Typically, process cheese formulations depend on the type of process cheese being manufactured and the type of end-use applications that the process cheese will be targeted for. Various chemical and compositional properties of process cheese affect the final functional properties of process cheese in a variety of ways. Therefore, while formulating a process cheese, manufacturers often try to control the final chemical properties of process cheese through appropriate selection of ingredients in order to achieve a process cheese formula that will have a specific functional property after it is manufactured. However, the availability of natural cheese (type and age), cost, availability of other ingredients, and presence or absence of rework varies from day to day. These are some of the constraints that manufacturers have to deal with while formulating their process cheeses in order to achieve a final product with consistent functional properties on a daily basis. As a result, there are numerous permutations and combinations involved in the selection of ingredients for the same process cheese formula. Traditionally, process cheese makers relied on their experience to select the ingredient blend for a specific formula. Over the years, process cheese manufacturers have started utilizing various computer-based formulation programs where they are able to set the desired chemical properties of a process cheese formulation and the formulation software determines the ingredient blend that produces the least cost formulation. Least cost formulation simply utilizes the cost and composition of an ingredient as a criterion for developing a formulation. As an example, various whey- and milkfat-based ingredients (whey powder, dried permeate, whey protein concentrate, butter, butter oil, dried cream, and so on) can be used as ingredients in process cheese. However, each of these ingredients has a different composition and current market price. A least cost formulation program uses the cost and composition information from each available ingredient and selects the ingredients used in a formulation based on their impact on the overall cost of the formulation. One such formulation program that we use in our laboratory is Techwizard™, which is an Excel-based formulation software program (Metzger 2003; Kapoor and Metzger 2004) provided by Owl Software (Columbia, Mo., U.S.A.).
As mentioned previously, various chemical and compositional properties have an effect on the functional properties of process cheese. These include fat content (Hong 1990), moisture content (Hong 1989; Lee and others 2004), pH (Templeton and Sommer 1930, 1932a; Marchesseau and others 1997; Lee and Klostermeyer 2001), total calcium content (Cavalier-Salou and Cheftel 1991), intact casein content (Vakaleris and others 1962; Meyer 1973; Piska and Štětina 2003; Garimella Purna and others 2006), lactose content (Templeton and Sommer 1932a, 1934; Thomas 1973; Berger and others 1998), and whey protein content (Gupta and Reuter 1992; Thapa and Gupta 1992a, 1992b). Due to the regulations set by the FDA (2006) for moisture and fat contents for PC, PCF, and PCS, manufacturers generally keep the moisture and fat contents in their product constant. However, due to the day-to-day variations in inventory of the natural cheese they use, the source and age of natural cheese tend to be different. These differences in the source and age of natural cheese on a daily basis lead to variations in the total calcium content, pH, and intact casein content of the process cheese and hence the functional properties of the process cheese. In addition to natural cheese variations that affect the total calcium content, pH, and intact casein content of the final process cheese, the type and amount of emulsifying salts that are added to process cheese influence the state of calcium in process cheese and the process cheese pH. Moreover, other ingredients (nonfat dried milk, dried whey, whey protein concentrate, and so on) influence the amount of whey protein and lactose in the final process cheese. The variations in the chemical properties of a process cheese that arise during a process cheese formulation significantly influence its functional properties. The type and amount of rework that is added to a process cheese formulation also have an effect on the final functional properties of process cheese. Therefore, it is important to control the formulation parameters of process cheese to achieve a product with consistent functional properties.
From the discussions above, it is evident that just standardizing the moisture and fat content of a process cheese formulation does not ensure a product with the desired functional properties. It is very important for the process cheese manufacturers to control and monitor the total calcium content, intact casein content, pH, type and amount of emulsifying salts used, lactose content, whey protein content, and type and amount of rework added while formulating their process cheese in order to produce a process cheese with specific physicochemical and functional properties. The individual influence of these formulation parameters on the functional properties of process cheese is discussed subsequently.
Intact casein contentMeyer (1973) and Shimp (1985) highlighted the importance of total intact casein of a process cheese on the quality of process cheese. Once again, the major ingredient that contributes to the intact casein in a process cheese formula is type and age of natural cheese utilized in the formula. The intact casein content of natural cheese is inversely related to the age of the natural cheese. As a natural cheese is ripened, its intact casein content decreases (Arnott and others 1957; Vakaleris and others 1962; Meyer 1973; Garimella Purna and others 2006). This occurs because, as the natural cheese ages, the enzymes and residual starter or nonstarter lactic acid bacteria present in the cheese hydrolyze the proteins present in natural cheese into peptides, thereby reducing the amount of casein that is still present in the intact (unhydrolyzed) form. Various researchers have indicated the effect of the age of natural cheese on the functional properties such as body and texture of the resulting process cheese (Arnott and others 1957; Vakaleris and others 1962; Garimella Purna and others 2006). The effect of the age of natural cheese utilized on the functional properties of process cheese is discussed in the subsection “Natural cheese.” In a recent study performed in our laboratory, Garimella Purna and others (2006) manufactured PCF with the same cheddar cheese at 2, 4, 6, 12, and 18 wk of ripening with 2.0%, 2.5%, and 3.0% trisodium citrate as the emulsifying salt. The results indicated that as the intact casein content of the natural cheese base used for PCF manufacture decreased, its viscosity immediately after manufacture and its firmness decreased whereas its meltability increased. However, interestingly, we found that with a decrease in the intact casein content of the cheddar cheese, the flowability of the resulting PCF increased initially up to 12 wk of ripening, but in the case of PCF manufactured with the cheese at 18 wk of ripening, its flowability decreased as compared to 12 wk of ripening. We attribute this change in flow properties of the PCF with a low level of intact casein to a phenomenon called “overcreaming.” Previous researchers have reported that overcreaming occurs when the natural cheese used in a formulation is excessively ripened (Meyer 1973). The casein in natural cheese that is excessively ripened is hydrolyzed into small peptides that are easily hydrated and dispersed during process cheese manufacture; and under normal process cheese manufacturing conditions extensive protein-based interactions occur that lead to a strong protein network that has restricted flow properties (Meyer 1973; Gerimella Purna and others 2006). The concept of overcreaming is discussed in more detail in the “Rework” subsection of this article.
pH The final pH of a process cheese has been found to have a significant effect on the quality, microstructure, and the type of protein interactions in the resulting process cheese emulsion (Palmer and Sly 1943; Meyer 1973; Marchesseau and others 1997). Various researchers have indicated that the pH range of a good-quality process cheese should be between 5.4 and 5.8 (Palmer and Sly 1943; Marchesseau and others 1997). According to Palmer and Sly (1943), the stability of the process cheese emulsion is decreased when the pH of the process cheese is below 5.4 or above 5.8. Marchesseau and others (1997) evaluated the microstructure of process cheese manufactured with different final pH. They found that a process cheese with a lower pH (5.2) had increased protein–protein interactions since the proteins were closer to their isoelectric point, thereby promoting the aggregation of proteins leading to a weaker emulsification of the fat phase in the process cheese. At higher process cheese pH (6.1), they found that process cheese had an open structure and therefore a weaker emulsion. In their study, process cheese with a pH of 5.7 produced a uniform fat emulsion with a closely knit protein network. Consequently, the final pH of process cheese is an important factor controlling the final structure and therefore the final functional properties of the process cheese. A previous study has shown the effect of the final pH of process cheese on its firmness (Templeton and Sommer 1932b). They found that as the final pH of the process cheese increased from 5.0 to 6.2, its firmness initially increased up to approximately pH 5.8 (where it had the highest firmness); however, with a further increase in pH (5.8 to 6.2), the firmness began to decrease. Alternatively, previous work has also indicated a very low correlation between the process cheese pH and melting properties of process cheese (Arnott and others 1957). The type and level of emulsifying salts (subsection “Emulsifying salts”) (Gupta and others 1984; Shirashoji and others 2006a) and the type and age of natural cheese (subsection “Natural cheese”) used during process cheese manufacture have a marked influence on the final pH of the resulting process cheese.
Although previous reports indicate the individual importance of total calcium, intact casein, and pH on the functional properties of process cheese, they do not indicate the combined effect of these factors on the process cheese properties. They also do not highlight the magnitude and nature by which these factors influence the final properties of process cheese. Currently, a study is being performed in our laboratory to evaluate the combined effect of these factors on the functional properties of process cheese.
Emulsifying salts As discussed previously, emulsifying salt is a major ingredient in process cheese manufacture. Emulsifying salts are ionic compounds made up of monovalent cations and polyvalent anions. The 2 primary functions of emulsifying salts in process cheese are “calcium sequestering” (to help disrupt the calcium–phosphate-linked protein network present in natural cheese during process cheese manufacture) and “pH adjustment.” Both of these functions help in hydrating the caseins present in natural cheese so that they can easily interact with the water and fat phases, thereby producing a homogeneous process cheese emulsion (Ellinger 1972; Meyer 1973; Caric and others 1985; Guinee and others 2004; Mizuno and Lucey 2005). According to the CFR, there are 13 emulsifying salts that are approved for use (either alone or in combination) in process cheese manufacture (21CFR133.169 to 133.180). These include mono-, di-, and trisodium phosphates, dipotassium phosphate, sodium hexametaphosphate, sodium acid pyrophosphate, tetrasodium pyrophosphate, sodium aluminum phosphate, sodium citrate, potassium citrate, calcium citrate, sodium tartrate, and sodium potassium tartrate. The most common emulsifying salts used for process cheese manufacture in the United States are trisodium citrate and disodium phosphate. Trisodium citrate is the preferred emulsifying salt for slice-on-slice process cheese varieties, whereas disodium phosphate (or appropriate combinations of di- and trisodium phosphates) is used in loaf-type process cheese and process cheese spreads. Sometimes, low levels of sodium hexametaphosphate are also used along with these emulsifying salts (in certain applications). Another emulsifying salt that has become popular recently is sodium aluminum phosphate. It is commonly used in rennet casein-based mozzarella type imitation process cheese varieties since it provides desirable functional properties for imitation process cheese that is used to replace mozzarella on frozen pizzas.
One of the most common questions that arise during the selection of emulsifying salts for process cheese manufacture is their calcium sequestering ability and their mechanisms of calcium removal from caseins. There have been various studies using different model systems to answer these questions (Cavalier-Salou and Cheftel 1991; Guinee and others 2004; Mizuno and Lucey 2005). However, the interaction of emulsifying salts with calcium and casein in process cheese is not fully understood. Important properties of different emulsifying salts and the influence of the type and amount of different emulsifying salts on process cheese properties have been extensively studied (Templeton and Sommer 1936; Ellinger 1972; Meyer 1973; Rayan and others 1980; Thomas and others 1980b; Gupta and others 1984; Caric and others 1985; Cavalier-Salou and Cheftel 1991; Molins 1991; Berger and others 1998; Awad and others 2002; Dimitreli and others 2005; Shirashoji and others 2005, 2006a, 2006b). However, due to the differences in experimental conditions among these studies, including type and age of natural cheese used, differences in process cheese formulation and composition, and differences in processing conditions used for process cheese manufacture, these studies show large variations in their results and, consequently, comparisons and interpretation of the results from these studies can be difficult. Detailed physicochemical properties of various emulsifying salts can be found in the literature (Ellinger 1972; Molins 1991; Berger and others 1998; Guinee and others 2004). Gupta and others (1984) extensively studied the effect of the type and amount of various emulsifying salts on process cheese pH, meltability, hardness, and body and texture. In Table 5, we have compiled some relevant data from previous research on the important properties of certain selected emulsifying salts along with their influence on process cheese properties. With relevance to Table 5, Rayan and others (1980) manufactured PC (40% moisture) with 4 different emulsifying salts at a 2.5% level without pH adjustment. Their results were in agreement with those of Gupta and others (1984) with PC manufactured using trisodium citrate and PC manufactured using sodium aluminum phosphate indicating a higher meltability than the PC manufactured using disodium phosphate; however, the firmness values of the 3 PC productions were similar. Thomas and others (1980b) manufactured process cheese (45% moisture) using trisodium citrate, disodium phosphate, and sodium hexametaphosphate at a 3% addition level (without pH adjustment) and evaluated the emulsion strength (by measuring oil separation), firmness, and meltability of all the process cheeses. They found that the emulsion strengths of all the process cheeses were not significantly different. The meltability of the process cheeses manufactured using trisodium citrate and disodium phosphate were not significantly different; however, process cheese manufactured using sodium hexametaphosphate had a significantly lower meltability. They also found that the firmness of the process cheese manufactured using trisodium citrate was significantly lower than both the process cheese manufactured using disodium phosphate and sodium hexametaphosphate. A more recent study also showed the effect of the type and amount of 3 different emulsifying salts on the hardness and meltability of process cheese (Shirashoji and others 2005). Shirashoji and others (2005) manufactured PC (38% to 39% moisture and 33% fat) with 3 different emulsifying salts, trisodium citrate, disodium phosphate, and sodium hexametaphosphate, at 0.25%, 1.5%, and 2.75% of the final process cheese. All of the PC was adjusted to pH 5.6 to prevent any effect on process cheese functional properties due to pH. They found that as the concentration trisodium citrate, disodium phosphate, and sodium hexametaphosphate in PC increased its firmness increased and its meltability decreased. Also, at 2.75% emulsifying salt concentration, PC made using sodium hexametaphosphate was the most firm and the least meltable followed by PC made using disodium phosphate and PC made using trisodium citrate. In another study on the effect of type of emulsifying salts in sliced process cheese, Shirashoji and others (2006a) produced sliced process cheeses (46% moisture, 19% protein) using 4 emulsifying salts (trisodium citrate, disodium phosphate, sodium hexametaphosphate, and tetrasodium pyrophosphate, at 2.5% each) without pH adjustment. They evaluated process cheese pH, flowability, and meltability, and created a “texture map” (by performing a large deformation burst test using a ball-type probe) of all the process cheeses. The pH of the process cheese made using sodium hexametaphosphate was significantly lower (pH 5.3) than the other process cheeses (pH 5.9 to 6.0). The meltability and flowability results showed that process cheese manufactured using tetrasodium pyrophosphate had the least meltability and lowest flowability followed by process cheese made using sodium hexametaphosphate. The meltability and flowability of the process cheeses made using trisodium citrate and disodium phosphate were similar. The results of the “texture map” indicated that process cheeses made using disodium phosphate and sodium hexametaphosphate tended toward a mushy and crumbly texture, whereas tetrasodium pyrophosphate provided the process cheese with a tough and rubbery texture (Shirashoji and others 2006a). In a study on the influence of the level of trisodium citrate on PCF properties performed in our laboratory, Garimella Purna and others (2006) manufactured PCF (44% moisture and 25% fat) using cheddar cheese (at 2, 4, 6, 12, and 18 wk of ripening) and trisodium citrate (at 2.0%, 2.5%, and 3.0% levels). All the PC batches were processed at 85 °C for 6 min at 2 different mixing speeds (450 and 1050 rpm). The viscosity was immediately measured after manufacture and the firmness, RVA hot apparent viscosity (a measure of the flowability of process cheese, Table 4), and RVA time at 5000 cP (a measure of the meltability of process cheese, Table 4) after PC manufacture. The results indicated a significant effect of the concentration of trisodium citrate on the RVA hot apparent viscosity and RVA time at 5000 cP. As the trisodium citrate level of the PCF increased, both the flow properties and the meltability of the PCF decreased; however, there was no significant effect of trisodium citrate concentration on the firmness of PCF.
Table 5—. Physicochemical properties of some emulsifying salts and their influence on process cheese (PC) properties.
|Emulsifying salt||Physicochemical propertiesa||Influence on PC propertiesc|
|Chemical formula||Formula weight, g/mol||Solubility, g/100 g H2O at 20 °C||pH, 1% solution||pH of the PC||Hardness (kg)||Meltability (mm)|
|Trisodium citrate (dihydrate)||NaH2C6H5O7.2H2O||294|| 75 || 8.6||5.9||32||131|
|Monosodium phosphate (monohydrate)||NaH2PO4.H2O||138|| 85b|| 4.5||5.1||27||NMd|
|Disodium phosphate (dihydrate)||Na2HPO4.2H2O||178|| 80 || 9.1||5.8||32|| 70|
|Trisodium phosphate (dodecahydrate)||Na3PO4.12H2O||380|| 11b||11.9||7.3||26|| 70|
|Dipotassium phosphate||K2HPO4||174||160 || 8.9||5.9||29|| 76|
|Sodium hexametaphosphate||(NaPO3)n (n= 10 to 15)||(102)n||157 || 6.6||5.2||33||NM|
|Sodium aluminum phosphate|| || 9.2||5.9||33||101|
Whey protein content Whey proteins constitute approximately 20% of the total proteins in bovine milk (Eigel and others 1984). Approximately 80% of whey proteins in milk are made up of the 2 major whey proteins (β-lactoglobulin and α-lactalbumin). One of the important characteristics of β-lactoglobulin from a processing standpoint is the presence of a “reactive” free sulfhydryl group in its primary structure (Wong and others 1996). Whey proteins are also highly susceptible to heat treatment and are found to denature between 60 and 70 °C. This temperature-induced denaturation of β-lactoglobulin exposes the free sulfhydryl group, which has the capability of crosslinking with other β-lactoglobulin and κ-casein molecules via disulfide bonds (Sawyer and others 1963; Wong and others 1996). NDM and whey protein concentrate (WPC), if used in a process cheese formula, may contribute to increased levels of whey protein in the process cheese. Since whey proteins can crosslink among themselves as well as with caseins at high temperatures, a high level of whey proteins in a process cheese formula not only influences its sensory properties but also may lead to an increase in the firmness of the final process cheese and a decrease in its meltability. The influence of whey protein incorporation in process cheese on its functional and sensory properties has been extensively studied (Gupta and Reuter 1992; Thapa and Gupta 1992a; Ido and others 1993; Abd El-Salam and others 1996; Al-Khamy and others 1997; Fayed and Metwally 1999; Mleko and Foegeding 2000, 2001; Laye and others 2004). Gupta and Reuter (1992) ultrafiltered whey to produce a liquid concentrate (26% total solids) with 20% protein and 5.8% lactose which was utilized as an ingredient to replace 20% of the solids in a PCF formula. They determined that addition of up to approximately 8% whey protein in the final PCF with average moisture content of 47% did not have an effect on the overall acceptability score of process cheese. However, in another study, Thapa and Gupta (1992a) have indicated that PCF (42% to 43% moisture, 2.5% emulsifying salt) containing WPC (at approximately the same final whey protein level) was firmer than PCF with no added WPC. Abd-El-Salam and others (1996) studied the effect of liquid WPC (28% total solids, 15% whey protein) addition (0%, 20%, and 40% of the final blend) on the compositional and the rheological properties of processed cheese spread (57% moisture, 3% emulsifying salt). In their study, addition of WPC increased the moisture by 0.8%, lactose by 2.5%, and pH by 0.3 in the final cheese spreads made from 40% WPC (6% whey protein in the final product) as compared to cheese spreads with no WPC added. They found an improved meltability (possibly due to the increase in moisture content), flavor, and other sensory properties of the PCS as the amount of WPC in the cheese spreads increased. In a recent study with a rennet casein process cheese model system (17% casein, 24% fat, 2% emulsifying salt), Mleko and Foegeding (2000) found that up to a maximum of 2% casein can be replaced with whey protein. However, they did find a slight increase in the firmness and a decrease in the meltability of the process cheese. They proposed that the heat-induced disulfide interactions involving the free sulfhydryl groups of β-lactoglobulins have a marked influence on the firmness and the melting properties of the process cheeses. Additional work by these researchers involved replacement of casein proteins with polymerized whey proteins in a rennet casein-based model process cheese system (Mleko and Foegeding 2000, 2001). They produced polymerized cross-linked whey proteins by heating whey protein dispersions to induce the disulfide cross-links between the whey proteins and added these polymerized whey proteins to a model process cheese system. Their work showed that, as the level of polymerized whey proteins was increased, there was an increase in the firmness and a decrease in the meltability of the process cheese analogs produced. However, they did conclude that replacement of 4% rennet casein (in a 17% rennet casein-based process cheese analog system) with 2% whey protein polymers could still produce process cheese analogs of identical texture and meltability (Mleko and Foegeding 2001).
Rework “Rework” is a term used to describe process cheese produced in a manufacturing facility that cannot be sold for a variety of reasons. The type of rework ranges from process cheese lost during production line changeovers; shavings and edge trimmings removed during slice line operations; residual process cheese that is removed from the cookers, lines, hoppers, and packaging machines, also referred to as “hot melt” (Kalab and others 1987); and process cheese that has been rejected by quality control due to improper weight, packaging, or on the basis of a quality defect (Kichline and Scharpf 1969; Zehren and Nusbaum 2000). Since rework poses economic challenges to manufacturers, it is incorporated into a fresh process cheese blend and reprocessed. According to Lauck (1972), the amount of rework produced in a process cheese manufacturing facility ranges from 2% to 15% of the total process cheese produced. Since rework has already undergone the emulsification process and also contains emulsifying salts, the addition of rework to a fresh blend during process cheese manufacture can cause difficulty during processing and also affects the final functional properties of process cheese (Kichline and Scharpf 1969; Lauck 1972; Kalab and others 1987). In general, the addition of rework tends to decrease the meltability and produce a firmer process cheese (Kalab and others 1987). Kichline and Sharpf (1969) specify that the maximum amount of rework that can be added to process cheese without any undesirable effects on its properties is 4% of the total cheese blend used for process cheese manufacture. Kalab and others (1987) found that the types of rework, as well as the amount used, have an effect on the final properties of PCF (43% moisture, 24% fat, pH 5.5 to 5.7, and 2.7% added emulsifying salt in the form of trisodium citrate). They studied the effect of type of rework on the apparent viscosity of PCF immediately after manufacture, its firmness, and its meltability. In their study, they used 3 sources of rework at 20% of the final PCF: fresh rework (quickly frozen PCF emulsion right after manufacture), regular PCF slices from older processing runs, and overcooked process cheese mass to simulate process cheese that had undergone excessive processing (which they referred to as “hot melt”). The “hot melt” was used at 2 levels: 10% and 20% of the final product. Their results indicated that when any kind of rework was used in PCF blends, the manufactured PCF was more viscous coming out of the cooker and had higher firmness and lower meltability relative to the control. They also found that the effect of the type of rework on the increase in apparent viscosity after manufacture and the firmness of the PCF was fresh rework < PCF slices from older process cheese runs < hot melt when used at 10% of the total blend < hot melt when used at 20% of the total blend. A similar trend was observed for the decrease in meltability of the PCF with the PCF with hot melt at 20% showing no melt. The microstructural results from their study showed that hot melt samples had a denser protein matrix and were overemulsified when compared to fresh rework samples (Kalab and others 1987). This phenomenon where excessive cooking of process cheese can increase the interactions among the caseins to such an extent that they attain a thick pudding-like consistency is also referred to as “overcreaming” (Meyer 1973).
In Europe, “precooked cheese” or rework obtained at different times during process cheese manufacture is sometimes used for enhancing the cooking and functional properties of process cheese (Meyer 1973; Berger and others 1998). According to Meyer (1973), fresh rework, which has a weakly dispersed (hydrated) protein structure (also referred to as “long structure”), can be effectively used to stabilize a process cheese that might show a tendency to overcream under normal cooking conditions. Normal rework with optimum protein dispersion and emulsification can be used from 2% to 30% in process cheeses where the creaming action is desired. However, Meyer (1973) cautions that hot melt should not be used at more than 1% of the process cheese blend in order to avoid adverse effects on the functional properties of the process cheese. In the United States, it is a general practice to collect shavings and edge trimming from slice-on-slice production lines and immediately use them as “fresh rework” at approximately 10% of the final blend.
The importance of controlling various formulation factors in a process cheese formula has been described previously. Also discussed previously are some of the important ingredients that affect these formulation parameters in a process cheese formula (for example, natural cheese affects the total calcium, intact casein, and pH; nonfat dried milk and whey ingredients affect the lactose content and whey protein level). Since the manufacturers use these ingredients to control the above-mentioned process cheese formulation parameters, the effect of natural cheese and other ingredients on process cheese properties is discussed subsequently.
Effect of ingredients. Natural cheese. Natural cheese is one of the most important ingredients used in process cheese. As discussed previously, natural cheese has a marked influence on total calcium, intact casein, and pH and, hence, the final functional properties of process cheese. Depending on the country of manufacture, availability, and market demand, various types of natural cheeses such as cheddar, Swiss, Gouda, and so on, are used to manufacture process cheese (Meyer 1973). However, in the United States, cheddar cheese is the major type of natural cheese used for process cheese manufacture. Depending on the type of process cheese manufactured, the amount of natural cheese in a process cheese formula varies from 51% to > 80% of the final process cheese (FDA 2006). Consequently, the characteristics of natural cheese utilized to manufacture process cheese have a major influence on process cheese characteristics and appropriate selection of natural cheese is critical in order to achieve a process cheese with the desired chemical and functional characteristics. The natural cheese used in a process cheese formulation is generally selected on the basis of type, flavor, maturity, consistency, texture, and pH (Zehren and Nusbaum 2000). Process cheese manufacturers, through years of experience, have realized and mastered the art of selecting the appropriate blend of young and aged natural cheese in order to achieve process cheese with desired flavor and textural properties (Meyer 1973; Thomas 1973). Numerous researchers have highlighted the importance of natural cheese characteristics on functional properties such as unmelted texture and meltability of process cheese (Barker 1947; Meyer 1973; Thomas 1973; Caric and others 1985; Shimp 1985; Zehren and Nusbaum 2000). The physicochemical characteristics of natural cheese that influence the process cheese properties include pH, calcium and phosphorus contents, and age, or the amount of intact casein present in the natural cheese (Templeton and Sommer 1930; Barker 1947; Olson and others 1958; Vakaleris and others 1962; Meyer 1973; Thomas 1973; Harvey and others 1982; Zehren and Nusbaum 2000; Kapoor and others 2007).
The importance of natural cheese pH on process cheese properties has been highlighted in a study performed by Olson and others (1958) in which they manufactured cheddar cheeses with modified manufacturing protocol so as to produce 2 cheddar cheese treatments with different final pH levels. The 2 cheddar cheeses were then used to manufacture PCS (at 10, 30, 60, 90, and 150 d of ripening), which were analyzed for unmelted texture using penetrometry and meltability using the tube melt test. Their results indicated that even after the final pH of the PCS was adjusted to 5.4 to 5.5, the PCS batches made using cheddar cheese with higher pH were harder and less meltable when compared to the PCS made using cheddar cheese with normal pH at all stages of ripening. However, the observed effect of natural cheese pH on process cheese properties was also related to the changes in the state and amount of calcium and phosphorus caused by the differences in natural cheese pH (Olson and others 1958; Zehren and Nusbaum 2000). The effect of natural cheese pH on the state and amount of calcium and phosphorus in natural cheese is discussed subsequently.
Minor changes in the manufacturing protocols during natural cheese manufacture, such as set pH, drain pH, and level of salting (salt-to-moisture ratio), can significantly change the state and amount of calcium and phosphorus in natural cheese (Dolby and others 1937; Czulak and others 1969; Upreti and Metzger 2006). In a study on low-fat mozzarella cheese, Metzger and others (2000) studied the effect of cheese milk set pH on the amount and state of calcium in the final cheese. It was found that as the pH of the cheese milk before rennet addition during cheese manufacture (set pH) was reduced to 6.0 or 5.8 (using acetic acid), the total calcium of the final cheeses decreased by 11% (set pH 6.0) or 23% (set pH 5.8), respectively, when compared to the control cheese that was set at a pH of 6.5. Czulak and others (1969) highlighted the effect of drain pH of cheddar cheese on the calcium content and the final pH of the cheese. They found that as the pH of the curd during whey drainage was decreased from 6.14 to 5.75, there was a 27% reduction in the total calcium content of the cheese curd at the time of whey separation.
Currently, another major thrust in the natural cheese industry is the utilization of concentrated milk to manufacture natural cheeses in order to increase the throughput of cheese plants. The type of concentration technique and the extent to which milk has been concentrated also influence the final pH, calcium content, and degree of proteolysis in the natural cheese (Sutherland and Jameson 1981; Anderson and others 1993; Acharya and Mistry 2004; Nair and others 2004). Consequently, utilization of these natural cheeses (manufactured using concentrated milk) as an ingredient in process cheese can also influence the functional properties of the process cheese manufactured using this natural cheese as an ingredient (Acharya and Mistry 2005). Acharya and Mistry (2005) manufactured PC using 5 different cheddar cheese treatments that were manufactured from cheese milk concentrated with different concentration techniques at different levels. The treatments were cheddar cheeses manufactured using control (normal cheese milk), ultrafiltered milk (concentration factor of 1.5×), ultrafiltered milk (concentration factor of 2.0×), vacuum-condensed milk (concentration factor of 1.5×), and vacuum-condensed milk (concentration factor of 2.0×). They found that, as the concentration factor of the milk utilized to manufacture cheddar cheese was increased to 1.5×, the calcium content of the cheddar cheese manufactured increased by 10% when the milk was ultrafiltered and by 4% when the milk was vacuum-condensed. Moreover, when the concentration factor of the milk utilized to manufacture cheddar cheese was increased to 2.0×, the calcium content of the cheddar cheese manufactured increased by 18% when the milk was ultrafiltered and by 13% when the milk was vacuum-condensed. Moreover, they found that the degree of protein hydrolysis in the cheddar cheeses was also affected by both the method and the level of concentration. When these natural cheeses were used to make PC, the hardness of PC increased and the melt and the flow properties of PC decreased, as the level of concentration of the cheese milk to manufacture the natural cheese increased (in both the concentration techniques). Consequently, utilization of natural cheeses manufactured using concentrated milk as an ingredient in process cheese can also influence the functional properties of the process cheese manufactured.
Sometimes, it is also common for process cheese manufacturers to freeze natural cheese that will eventually be used for process cheese manufacture (Zehren and Nusbaum 2000). Thomas and others (1980a) studied the effect of frozen natural cheese on process cheese functional properties. They manufactured 3 process cheese treatments utilizing the same natural cheese (regular or frozen). The natural cheese was split into 3 parts. One part was ripened for 6.5 mo at 10 °C, the 2nd part was ripened for 6.5 mo and then frozen at −20 °C for 3 mo before being used to make process cheese, the 3rd part was ripened for 3.5 mo followed by freezing at −20 °C for 3 mo and further repining for 3 mo. They found that when frozen natural cheese (in all the freezing treatments) was used to manufacture process cheese, the firmness of the resulting process cheese was higher and its meltability was lower when compared to the process cheese manufactured using the same natural cheese which was not frozen.
Nonfat dried milk/dried whey/whey protein concentrate. The levels of dairy-based ingredients other than natural cheese used in PC are specifically defined by the CFR (Table 1). Ingredients such as NDM and whey-based dairy ingredients such as liquid whey, whey powder, and WPC can be used in PCF and PCS (FDA 2006). Since the addition of these ingredients to process cheese formulation helps to reduce the cost of the product, manufacturers often try to maximize the addition of NDM and whey-based dairy ingredients in their products. The amounts of these ingredients typically added to process cheese are not known to cause significant changes in process cheese properties. However, since commercial NDM and dried sweet whey have an approximate lactose concentration of 50% and 75%, respectively, and commercial NDM and WPC have a significant amount of whey proteins, 2 important formulation factors need to be taken into account when using NDM and other whey-based ingredients in PCF and PCS manufacture. These factors are the level of lactose and the level of whey protein (as discussed in subsections on lactose content and on whey protein content) provided by these ingredients in the final process cheese.
Food gums/hydrocolloids. The CFR allows gums or hydrocolloids to be used in PCS at levels not exceeding 0.8% of the finished product (Table 1). These include carob bean gum, gum karaya, gum tragacanth, guar gum, gelatin, sodium carboxymethylcellulose (cellulose gum), carrageenan, oat gum, algin (sodium alginate), propylene glycol alginate, or xanthan gum singly or in combination. Gums do not directly affect any of the above-mentioned formulation parameters in the process cheese; however, since PCS has a high moisture content (up to 60%), the major function of gums in PCS is to bind water and to provide appropriate viscosity/thickening to the product and improve its mouthfeel. Therefore, gums in a PCS formula have an effect on melted textural properties. Zehren and Nusbaum (2000) indicate that the selection of gums depends on various factors. These include ease of dispersibility, solubility, hydration behavior, moisture holding ability, cook viscosity, compatibility with milk proteins and other compounds present in process cheese, and optimum working pH range. Phillips and Williams (2000) provide extensive information on the properties and uses of different gums used in the food industry. However, there is a lack of available literature dealing with the effects of various gums on the final properties of PCS. Another important area where the use of gums is gaining popularity is in low-fat and reduced-fat process cheese and imitation process cheese varieties (Brummel and Lee 1990; Swenson and others 2000).
As described in the subsection “Process cheese manufacture,” after the preparation of a desired formulation, the ingredient blend is processed using heat and mixing to produce a homogeneous mass, which is packaged and cooled. Although effective monitoring and control of the formulation parameters are critical to ensure the production of process cheese with specific functional properties, appropriate selection of the process cheese cook conditions is also very important, since differences in processing conditions during process cheese manufacture have a major influence on the functional properties of process cheese. The effect of processing conditions on process cheese functional properties is discussed subsequently.