A typical process for fromage frais is outlined in Fig. 2 (27). This process is highly automated and is used in large commercial processes. Each processing step is described in further detail below. It should be kept in mind that many alternative and more traditional processes are utilized in smaller plants.

A. Centrifugation and Thermization

A thermization or heat shock treatment (65 °C, 15 to 30 sec) generally follows the initial centrifugation that separates raw milk into skim milk and cream. This heat treatment is not a legal pasteurization but suffices to destroy most spoilage bacteria. In general, this heating increases process flexibility by allowing more time for protein standardization without deterioration of skim milk quality.

B. Protein Standardization

While cheeses traditionally require casein-to-fat standardization, fromage frais is prepared from skim milk and only protein standardization occurs. The reasons for standardizing protein content are threefold: (a) to ensure constant protein content throughout the year, (b) to increase cheesemaking capacity of the processing line, (c) to promote a firm coa-gulum (see Sec. III). The standardization can be done by adding protein or by concen-

Centrifugation + Thermization

Protein standardization Pasteurization (93°C, 120 sec)

Fermentation (until pH 4.5, 14-16 hr) Heat treatment (62°C, 120 sec)

Whey separation (centrifugation) Fat standardization {add pasteurized cream)

Inline mixing of sugar and fruit

Figure 2 Processing of fromage frais using curd centrifuge. (From Ref. 27).

trating the skim milk. Typically, proteins are added as nonfat milk powder or milk protein concentrates. It is important that time is available for the added powders to become properly hydrated. Hydration rates are increased if proper mixing equipment such as a triblender is utilized. The subsequent pasteurization and cooling also promote hydration.

Concentration of skim milk can be achieved through membrane technology or by evaporation. The choice of method affects composition of the standardized skim milk. Ultrafiltration (UF) retentates contain fewer low-molecular-weight compounds such as minerals and lactose than evaporated concentrates because low-molecular-weight compounds are removed in the UF permeate. Compositional differences can have profound influence on cheese characteristics and must be controlled by changing processing parameters such as rennet addition and fermentation rate and extent (28).

C. Pasteurization

It is important to pasteurize as quickly as possible to prevent growth of psychrotrophic spoilage bacteria. Pasteurization conditions are well above minimum requirements for destroying potential raw milk pathogens. The severe heat treatment creates a higher viscosity of the coagulum, in part because whey proteins are heat sensitive and denature at severe heat treatment. Denatured whey proteins are retained in the casein network, thus leading to increased cheese yield. Westfalia obtained a patent (29) in 1977 for the Thermo-Quark process, which is frequently used in fromage frais processing. The patent covers the process of pasteurizing cheesemilk to 90-95 °C for 2-3 min followed by fermentation and then a second heat treatment of the coagulum at 60°C for 1-2 min. The advantage of this process is a decrease in protein content of the whey by up to 50%.

Following the holding tube, the milk is cooled to the appropriate fermentation temperature in the regeneration section of the pasteurizer. Typical fermentation temperatures are around 25-27 °C.

D. Fermentation

The quantity of starter cultures added depends on the activity of the culture, the incubation temperature, time available for fermentation, and the sensory characteristics expected of the finished product. In general, 0.5-3% culture is added. Approximately 90 min after addition of starter culture, the milk/coagulum reaches a pH of 6.3 and rennet is added (.5-1 mL single strength/100 L cheesemilk). Fromage frais is a fresh acid-curd cheese and can be made without rennet. However, rennet firms the coagulum and subsequently minimizes the loss of shattered casein curd into the whey. If severe heat treatment has been utilized to denature whey proteins, it is beneficial to increase rennet quantity because n-casein is protected by the denatured whey protein. However, compared to traditional rennet cheeses such as Cheddar, the rennet addition to fromage frais is low.

The fermentation takes 14-16 hours and is complete when pH reaches 4.5, whereupon the coagulum is well mixed to assure a smooth texture. Fermentation time above 18 hr indicates slow cultures and could pose a safety risk. In contrast, overly fast fermentation may contribute to syneresis as described above (Sec. III). Fermentation time is a function of starter activity as well as fermentation temperature.

E. Heat Treatment

The coagulum is heat treated at around 62°C for 120 sec to improve whey separation in the subsequent step. In addition, this heat treatment destroys some bacteria and enzymes, thus increasing shelf life of the final product. The disadvantage of this heat treatment is that lactic acid bacteria from the starter culture are destroyed as well. It is estimated that only 10% of lactics survive the treatment (27).

F. Separation of the Whey

Syneresis is required to increase solids content of the cheese. Following fermentation the coagulum physically retains moisture within serum pockets. To permit moisture release it is necessary to break the aggregated protein strands around serum pockets. Breaking the strands releases moisture, and the broken strands will reaggregate into a more compact structure, thus encapsulating less moisture (13).

Traditionally, whey separation in cheesemaking is obtained by cutting the curd, cooking, and draining the whey. The automated and continuous whey separation step in fromage frais processing utilizes a centrifuge to mechanically remove whey (Fig. 3). The coagulum is cooled to around 40-44°C before entering the centrifuge. Within the centrifuge, curd is fed into the center of the bowl and through the distributor into the rising channels of the disc stack (27). The centrifuge separates the coagulum into curd (8 = 1.05) and whey (8 = 1.02). Curd centrifuges are specifically developed for viscous products and have capacities from 1000 L/hr to 10,000 L/hr. The centrifuge is cooled by circulating cold water through the hood. Solid content of the curd is determined by the feed rate as well as the nozzle discharge capacity. It is possible to install different diameter nozzles when changes in solids content are required. Solid-nonfat contents range from 13 to 24%, dependent on equipment and the cheese produced. Constant solid content is obtained by having a homogeneous coagulum composition and constant feed rate to the centrifuge. A well-installed centrifuge can keep variations in total solids within +/—0.05%. Besides variations in composition, failing to optimize operation of the centrifuge can lead to defects such as sandy mouthfeel. The whey should be clear and free of curd particles. Whey can be tested by placing a sample in a lab centrifuge and centrifuging at 3500 rpm for 9 min (27). If suspended particles are present, it is necessary to optimize variables such as temperature, feed rate, pretreatment, and solid content of the coagulum. For nonfat coagulums, the whey constitutes the light phase, which moves toward the interior while the curd is pushed outward. Highfat coagulums require different centrifuges because in this case the light phase is the fat-containing curd and the heavier whey is pushed outward.

To optimize yields, an ultrafiltration system can be connected to the whey stream to collect whey proteins, which are then reintroduced into the curd (30). However, the Thermo-Quark/Thermo-soft method, which denatures whey proteins, is often considered a more efficient process for yield improvement. Another approach to increasing yield is to minimize nonprotein nitrogen by selecting a starter culture with low proteolytic activity.

Ultrafiltration can also be used for concentrating milk solids in place of the curd centrifuge. The UF system can be installed either before or after fermentation. In the former scenario, only retentate is fermented because the concentration step occurs before fermentation. Concentration of cheesemilk up to final total solid content is utilized in the production of other cheeses as well, such as feta, Camembert, and Brie. This process was developed by Maubois, Mocquot, and Vassal in 1969 (31). To obtain the correct calcium balance and ultimately a comparable coagulum, it is necessary to slightly preacidify the milk prior to ultrafiltration. Traditionally, whey contains high calcium content because solubilization of colloidal calcium phosphate occurring during fermentation shifts calcium from the casein fraction to the serum fraction. UF retentate retains colloidal calcium phosphate unless the milk is acidified prior to filtration. Furthermore, UF retentate contains all whey proteins, which lead to a weak coagulum unless the proteins are denatured by severe heat treatment (26).

As stated above, UF can also be performed after fermentation. In this case, the calcium balance will likely be correct because the fermentation has caused solubilization of colloidal calcium phosphate. When installed after fermentation, the UF system basically replaces the curd centrifuge as indicated in Fig. 4. It has been stated that this process gives a higher yield than the thermoquark method (32) and provides higher process flexibility because higher fat coagulums can be processed (33) (Fig. 5).

Pump For Cheese Coagulum

Figure 3 Curd separator type KDB 30-02-076.1: Feed; 2: Discharge—whey; 3: Centripetal pump— whey; 4: Hood overflow; 5: Segmental insert; 6: feed—hood cooling; 7: Discharge, concentrate collector, and brake ring cooling; 8: Feed, concentrate collector, and brake ring cooling; 9: Frame drain; 10: Sterile air/CIP connection; 11: Cheese hopper; 12: Concentrate collector; 13: Brake ring, cooled; 14: Rising channels; 15: Nozzles; 16: Disc stack; 17: Discharge, hood cooling. (From Ref. 27.)

Figure 3 Curd separator type KDB 30-02-076.1: Feed; 2: Discharge—whey; 3: Centripetal pump— whey; 4: Hood overflow; 5: Segmental insert; 6: feed—hood cooling; 7: Discharge, concentrate collector, and brake ring cooling; 8: Feed, concentrate collector, and brake ring cooling; 9: Frame drain; 10: Sterile air/CIP connection; 11: Cheese hopper; 12: Concentrate collector; 13: Brake ring, cooled; 14: Rising channels; 15: Nozzles; 16: Disc stack; 17: Discharge, hood cooling. (From Ref. 27.)

Industrial Sugar Breaking Triblender
Figure 4 Thermo-Quark cheese line with KDB 30 separator. (From Ref. 27.)

Skim milk/mi Ik/cream Pasteurization (90°C, 5 min)

Fermentation (until pH 4.5-4,6)

Heat treatment ^C, 130 sec) t


UF Retentate UF Permeate

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