Decontamination of collected milk is generally achieved through heat treatments that use various combinations of time–temperature parameters to obtain the desired bactericidal effect: thermization, pasteurization, or sterilization by autoclaving or ultra-high-temperature (UHT) treatment. Even though these heat treatments ensure the safety of milk and dairy products, they almost always induce irreversible modi-fications of milk components, alter physicochemical calcium salts and protein equi-librium, and also adversely affect the organoleptic quality of fluid milk and dairy products as well as the cheesemaking ability. Moreover, the dead cells of killed bacte-ria remain in heated milk with their potentially active enzymes, which with the meta-bolic activity developed by the growth of the remaining thermoduric bacteria will cause alterations of liquid milks during storage, thus reducing commercial shelf life.
Membrane MF offers an interesting alternative to heat treatments. Initially proposed by Holm et al. (1984), it has led to the technology and equipment called Bactocatch® (Tetra Laval Co.). Numerous studies conducted in Sweden and in France, summarized by Saboya and Maubois (2000), have optimized the original parameters described in the patent of Holm et al. (1984). Skim milk heated to 50°C is circulated at a velocity of 7.2 m.s–1 along a membrane having an average pore size of 1.4 mm (Sterilox® or equivalent) according to the hydraulic concept of an uniform transmem-brane pressure (UTP), in the range of 0.5 bar, obtained either by recirculation of the permeate (Meershon, 1989) or by a specially designed MF membrane having contin-uous variation of the porosity of its support (Membralox GP®) or continuous varia-tion of the thickness of the membrane layer (Isoflux®). All the somatic cells and most of the residual fat and the contaminating microorganisms are concentrated 20 times in the MF retentate. In MF industrial equipment, this retentate is then concentrated 10 times more in a second MF apparatus, thus leading to a volumetric concentration factor (VCF) of 200. Fluxes obtained industrially are in the order of 500 L.h–1.m–2 during 10 h. According to VCF of 20 or 200, the observed permeation rates are, for proteins 99% and 99.4% respectively, and for total solids 99.5% and 99.9%. Average observed decimal reduction (DR) of bacteria is above 3.5 for the milk collected in the developed dairy countries (initial Total Count [TC] < 200 000 CFU.mL–1); it can be higher than 6 in milks with a poor bacteriological quality collected in some emer-gent countries. Spore-forming bacteria that represent the main surviving species to pasteurization are highly retained by MF membrane (DR > 4.5) because of their large apparent cellular volume when they are in milk (Trouvé et al., 1991). Synthesis of the studies done by Madec et al. (1992), the Pasteur Institute, and the Institut
Applications of Membrane Technologies in the Dairy Industry 37
National de la Recherche Agronomique (INRA) has shown for Listeria monocyto-genes, Brucella abortus, Salmonella typhimerium, and Mycobacterium tuberculosis DR of 3.4, 4.0, 3.5, and 3.7, respectively. Considering the usually described contami-nations of milk at the farm level, such results will assure that MF 1.4 mm skim milk will contain less than 1 CFU.L–1 of these pathogenic bacteria (Saboya and Maubois, 2000) which means 1.4 mm MF milk can be considered as safe as pasteurized milk.
France is the only country that has officially allowed the commercialization of extended shelf-life (ESL) MF raw milk. The MF skim milk is mixed with the amount of heated cream (95°C–20 s) requested for fat standardization; the mixture is homog-enized and aseptically filled. The authorized shelf life at 4°C to 6°C is 3 weeks. The yearly volume of this MF milk proposed, to our knowledge, by only one dairy com-pany under the trademark Marguerite®, (see Figure 3.1) reached in 2008, 10 million liters. Other plants in many countries apply to the homogenized mixture before con-ditioning a high-temperature short-time (HTST) (72°C–20 s) pasteurization leading to a claimed shelf life of 5 weeks (Eino, 1997). In many countries, the commercial success encountered by these MF milks is high because of their improved flavor (no cooked taste) and storage ability (Eino, 1997). In some plants, use of 1.4 mm MF has been extended as a pretreatment in the production of UHT milk in order to decrease the intensity of heat treatment (decreased to 140°C–4 s or less) with, consequently, a less cooked taste and an improved storage capability coming from the removal by MF of thermoduric enzymes present in dead bacterial cells and in somatic cells.
Use of MF membrane with a smaller pore diameter (0.8 mm instead of 1.4 mm) proposed by Lindquist (1998) was studied in Sweden, France (AFSSA, 2002), and Canada. At 50°C, the obtained flux was in the range of 400 L.h–1.m–2,and the observed DR with this MF 0.8 mm membrane was higher than 13 on Clostridium botulinum, a value that means sterility of the product. After mixing with UHT cream FIGuRE 3.1 Raw MF Marguerite® liquid milk.
38 Engineering Aspects of Milk and Dairy Products
(142°C–4 s) for fat standardization, homogenization at 80°C, a heat treatment lim-ited to 95°C–6 s is applied with the only purpose to inactivate endogenous milk enzymes, followed by aseptic conditioning and packaging at 20°C. The obtained milk, called Ultima® milk by the Tetra-Laval Co., was recognized as commercially sterile (AFSSA, 2002). It is stable at 40°C for 62 days and for more than 8 months at room temperature. Its organoleptic quality was judged as similar to that of an HTST pasteurized milk. Its lactulose content was reduced by 71% compared to UHT milk.
But until now, to our knowledge, this Ultima process was not commercially devel-oped by the Tetra Pak Co., for unknown reasons. Nevertheless, today, in some dairy plants, the MF 1.4 mm membrane is substituted by the 0.8 mm membrane for the production of ESL MF pasteurized milk in order to extend storage ability.
Ultrafiltration offers the possibility of adjusting the protein content of consumer milks either by their specific concentration or by addition of UF milk permeate to the collected milk in order to overcome natural variations in milk composition depend-ing on the cow’s breed, its feed, the season, and its stage of lactation. Surprisdepend-ingly, although fat standardization is commonly accepted and has been legally authorized for many years, the proposal to deliver consumer milks with defined protein content has encountered incomprehensible and illogical (protein content is one of the pay-ment criteria to the milk producers) opposition, and until now, to our knowledge, no country in the world has modified its legislation for allowing protein standardization of consumer milks despite the fact that adjustment is allowed for milk and whey powders. Questions that arose by protein standardization of consumer milk were summarized by Maubois (1989): ethical acceptance and logic face to fat standardiza-tion, one unique level (for example, 32 g.L–1) or several ranging from 29 g.L–1 (mini-mum defined in EU [J.O.U.E., 2007] and required on a nutritional point of view) to 34 g.L–1 (content found in many developed countries), technologies to be used, and economical consequences.
Somatic cells (SCs) that range in size from 15 to 6 mm contain numerous ther-moresistant enzymes (protease, lipase, catalase). They are very sensitive to mechani-cal treatments and consequently are able to release their enzymes into the milk with potential impacts on the quality of the dairy products derived from that milk (pas-teurized and UHT milks). They have been shown to protect Listeria monocytogenes during heat treatment, and it has been suggested that milk leukocytes could also contain bovine spongiform encephalopathy (BSE) prions, but no demonstration of this hypothesis has been made either in milk or in colostrum (Maubois and Schuck, 2005). Specific removal of SC from raw whole milk by MF membranes having an average pore size ranging from 12 mm (Le Squeren and Canteri, 1995) to 5 mm (Maubois and Fauquant, 2004) was studied by the group of one of the authors of this chapter. Permeation fluxes between 2000 L.h–1.m–2 and 1460 L.h–1.m–2 were respec-tively obtained over a running time of 8 h. In the MF retentate, 93% to 100% of the SC were retained which represented 4% to 5% of the volume of treated milk. Permeation rates of the globular fat were, respectively, 89% and 83%. In addition to being the solution for treating milk if the presence of prions was eventually demonstrated, these results open new avenues for researching, for example, the specific effects of varied numbers of SC in normal milks (most of the published studies have been done
Applications of Membrane Technologies in the Dairy Industry 39
with mastitis milks of which the composition is highly modified) on the stability of UHT milk in comparison with the residual activity of the endogenous milk plasmin or the proteases of Pseudomonas, the potential creation of microheterogeneity in the microstructure of cheese and, on the other hand, its use as tracers for identifying cows or herds that have produced the used milk raw material, as all their genetic patrimony is contained in the SC.
For fermented milks such as yogurts, enrichment of milk either by RO or by NF has led to products considered as better in terms of texture and flavor than those made from milk added with milk powder (Tamime and Robinson, 1985). Such results prob-ably originated by a drastic reduction of the Maillard reaction always initiated in milk powders and the absence of insoluble particles that are more or less present in even high-quality powders. The specific increased flavor improvement found in yogurts made from milk concentrated by NF likely originates from the specific decrease in monovalent ions (Na and Cl) to which consumers are particularly sensible.
3.5 APPlICATIONs OF MEMbRANE TEChNOlOGIEs