied by a titratable acidity increase was observed over 15 days of storage. Milk samples processed by high pressure at 600 MPa and for 13 and 20 minutes allowed higher and stable coagulation yield values over the entire storage time, again without significant impact on color.
High pressure is recognized as one of the most promising low temperature processing technologies and can be applied across a wide range of pressure/time/temperature conditions trinomials. The processing of raw milk by high pressure technology can cause changes in its microbiological, physicochemical and technological properties. Thus, the objective of this research was to evaluate the impact of non-thermal processing by high-pressure technology on milk, as an alternative to conventional thermal processing, in order to produce microbiological safe milk, but with greater technological aptitude for the manufacture of cheese, expressed in the form of higher yield. Thus, different pressure/processing time (p/t) of milk were tested in order to establish the best p/t combination, to allow a better microbiological quality, and consequently a longer shelf life, as well as curd yield. In a first phase, 3 pressures (300, 450 and 600 MPa) were tested for 6 minutes. In a second phase the pressure was set at 600 MPa and tested different processing times (6, 13 and 20 minutes). In all trials the milk, before and after treatment, and during storage under refrigeration up to 15 days, was analyzed in terms of microbiological parameters (aerobic mesophiles, aerobic psychrophils, lactic bacteria, enterococci, staphylococci, enterobacteria and molds and yeasts), physical-chemical (color, pH and titratable acidity) and technological aptitude (coagulation yield). From the first phase it was observed that pressures higher than 300 MPa allowed a greater inhibition of microbial growth. The most sensitive microbial groups at 300 MPa were enterobacteria and enterococci, but the application of pressures at 450 and 600 MPa resulted in inhibition of all target microbial groups. Regarding physical-chemical parameters, these were positively affected by the increase of pressure. On the other hand, high pressure application in milk processing had a variable impact throughout the storage time; after 7 days, 24 to 46% of coagulation yield losses were observed. Regardless of the order of magnitude of the High Pressure, only slight changes in luminosity were observed, and the values of a * and b * were observed. From the second phase, it was found that the increasing of the processing time from 6 to 20 minutes at 600 MPa allowed maintaining a viable number of lactic acid bacteria, enterococci, staphylococci and enterobacteria less than detection limits over 7 days and staphylococci and enterobacteria up to 15 days of storage at 4 ° C. Slight decrease in pH values accompanied by a titratable acidity increase was observed over 15 days of storage. Milk samples processed by high pressure at 600 MPa and for 13 and 20 minutes allowed higher and stable coagulation yield values over the entire storage time, again without significant impact on color.
Descrição
Palavras-chave
Leite Processamento por alta pressão Qualidade microbiológica Aptidão tecnológica Milk High pressure processing Microbiological quality Technological adequacy