Using chromatographic techniques, a number of compounds was isolated from the steam distillates of skim milk containing oxidized (cardboard) flavour. The following compounds were definitely identified: acetone, acetaldehyde, n–hexanal, crotonaldehyde, and the C5 to C11 2-unsaturated aldehydes. Presumptive evidence for the presence of several 2,4-diunsaturated aldehydes of medium chain length was obtained also. Acetaldehyde and acetone were also isolated from normal skim milk; these two compounds appear to play no part in the defect.2-Enals, in particular the C8 and C9, were considered to be the principal flavour-determining compounds. In milk at dilutions of 1 part in 107 to 109 they impart an oxidized type of flavour resembling that known as cardboard. Evidence was obtained that 2,4-dienals were also present and they produced a similar flavour.It may further be deduced that cardboard flavour in milk is caused by the preferential oxidation of di- and polyunsaturated fatty acids.
The C6–C11 2,4–dienals have been identified in distillates from skim milk with oxidized flavour. The C4–C11 2–enals were isolated from the same distillates in a previous investigation. The most abundant individual compounds in the two groups were 2–octenal, 2–nonenal, 2–4–heptadienal and 2,4–nonadienal. These compounds when flavour–tested in skim milk in dilutions of 10–7–10–9 closely resembled oxidized (cardboard) flavour. It is concluded that they are the main constituents of this flavour defect and that they originate from the oxidation of the more highly unsaturated fatty acids in milk lipids.
Frozen concentrated cheese starters were prepared from 7 Streptococcus cremoris and 2 Str. lactis strains by continuous culture. pH control with NH 4 0H and Ca(OH) 2 increased the yield of Str. cremoris strains as compared with NaOH, but the resultant cells were smaller and less active. Maximum yields for both species were obtained at a dilution rate of 0-6 and pH 6-3, but with Str. cremoris cell activity was reduced above pH 60.Degeneration in continuous culture was observed in the 2 Str. lactis strains, due apparently in one case to the action of a bacteriophage and in the other possibly to selection or to loss of adaptive enzymes.Concentrated starters retained full viability and activity when stored for 10-12 weeks at -110 °C, and 94% activity after storage for 4 weeks at -40 °C.Cheese made with the concentrated starters ripened normally and was comparable in quality with control cheese made with normal bulk starter.Cheese-making in Australia is mostly confined to the Cheddar variety and the starter cultures used are single strains of the mesophilic lactic streptococci.Two strains of Streptococcus lactis and 10 strains of Str. cremoris are currently used in the manufacture of Cheddar cheese in Australia. These strains have been selected because there is no evidence that they are phage-related. A daily rotation of such strains is advised to help control phage levels in the factory. The commercial development of frozen concentrated starters in Australia would therefore need to be based on the supply of a number of strains to allow for at least a 4-day rotation (Czulak, 1966) and some degree of preference by individual factories.Concentrated starters prepared from Str. lactis grown in continuous culture in a trypsin-digested whey medium gave promising results, but with Str. cremoris poor yields were obtained (Lloyd & Pont, 1970). As several Str. cremoris strains would be required for a 4-day starter rotation, we have attempted to improve their yields in continuous culture.Using a trypsin-digested milk medium, some of the more important parameters affecting the growth of Str. cremoris and Str. lactis in continuous culture were studied, particularly the yield and biological activity and storage stability of frozen concentrates. The results are presented in this paper .
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.