Sustainable Approaches in Whey Cheese Production: A Review

: Whey cheeses have been produced from the very early steps of cheesemaking practices as a sustainable way to utilize whey, which is the main by-product of cheesemaking. Traditional whey cheeses, manufactured with similar processes, are Ricotta, Ricotta salata or Ricottone, and Ricotta fresca in Italy; Anthotyros, Myzithra, Manouri, Xynomyzithra, and Urda in Greece; Urda in Serbia and Romania as well as in other countries such as Israel; Lor in Turkey; Anari in Cyprus; Skuta in Croatia and Serbia; Gjetost and Brunost in Norway; Mesost and Messmör in Sweden; Mysuostur in Iceland; Myseost in Denmark; Requeij ã o in Portugal; and Reques ó n in Spain and Mexico. The production of whey cheese is based on the denaturation of whey proteins by heating to 88–92 ◦ C. The speciﬁc processing conditions and aspects of the microbiology of whey cheeses are discussed. The special characteristics of whey cheeses, which are high pH and high moisture content, make them susceptible to microbial growth. Due to the limited shelf life of these products, extended research has been carried out to extend the shelf life of whey cheese. The sustainable preservation approaches, such as modiﬁed atmosphere packaging, addition of herbs and/or plant extracts, and bio-preservation methods are reviewed. Moreover, novel whey cheeses focused on functional properties have developed during the last 10 years.


Introduction
The nutritional properties of whey were very early recognized, and from the fifth century BC, the Greek physician Hippocrates prescribed cheese whey for gastrointestinal disorders and to enhance the immune system [1,2]. It is not known when exactly whey was first utilized to produce whey cheese. From the seventh century BC, rennet-coagulated Caprino and Pecorino were produced in Tuscany; Columella, in his treatise on agriculture, De Re Rustica (50 AD), describes the production of Ricotta cheese [3][4][5][6]. Whey cheese was very common in Roman times for breakfast and Caseum sanctuarie was the cheese made with the milk provided on the celebration of St. J. Baptist [5,6]. Anari cheese is produced from the whey released during the production of Halloumi and its manufacture dates back to Halloumi making [7]. Norwegian whey cheeses have their origins back to the Vikings, about a thousand years ago [8].
Whey cheeses are, nowadays, produced globally, under various names: Ricotta, Ricotta salata or Ricottone, and Ricotta fresca in Italy; Anthotyros, Myzithra, Manouri, Xynomyzithra, and Urda in Greece; Urda in Serbia and Romania as well as in other countries such as Israel; Lor in Turkey; Anari in Cyprus; Skuta in Croatia and Serbia; Gjetost and Brunost in Norway; Mesost and Messmör in Sweden; Mysuostur in Iceland; Myseost in Denmark; Requeijão in Portugal; and Requesón in Spain and Mexico [5,[7][8][9].
Great variation in the chemical composition of whey is reported, and this differs according to the type of cheese produced, which is the main cheesemaking product, and the kind of milk from which the main cheese is made [35][36][37][38]. Additionally, seasonal differences due to lactation or other reasons or feed differences also cause the observed variations [39]. Due to the composition of sheep milk, sheep whey consists of significantly higher levels of total solids, lipids, and proteins than cow whey [14,40]. Whey proteins have a higher nutritive value than casein and are considered as one of the richest proteins available. Whey contains essential amino acids, e.g., branched-chain amino acids (isoleucine, leucine, and valine) and sulfur amino acids (cysteine and methionine), and antioxidant properties have been reported [41]. Sheep whey contains more β-lactoglobulin, about the same proportion of α-lactalbumin, and lower proportions of serum albumin and immunoglobulins compared to cow whey [42]. Goat whey contains twice as much α-lactalbumin and lower levels of β-lactoglobulin, serum albumin, and immunoglobulins compared to cow whey [43,44]. Nudda et al. [45] observed seasonal variation in conjugated linoleic acid and vaccenic acid in milk fat of sheep whey used to make Ricotta cheese; conjugated linoleic acid and vaccenic acid decreased as lactation progressed, probably due to variation in pasture availability and fatty acid composition of grass.

Sustainable Valorization of Whey
Based on the principles of circular economy, whey is nowadays used in the manufacturing of various products, such as whey cheeses, whey powder, edible films, beverages, as well as feed supplement and biogas production. Whey has been proved to be a valuable product due to its components and their functional properties, and food-related applications containing whey components, mainly proteins, have been developed [21,22,61,62]. The valorization of whey has been reviewed by Smithers [2], Papademas [70], Barba [16], Arshad et al. [71], Osorio-González et al. [72], León-López et al. [73], Buchanan et al. [74], Gergihon et al. [75], and Rocha and Guerra [76]. Rama et al. reviewed the potential applications of whey, as cryoprotectants for freeze-drying and as encapsulating agents for the spray-drying used to produce lactic acid bacteria (LAB) cultures [77]. Valta et al. [78] investigated the technologies of treatment of cheese whey applied by Greek dairies and suggested potential whey valorization opportunities.
Membrane processing, such as ultrafiltration, microfiltration, and reverse osmosis technologies, has been used in the cheese industry to fractionate whey components and to enable whey proteins and other valuable constituents to be retained in the cheese [79][80][81][82][83]. New or improved products derived from whey have gained commercial success due to the enhanced functional properties, which have enabled their incorporation into a number of food and drinks [63,79]. Whey cheeses such as Ricotta cheese, have been produced using ultrafiltrated whey, with reduced cost and improved cheesemaking efficiency [26].
Cheese whey exploitation by the Brazilian dairy industry was studied using a questionnaire survey [84]; most of the dairy industries produced and processed sweet cheese whey, using it in whey dairy beverages, Ricotta, and different types of whey concentrates. Kotoupas et al. [85] compared five alternative scenarios for the treatment of cheese whey wastewater resulting from a cheese factory and concluded that the production of Manouri cheese was the most profitable one. Masotti et al. [86] reviewed the effect of different technologies used for the fortification of cheese with whey proteins: heat treatment of cheese milk, membrane separation technologies, and the direct addition of milk-or whey-based derivatives and more recent technologies such as high hydrostatic pressure, ultra-high-pressure homogenization, transglutaminase treatment of cheese milk, and direct addition of buttermilk.

The Manufacture of Traditional Whey cheeses
Whey cheeses belong to a special category of cheeses that are produced by heating the whey at high temperatures to form a coagulum following the denaturation of whey proteins. Whey cheeses are globally produced by traditional methods either as artisanal or standardized industrial processes as a sustainable method of utilizing the whey according to the principles of circular economy [14,32]. There are many whey cheeses that were originally produced from sheep or goat whey as by-products. Nowadays, since their consumption has increased, dairies have started to produce whey cheese from cows' whey or add cows' milk. Most of them are heat-and/or acid-coagulated cheeses, and their technology varies in respect of the raw material (type of whey) used, the quantity and type of the milk and/or cream added, and the cheesemaking recipe followed [87,88]. The most important whey cheeses and their composition and processing characteristics are shown in Table 2.   In general, to produce whey cheese, the whey is heated at a temperature of 88-92 • C while stirred. The rate of heating is important to be such as to attain the selected temperature in 40-45 min. Another critical step is the rate of stirring. The first small particles of denaturated whey proteins appear at temperatures around 80 • C. This temperature could be lower according to the type of whey, the acidity and the quantity and proportion of milk and/or cream added [14,112]. The final temperature is lower when whey cheeses are to be consumed fresh, whereas higher temperature heating is applied for dried whey cheeses. The curd obtained is left for 15-20 min on the surface at the selected temperature to lose part of its moisture and then it is scooped and molded for drainage; whey cheese is mainly consumed fresh [14,112].
In the cases in which only the whey is used for the production of whey cheese, the particles of denatured proteins floating on the surface are small and the yield is low. Usually, a certain amount of milk and/or cream is added during heating; the quantity of milk is 30-50 mL/L; however, for higher quality cheeses, milk percentage could be 10% or higher, and cream may also be added [112]. Whey cheese, in most cases, is consumed fresh, that is, without fermentation and ripening; only a few whey cheeses, e.g., Ricotta forte and Xynomyzithra, undergo the ripening process [111,113]. Coprecipitation of caseins may be improved by acidification; the yield is improved when addition of acid is coupled with the addition of calcium chloride [112].
Salt may also be added to the whey, as well as an acidification agent; the latter is usually needed in the case of cow whey to help denaturation and final precipitation of the whey proteins [114]. Acidification is applied in the form of an aqueous solution of citric acid at the rate of 6 mL/L and this quantity decreases the pH to about 5.2 [112,114].
Kaminarides et al. [115] studied the effect of the addition of milk and cream on the characteristics of Greek whey cheeses and reported that the addition of milk and cream led to higher yields; cheese produced with 79% whey, 15% milk, and 6% cream had the most preferred sensory characteristics. More recently, Kaminarides et al. [27] studied the effect of whey fat content on the yield, physicochemical, textural, and sensory properties of cheeses made from sheep whey; cheese with 5% added cream of 50% fat was the most preferred in sensory evaluation. Similarly, Ricotta can be produced by using whey resulting from cow, buffalo, sheep, and goat cheese manufacturing, as well as mixtures of them (Table 2). Biancolillo et al. [116] investigated the fatty acid profiles of 240 samples of Ricotta cheese made from sheep, goat, cow, or buffalo milk. Low-salt whey cheeses can be manufactured with a combination of NaCl and KCl salts to lower the sodium content in Requesón while maintaining the accepted sensory characteristics [117].
As shown in Table 2, there is a great variability in the composition of whey cheeses. The variability is mainly reflecting the differences in composition of the whey (Table 1), differences in the cheese-making technology, the kind of cheese milk used for the main cheese, the kind of milk and/or cream added, or the use of sweet or acid whey; these factors lead to a large variability in the characteristics of whey cheeses. Sensory properties of Ricotta cheese, such as softness, granularity, and greasiness, as well as the fatty acid profile, were significantly different among goat breeds [118].

Aspects of the Microbiology of Whey Cheese
Whey cheeses are undergoing a high temperature heat treatment at 88-92 • C for the denaturation of whey proteins, and this treatment has a severe impact on the microbiota of the whey or of the milk and/or cream added to the whey. The indigenous microbiota is inactivated and no starter cultures are applied after heating [7][8][9].
Escherichia coli O157:H7 was reported to grow in Myzithra, Anthotyros, and Manouri whey cheeses at 12 • C and survive at 2 • C storage [119]. The growth of L. monocytogenes in the same whey cheeses was reported to be very rapid; maximum populations were significantly lower in Myzithra cheese at 5 and 12 • C than maximum populations in Anthotyros and Manouri cheese at the same temperatures independent of the inoculated strain. In addition, Aeromonas hydrophila was recovered in 4 (10.2%) and 3 (8.3%) of the Anthotyros and Manouri cheese samples analyzed, respectively, but not countable populations were noted [120]. A. hydrophila grew in Myzithra and Anthotyros, but no growth was observed in Manouri [121].
Theodoridis et al. [125] reported 4% and 2% of Manouri and Anthotyros cheese samples, respectively, positive for Listeria spp. Additionally, 2% of samples of Anthotyros were reported positive for L. monocytogenes. Later, Angelidis et al. [126] in a survey of L. monocytogenes in soft cheeses for sale in retail outlets found no positive soft cheeses, including Anthotyros, Manouri, fresh Myzithra, dry Myzithra and Ricotta, and only 1 sample out of the 137 cheese samples examined contained Listeria innocua at levels of 4.5 × 10 2 cfu/g. Different physicochemical and microbiological characteristics of cheeses may affect L. monocytogenes potential to grow and/or survive during storage [127]. A study conducted on Sardinian dairies revealed that the prevalence of L. monocytogenes on Ricotta salata rind was approximately 18% [128]. Requesón showed a 6.7% prevalence of L. monocytogenes [129,130], whereas, 30 samples of Ricotta examined were free of L. monocytogenes [131]. Villarruel-Lopez et al. [99] studied the presence of aerobic mesophilic bacteria, coliform bacteria, yeasts, molds, and certain pathogenic bacteria and staphylococcal enterotoxins in Mexican commercial Requesón cheeses; although they reported samples at high risk of contamination, neither L. monocytogenes nor staphylococcal enterotoxins were detected. Scatassa et al. [132] reported that during a 15 year (2002-2016) investigation of the microbiological profile of Sicilian Ricotta, made from sheep whey, Ricotta represented a safe cheese from a hygienic point of view, and during the entire period of investigation, the pathogenic bacteria (L. monocytogenes, Salmonella spp. and Brucella spp.) were never detected, while the levels of undesired microorganisms such as Enterobacteriaceae, E.coli, and coagulase-positive staphylococci were found in reduced numbers.
Whey cheeses have reported to be able to support the growth of L. monocytogenes and low initial levels of contamination can lead to concentration potentially threatening to human health [133][134][135][136][137]. Evidence that L. monocytogenes can contaminate whey cheeses and that these cheeses support its growth, even during refrigerated storage, has been documented [133][134][135][136][137][138].
Adame-Gómez et al. [139] reported 9 positive samples of Requeson cheese out of 16 examined with Bacillus cereus contamination at a 5.52 log cfu/g, whereas Sattin et al. [140] identified a Bacillus strain that caused pink discoloration in the same cheese. Cosentino et al. [141] reported 78% positive samples for Bacillus spp. in Ricotta cheese, with counts rarely exceeding 10 cfu/g, but also counts higher than 1000 cfu/g were found (16% of positive samples). The level of B. cereus vegetative cells in naturally contaminated Ricotta salata decreased during refrigerated storage, while the presence of spores was rare [142,143].
Post-process handling of the curd results in cross-contamination with a diverse native microbiota, mainly psychrotrophic Gram-negative bacteria, lactic acid bacteria (LAB), staphylococci, spore-forming bacilli, clostridia, yeasts, and molds [14,[144][145][146][147]. Considering that whey cheeses have a high pH, high moisture content, and a low salt content (Table 2) these dairy products are very susceptible to microbial spoilage, especially under abuse temperatures. Additionally, whey cheeses constitute a major food safety concern due to their lower hurdle effect compared with ripened cheeses, showing a higher incidence and potential for survival/growth of pathogens [14,[144][145][146][147]. Psychrotrophic food-borne pathogenic bacteria possessing elaborate cold-adaptation mechanisms can grow in milk and whey products during refrigerated storage [148]. The main route of contamination is the contact with surfaces of the processing plant environment and equipment [149]. Spanu et al. [135] studied the effect of a post-process heat treatment on the survival of L. monocytogenes cells in artificially contaminated Ricotta salata and demonstrated that the reduction in L. monocytogenes count was entirely accountable to the hot water treatment.

Sustainable Preservation Approaches of Whey cheeses 6.1. Modified Atmosphere Packaging
The use of effective and sustainable methods for the preservation and safety of whey cheeses is necessary to control cross-contamination of fresh whey cheeses throughout their storage. The increased consumer demand for fresh, preservative-free foods has led to extended research on advances in cheese preservation focusing on 'clean labels' [150]. The use of modified atmosphere packaging (MAP) as a technique to improve product' safety and extend the shelf life satisfies the chemical preservative-free strategies for various foods [151]; MAP is widely used as a storage method, extending the shelf life and improving the appearance of several foods. The potential of MAP and active packaging for extending shelf life of dairy products, including cheese, has been demonstrated [152][153][154].
Several MAP conditions were used to assess the effect of MAP on the characteristics, that is, free fatty acids, lactose, lactic acid, moisture, pH, and texture of whey cheeses [24,[155][156][157][158][159][160][161][162][163][164]. The MAP conditions and the best practice and/or effect on whey cheeses is presented in Table 3. Different mixtures have been applied in different whey cheeses; in most studies, a specific mixture of gases provided an extension in the shelf life. Table 3. Modified atmosphere packaging (MAP) conditions and the best practice and/or effect on whey cheeses.

Addition of Herbs and/or Plant Extracts
There are a number of studies in the literature on the addition of various herbs and/or plant extracts and their potential use in combination with different packaging materials to extend the shelf life of whey cheeses [20,[164][165][166][167][168][169][170]. Black cumin, thyme, and rosemary are frequently added to cheeses because of their sensory and antioxidant properties [20,166]. Akpinar et al. [20] studied whey cheeses packaged in goat-skin bags used for the storage of traditional cheeses with a long shelf life (semi-hard and hard cheeses) and alternatively studied cheeses in artificial casings. In addition to different packaging materials, the potential usage of different plants (black cumin, thyme, and rosemary) in Lor cheese production was evaluated.
Christaki et al. [170] studied the effect of oregano essential oil and extracts on two Greek whey cheeses. The authors evaluated the feasibility of employing oregano essential oil and extracts in whey cheese to control fungal contamination that causes cheese spoilage and important economic losses and reported that the combination of oregano essential oil and extracts produced nanoemulsions with potent antioxidant and antifungal activity against Penicillium expansum in vitro.
Samelis et al. [179] investigated the use of nisin as a bio-preservative to control L. monocytogenes introduced post-processing on Anthotyros stored in refrigerated storage in vacuum packages for up to 45 days; they reported that this treatment suppressed L. monocytogenes growth below the inoculation level for 30 and 45 days [179]. Interestingly, nisin resulted in a switch in the natural spoilage flora of Anthotyros from Gram-positive to Gram-negative [179]. Aspri et al. [180] applied a bacteriocin-producing Enterococcus faecium isolated from donkey milk in the bio-control of L. monocytogenes in fresh whey cheese. Spanu et al. [181] investigated bio-preservatives to control the growth of spoilage microorganism, that is Pseudomonas spp., on the surface of MAP-packed Ricotta fresca during refrigerated storage and Carnobacterium spp. inoculated on the surface of the finished product; they reported promising results in controlling contamination of Ricotta fresca with certain spoilage microorganisms. In another study, Carnobacterium spp. significantly reduced the growth of Pseudomonas spp. at 1.28 log and 0.83 log after 14 and 21 days of refrigerated storage, respectively [182]. Fernández et al. [187] studied the effect of nisin alone and in combination with Microgard, that is bacteriocin-like inhibitory products, on the microbial flora of Ricotta cheese.
Bacteriocin production in situ from several LAB and its impact on food preservation have been examined; thermophilin T, a bacteriocin produced by Streptococcus thermophilus ACA-DC 0040, had an inhibitory activity against a large number of related LAB as well as Clostridium spp. [189]. Kaminarides et al. [190] evaluated the effect of thermophilin T, previously produced in fermented milk, on the microbiological and physicochemical characteristics of Myzithra cheese.
More recently, Sameli et al. [185] studied the effect of an enterocin A-B-P crude extract on the spoilage microbiota in fresh Anthotyros whey cheeses stored at refrigeration temperatures in vacuum for 40 days; Pseudomonas spp., Aeromonas spp., Hafnia spp., and Serratia spp. grew faster than LAB during early storage. Later, LAB outgrew the Gram-negative bacteria and prevailed by mid-to late storage in all cheese batches, causing an acidification effect [185]. Sameli and Samelis [144] assessed the growth and bio-control of inoculated L. monocytogenes in Anthotyros whey cheeses, without or with 5% of a crude enterocin A-B-P extract during storage at 4 • C. From day 15 to the sell-by date (days [35][36][37][38][39][40], L. monocytogenes growth ceased, and progressively, the populations of the pathogen declined in most cheeses; this was due to an unmonitored, batch-dependent natural acidification by spoilage lactic acid bacteria, predominantly Leuconostoc mesenteroides, which reduced the cheese pH to 5.5 and finally to a value of 5.0 [144].

Novel Treatments for Extending the Shelf Life
During the last 10 years, a number of novel treatments have been applied to whey cheeses in order to extend the shelf life and to enhance their functional and sensory characteristics. Duarte et al. [191] proposed an alternative to refrigerated storage technology for Requeijão; they stored the cheese under 100 MPa hyperbaric storage at variable room temperature, and total aerobic mesophiles and Enterobacteriaceae growth was inhibited. For LAB, yeasts, and molds, hyperbaric storage showed an additional microbial inactivation effect. This technology is an environmentally friendlier technology, with a carbon footprint estimated to be about 26-fold lower compared to refrigerated storage [191]. The possibilities of using the continuous type of UV-C light on the surface of Lor cheese was studied, and it was found that the application of UV-C light (1.617, 4.018, and 36.832 kJ/m 2 ) to the cheese surface allowed delaying mold growth during storage; however, extreme doses could induce lipid and protein oxidation reactions, leading to quality deterioration [192].
Two novel food preservation technologies were applied to Ricotta, that is pulsed light [193] and plasma treatment [194]. The authors concluded that both methods could be used to extend the shelf life of Ricotta cheese. To extend the shelf life of buffalo Ricotta cheese, a process was assessed that included a second heat treatment followed by homogenization and hot packaging. In another study, Tripaldi et al. [101] reported that homogenized buffalo Ricotta cheese had a longer shelf life than traditional Ricotta cheese, although the process could be optimized to reduce the total bacterial load during storage.

Novel Whey Cheeses and Whey Products
A modified form of Myzithra cheese was produced by substituting the fresh cheese whey by dried whey protein concentrate 65% [195]. The modified Myzithra cheese had a higher lactose, potassium, and sodium content and lower total bacteria, protein, ash, calcium, hardness, and adhesiveness than control Myzithra cheese, while yeasts and molds were absent. Attempts to incorporate fresh whey protein concentrates in Queso fresco showed good results in terms of yield and acceptance, although excessive levels may cause detrimental changes in the texture and flavor of Cheeses and the process needs to be optimized [38]. The incorporation of whey cheese in milk used to make Queso fresco was suggested as an alternative technology for small and medium scale producers for revalorizing cheese whey, increasing yield, and lowering production costs, although physicochemical characteristics were affected [196].
An application for salted whey produced from Domiati, Ras, and Cheddar cheese making is to use it as an ingredient in processed cheese [197].
An added-value whey cheese through addition of pine nuts was developed by Semenium et al. [198]; different concentrations of pine nuts were added to whey cheese, and the whey cheese sample with 4% pine nuts was the most appreciated by the sensory panel [198]. A whey cheese analogue was made from 65% whey protein concentrate and vegetable fat with 15% olive oil, and the product was enriched with thyme leaves, film-packed, and then subjected to heat treatment; the product was characterized as a new whey cheese easy to produce, with improved health enhancing properties, acceptable textural characteristics, a rich aroma profile, and extended shelf life [199].
In addition, novel dairy products using whey cheese or whey components have been studied. Ice cream made from sheep's milk was manufactured with addition of whey protein concentrates [200]. Ricotta cheese powder, prepared from different blends of whey and milk, was used for the manufacture of dried cheese [201]. Marnotes et al. [202] manufactured frozen yoghurts using goat and sheep liquid whey concentrates produced by ultrafiltration; different concentrations of inulin were also added. Maragkoudakis et al. [203] studied the use of scotta, that is, the second whey remaining after the manufacture of Ricotta, as substrate for bacterial fermentation with the objective of obtaining a novel fermented drink and a selection of a starter consisting of S. thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, and L. acidophilus, was chosen; the authors speculated that further studies are needed to optimize the organoleptic aspects of the final product. Moreover, an economic and feasible option for dairy products is the production of butter using cheese whey; whey cream is separated from the aqueous fraction of cheese whey and processed to obtain whey cream butter [204][205][206]. Costa et al. [206] reviewed the technology of production of butter from whey cream and the physico-chemical, sensory, and nutritional characteristics of the product.

Functional and Probiotic Products
Although Hippocrates (460-370 BC) said 'Let food be thy medicine and medicine be thy food', and the term 'functional food' was first mentioned in Japan [207], nowadays, there is an increasingly awareness that a good health is directly associated with food and nutrition habits, and functional foods are increasingly demanded by the consumers. Dairy products are leading the research and development of health-promoting and therapeutic products [23,[208][209][210][211]. Madureira et al. [212][213][214] effectively applied probiotic cultures in whey cheese, and this type of cheese has proven to be a good vehicle to deliver probiotic bacteria into the gastrointestinal tract. Whey cheese and whey products have several functions to be classified as functional foods [215][216][217]. Whey-protein-derived peptides have several beneficial effects, e.g., ACE-inhibitory peptides in the treatment of hypertension and β-lactotensin peptide in cholesterol-lowering, anti-stress, and memory-enhancing activities [91,[215][216][217].
Dimitrellou et al. [218] evaluated the feasibility of producing a novel whey cheese with extended shelf life using probiotic lactobacilli immobilized on casein. Pires et al. [219] evaluated the application of kefir culture and probiotics, i.e., Bifidobacterium spp., which has previously added and fermented the cream added to the whey for the manufacture of Requeijão cheese. In another study, the addition of L. delbrueckii ssp. bulgaricus NCFB 2772 was investigated to manufacture a functional acid whey cheese [220]. Nzekoue et al. [221] suggested Ricotta cheese as a cheese matrix for vitamin D fortification.
By considering the potential of whey cheese as a probiotic carrier, the combination of traditional Requeijão with second cheese whey protein, probiotic cultures, and ingredients improving the organoleptic characteristics of the final product was studied [222]; this approach enabled minimizing the environmental impact of the cheese whey and especially whey by-products, assuring a sustainable cheese supply chain. Rosa et al. [223] evaluated the effect of probiotic strains of L. acidophilus, Bifidobacterium Bb-12, and Lacticaseibacillus casei-01 on the characteristics of fermented whey/milk beverages during refrigerated storage for 30 days [221]. Schlabitz et al. [224] investigated the shelf life of a symbiotic fermented dairy beverage using Ricotta cheese whey, prebiotics, and probiotics; since the aim of the study was to reduce the amount of whey incorporated into the dairy wastewater, the authors concluded that the whey drink proved to be a viable way to use whey, reducing thus the effluent generated at dairy industries. In addition, Mileriene et al. [225] suggested a valuable way to utilize acid whey and apple pomace with the fermentation from L. lactis LL16, with the potential to produce a sustainable cheese with functional properties.

Conclusions
Whey cheese production has a long history as a sustainable way to utilize the released whey after the milk curdling. Whey cheeses are produced and consumed worldwide due to their nutritional value and acceptable sensory characteristics. There is a great variation in the technology used, the kind of whey, and thus the variation in composition and characteristics common among different types. In any case, the exploitation of whey for the manufacture of whey cheeses has many practical and financial benefits.
The special cheesemaking processes and the composition have an impact on the type of microorganisms present in the final product. Because whey cheeses have a neutral pH and high moisture content, they may be exposed to contamination and should be consumed in a short period after production. Initially, after the coagulation of whey proteins, and due to the heating of the whey, the natural milk and whey microbiota is inactivated, starter cultures are not applicable and thereby whey cheeses present very low counts of microorganisms after manufacture. However, handling of the fresh curd post-heating results in cross-contamination with a large variety of spoilage, mainly yeasts, molds, coliforms, pseudomonads, and potentially pathogenic bacteria. Therefore, hygienic practices throughout the handling, packaging, and storage are of high importance.
The functional properties of the whey have forced researchers to investigate novel and more attractive products based on whey, some of them with health-promoting potential.
Although the production of whey cheese is one sustainable way to utilize the valuable whey, the current review presents specific knowledge concerning whey cheeses to help further research for the development of novel cheeses and other dairy products for sustainable valorization of whey, a proven valuable by-product of the cheesemaking process in a sustainable way.
Author Contributions: Conceptualization, T.B. and P.P.; writing-original draft preparation, T.B. and P.P.; writing-review and editing, T.B. and P.P.; visualization, T.B. and P.P. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.