Break curd by agitation after pH has reached 4.5 and cool to 10°C with gentle agitation
Figure 4 Steps for the manufacture of cultured buttermilk. (From Kosikowski and Mis-try, 1997.)
Cultured buttermilk is produced with combinations of mesophilic lactic acid bacteria that will produce lactic acid as well as diacetyl. Species used include Lactococcus lactis subsp. cremoris, Leuconostoc mesenteroides subsp. cremoris, and Lc. lactis subsp. lactis (biovar. diacetylactis). The latter two produce diacetyl and small amounts of carbon dioxide. Lc. lactis subsp. lactis (biovar. diace-tylactis) also produces acetaldehyde, which is not desirable in buttermilk, and therefore this bacterium should be used with caution. The lactic acid producers thrive on lactose, whereas the flavor producers require the presence of sufficient citric acid to produce diacetyl. The naturally present citric acid in milk should be supplemented by the addition of sodium citrate (0.1-0.15%). The flavor producers do not produce an appreciable amount of lactic acid but do require acidic conditions for proper growth and fermentation of citrate. Therefore, sufficient activity by the lactic acid producers is necessary (pH 5) before flavor producers can function. Levata-Jovanovic and Sandine (1997) have reported on the use of a Leuc. mesenteroides subsp. cremoris strain in combination with a ropy Lc. lactis subsp. cremoris culture for improving the flavor and texture of buttermilk.
An important advantage of using leuconostocs is that these organisms are relatively insensitive to phages.
Flavor producers are rather temperature sensitive. If the temperature of incubation is maintained at 27°C instead of the optimum 22°C, they will not produce sufficient diacetyl and consequently acid rather than a balance of acid and diacetyl flavor will dominate the finished product (Kosikowski and Mistry, 1997). Diacetyl-producing bacteria also possess an enzyme that converts diacetyl to acetyl methyl carbinol (acetoin). This results in a loss in the quantity of diacetyl in buttermilk (Vedamuthu, 1994). Hence, production of cultured buttermilk requires proper selection of culture bacteria as well as manufacturing conditions that will induce balanced growth of acid and flavor producers.
Cultured buttermilk typically has a thick, homogeneous body. Vedamuthu and Shah (1983) patented a procedure for manufacturing such a product using a mixture of slime-producing Lc. lactis subsp. cremoris and non-slime producing Lc. lactis subsp. cremoris and/or Lc. lactis subsp. lactis. Ropiness occurred only if >80% of the culture mixture was a slime producer.
In many respects, cultured buttermilk is a delicate product that can have defects if proper care is not taken during manufacture. On the other hand, culture characteristics and proper manufacturing conditions have been well documented, and, if employed, good-quality product can be readily obtained. Many defects of cultured buttermilk can be linked to improper culture usage, whereas others are related to manufacturing procedures. Culture-related defects can be flavor defects and may indirectly also lead to body defects. Even buttermilk produced under the best sanitary conditions may lack flavor (flat flavor) if the environment is not optimal for the growth of flavor producers. For example, a high incubation temperature (27°C) discourages growth of flavor producers; therefore insufficient diacetyl will be present. Such defects can be prevented by ensuring that the acid-producing culture is active, because the flavor producers will be activated only after sufficient acid has been produced (0.8-0.85%, pH 5) and incubating at 22°C. Milk should be supplemented with citrate, and after the curd has been broken at the optimum pH, the product should be rapidly cooled with gentle agitation. This will prevent degradation of diacetyl. If incubation is not monitored and if fermentation is not halted by cooling, acid production will continue and may even exceed 1%. This process is not reversible and produces a highly acidic product with a loss of diacetyl (Vedamuthu, 1994). Excessive acidity will also lead to wheying-off because of a lowered water-holding capacity of the proteins. Such wheying-off may also result from excessive and high-speed agitation during cooling after fermentation is completed (Kosikowski and Mistry, 1997). During storage such a product will separate into whey and a heavy protein mass that settles to the bottom.
A weak culture that is contaminated with organisms such as psychrotrophs and coliforms will lead to unclean, and, in extreme conditions, bitter flavors. Contaminating bacteria such as coliforms and Pseudomonas spp. possess a relatively high level of diacetyl reductase which degrades diacetyl (Elliker, 1945; Seitz et al., 1963). One strain of Enterobacter aerogenes had an activity of 345 units of enzyme protein per milligram compared with 100 units for Lc. lactis subsp. lactis (biovar. diacetylactis) (Seitz et al., 1963). Such enzyme activity leads to a product that lacks flavor. Good manufacturing and sanitation practices are therefore vital and can easily prevent such defects. Proper starter maintenance, including replacement of the mother culture at regular intervals, is also a good practice to ensure continued high activity of the starter culture.
Some of the aforementioned culture-related defects will eventually lead to body and texture defects. For example, if the culture lacks adequate activity and if the product is cooled at low acidity, the finished product will not have optimum viscosity. In contrast, excessive viscosity can result from cultures such as Lc. lactis subsp. lactis, which form long chains. Some contaminants produce slime, which results in a highly viscous product.
The two main fermented cream products are sour cream and crème fraîche. The later originated in France but is now also used in other countries. Because of their high fat content, 18 and 50%, respectively, they are used for dips and toppings rather than for direct consumption. Cultures used for these products and manufacturing procedures are similar to those for cultured buttermilk (Kosikow-ski and Mistry, 1997). The high-fat and solids contents provide these products with a thick and heavy body. The manufacturing procedure for sour cream is especially designed to produce a very thick body. Sour cream typically has a clean acidic flavor with hints of diacetyl. Mesophilic lactic acid and flavor-producing cultures are used along with double homogenizing and a small amount of rennet for developing body (Fig. 5). Crème fraîche, on the other hand, is also manufactured with the same cultures but the pH is higher (6.2-6.3).
As sour cream is a high-fat product (approximately 70% fat on dry basis), manufacturing a low-fat, and, particularly a fat-free product, is challenging. Simply replacing the fat with moisture, as is done in most low-fat cheeses, does not provide the required thick and smooth body of sour cream. Thickening agents such as starches, stabilizers, and fat replacers therefore play an important role in these products. Lee and White (1991) demonstrated that good body and texture in sour cream of 5 and 10.5% fat could be obtained with gelatin, modified food starch, or methoxyl pectin. Addition of rennet helps firm the body but also leads
Preparation of mix: Standardize cream to 18-19% fat and add 0.2% citric acid and 0.5%
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