Today we are we are going to discuss the processes used to turn raw milk into dairy products and describe how these processes affect their microbial, chemical, and sensory the quality. We will be covering…
Shelf life can be evaluated in three ways: microbial tests sensory evaluation, and biochemical tests, but for the purposes of this lecture we will use a more broad definition: Where do these definitions come from? Present the PMO
The PHS and FDA (a law enforcement agency) has set up set of rules that when followed will ensure sefety and maximize quality of dairy products, they are known as the PMO. These are rules that are enforceable by law, however, they are not strictly enforced. Now that we know some of the definitions and where they come from, let’s look at the process of making fluid milk.
One continuous theme throughout this process is Temperature Control. Whether it’s low temperatures to slow microbial growth or high temperatures to decrease risk of pathogens surviving. Bacteria are ubiquitous in the dairy environment there is no silver bullet to eliminate spoilage and contamination but, we can control temperature practice proper sanitation optimize quality and reduce health risks.
The first step in the process starts off with raw milk collections and cold storage, and the PMO has specifications for the chemical, microbial quality as well as temperature limits. (Research has suggested that the numbers of bacteria present are critical, because the bacteria are capable of sensing each other (referred to quorum sensing) and it has been shown that they produce the bad enzymes when they reach specific levels. Therefore it is wise to keep them low at all times.)
What are the bacteria that limit shelf-life? We mentioned some of these the other day. Some of these enzymes are exquisitely resistant to heat. They have been isolated from UHT milk (Law, 1979). I saw a paper that showed that 10,000 active units of a lipase produced by Pseudomonas fluorescens would only be reduced to 1,000 units if treated at 110°C for 17 min (Vercet, 1997). Pasteurization as I’ll describe in a few minutes is 72°C for 15 seconds. It can’t touch this enzyme. And these enzymes don’t need the bacteria to be around to be active. The bacteria produces them and excretes them into the milk in order to break down the proteins and fat as food source. The spore formers are as resistant as the enzymes and in some cases the heat treatment of pasteurization revives spores from their dormant state. So the strategy is to minimize contamination with bacteria by following the sanitary regulations laid out by the PMO, and control temperature to minimize growth.
One temperature treatment that is used to control bacterial numbers is Thermization. I mentioned this last week. It turns out to be more more common in Europe than in the US. 135-155°F 40°C
Once the milk has been collected, cooled, and transported to the processing plant, it is first clarified,and that means all particulate matter is removed. This takes advantage of the different densities of the particulates and the milk matrix; essentially a decanting process. Can be quiescent (settling out) or removal by centrifugation. Centrifugal force is used to remove bacteria and their spores. Re. Microfiltration: the tradeoff is that it takes a lot of energy to pump the milk through these small membranes.
This process takes advantage of the fact that milk is composed of 2 phases with different densities. DRAW diagram. Centrifugal force is used to separate these two phases. A pairing-disc separator is used and it is essentially a cone with a series of baffles that spins around and the skim which is more dense than the cream (or fat) moves to the outside of the cone and is collected through little openings and pooled together. The cream which is less dense moves inward toward the apex of the cone and along the axis of rotation and is collected through a channel at the tip. These two components form streams are then mixed in specific ratios depending upon the product being processed. The exact amounts are monitored using IR analysis which controls the valves that connect the 2 streams.
We’ve used centrifugal force to separate these two phases, and now we need to use some force to put them back together, which is the process of homogenization. Regarding color mention why skim milk is not as white as whole milk.
Shear force smashes the fat globules into smaller particles.
Pasteurization was engineered to minimze the risk of survival of the most thermally resistant pathogen. This organism is Coxiella Burnettii. It was not designed to extend shelf-life, however it does significantly reduce the number of spoilage organisms present. Remember that there are enzymes and spores that will remain active. The temperature has to be recorded so that there is a record that the process is achieving the lethality that it was designed to achieve.
So the record shows that pasteurization has been quite effective with respect to safety. What are the standards regarding the quality of milk?
UHT milk you’ve seen at the supermarket, Parmalat for example. Shelf-stable; also referred to as commercially stable. Long shelf-life but has cooked flavors
The packaging can have a major effect on the quality of milk.
So this processing allows for the production of a consistent product, has a positive overall effect on safety, and extends shelf-life. The extent to which shelf-life is improved is dependent upon the quality of the raw product, the conditions under which it is processed that is temperature control, and post process handling with the goal to minimize contamination and maintain low storage temperatures. We use packaging materials to minimize the negative effects of light oxidation.
Cheese making is a very old process It is both a science and an art Due to the natural variation in milk between cows and regions, and the different micrflora found in those environments, many different types of cheeses have evolved. They can classified into 6 major groups based on their texture and ripening
Cheeses are classified based on their textures and the kind of ripening ie. bacteria or mold, on the surface or in interior. For example: soft cheeses… semisoft cheeses such as Munster are ripened by bacteria within the cheese matrix whereas Limburger is ripened by bacteria within and on the surface. Blue cheese is ripened by mold on the interior Raw milk cheeses need to be aged a minimum of 60 days however there is data suggesting that pathogens may be able to survive this ageing process. The down side of pasteurized milk cheeses is that they may lack sensory properties that only the natural microflora can provide. We are going to focus on the cheddar cheese process as to model the cheese making process.
Through out each of these stepe we are conditioning the cheese such that it less susceptbile to spoilage and less likely to support the survival of pathogens. We do this by continuously removing water, in the for of whey, or binding it up so that the “bad” bacteria don’t have access to it. Chemical changes also inhibit growth such as the addition of salt, and the production of lactic acid .
Sweet whey has greater value because subsequent products made from it have better flavor and color
Dairy Products and Processing• Definitions and standards• Processing steps• Shelf-life• Fermented dairy products
Definitions• Raw milk: The lacteal secretion , practically free from colostrum, obtained by the complete milking of one or more healthy cows (PMO).• “Consumer Milk” products: - Homogenized milk: ≥3.25% fat - Reduced fat milk: 2% fat - Low fat milk: 1% fat - Fat-free milk: skim milk, <0.5% fat (all with 8.25% solids-non-fat)• Other “milk products”: lactose reduced milks, heavy cream, cultured milks, yogurt, cottage cheese.
Shelf-life: Time for which a product can be stored without the quality falling below a certain acceptable minimum level Consumer milk: 14 days, under refrigeration (Muir, 1996)
Pasteurized Milk Ordinance (PMO)• produced by Public Health Service/Food and Drug Administration• sanitary regulations for milk and milk products• specifies sanitation measures throughout production, handling, pasteurization, and distribution of milk(http://vm.cfsan.fda.gov/~ear/p-nci.htm#pmo96)
Raw Milk Quality and Storage Chemical, bacteriological, and temperature standards for Grade A raw milk for pasteurization, ultrapasteurization or aseptic processing (PMO) - Temperature: 45ºF or less within 2 h after milking - Bacterial counts: <100,000 cfu/ml for individual farm milk and <300,000/ml as commingled milk prior to pasteurization - Somatic Cell Counts: <750,000/ml - Antibiotic presence: negativeStorage time at plant max. 72hLonger holding times allow growth of psychrophilic bacteriawhich can secrete heat-resistant proteases and lipases
Thermization (Lewis and Heppell, 2000)• 57-68°C for 15 seconds• only effective if cooled to 4°C after treatment• applied to raw milk that needs to be stored for several days prior to use• purpose: reduce gram-negative psychrotrophic spoilage organisms (enzyme production)
Clarification and Clearing Clarification: removal of small particles - straw, hair etc. from milk; 2 lb/2,642 gal - based on density“Bactofugation”: Centrifugal separation of microorganisms from milk: - Bacteria and particularly spores have higher density than milk - Two-stage centrifugation can reduce spore loads up to >99% - Optimal temperature for clarification is 55-60ºC Microfiltration - Microfilter membranes of 1.4 µm or less can lead to reduction of bacteria - and spores up to 99.5-99.99%.
Milk Fat Standardization/Decreaming Separation of skim milk (about 0.05% fat) and cream (35-40% fat) Based on the fact that cream has lower density than skim milk Centrifugal separators are generally used today Standardization of fat content: Adjustment of fat content of milk or a milk product by addition of cream or skim milk to obtain a given fat content
Homogenization• Definition: Treatment of milk or a milk product to insure breakup of fat globules such that no visible cream separation occurs after 48 h at 40ºF (4.4ºC)• Effects of homogenization: – No cream line formation due to smaller fat globules – Whiter color – More full-bodied flavor, better mouthfeel• Process requirements: – Homogenization most efficient when fat phase is in a liquid state – Cream >12% fat cannot be homogenized at normal pressure, high pressure homogenization process is necessary• Homogenization is a mechanical process where milk is forced through a small passage at high velocity
From the “Dairy Processing Handbook” 1995. Tetra Pak
Pasteurization• Purpose: Inactivation of bacterial pathogens (target organisms Coxiella burnettii) - assurance of longer shelf life (inactivation of most spoilage organisms and of many enzymes)• Pasteurization – Heat treatment of 72ºC (161°F) for 15 sec (HTST) or 63ºC (145°F) for 30 min (or equivalent) – does not kill all vegetative bacterial cells or spores (Bacillus spp. and Clostridium spp.) – Pasteurization temperature is continuously recorded
Efficacy of Pasteurization• prior to pasteurization (1938) : milkborne outbreaks constituted 25% all disease outbreaks• Today: milk products associated with < 1%
Heat Treatment (Con’t)• Standards for Grade A pasteurized milk and milk products (PMO) – Temperature: Cooled to 45ºF or less – Bacterial counts: <20,000 cfu/ml – Coliform Counts: <10/ml – Phosphatase: < 1µg/ml – Antibiotic presence: negative
Heat Treatment (Con’t)• Ultra pasteurization: Thermal processing at 138ºC (280ºF) for at least 2 seconds - UP milk: ultrapasteurized and “non-aseptically” packaged, refrigerated storage - UHT milk: ultrapasteurized and aseptically packaged, storage at room temperature; avoid recontamination• Standards for Grade A aseptically processed milk (UHT) - Temperature: none - Bacterial counts: no growth - Antibiotic presence: negative
Vitamin Fortification• Preferably after separation• Has to occur before pasteurization• Can be continuous (using a metering pump) or batch addition
Filling/Packaging• Functions of packaging: – Enable efficient food distribution – Maintain product hygiene – Protect nutrients and flavor – Reduce food spoilage – Convey product information• Different containers: – Glass bottles (translucent vs. dark): can be reusable or recyclable – Plastic containers – Cartons – Plastic bags
Shelf Life of Heat Treated Fluid Milk• Shelf life depends on: – Raw milk quality (bacterial and chemical quality) – Processing conditions – Post-processing storage• Loss of taste and vitamins by light exposure: – Light-impermeable containers• Extended Shelf life (ESL) milk – No single, specific definition of ESL – Pasteurized milk with a shelf life beyond the current typical shelf life of these products (10 - 14 days) – Generally involves measures to eliminate or minimize “post-pasteurization” contamination
Fermented Dairy products• Fermented foods: – Food products produced by biological transformation (by bacteria or fungi) – Carbohydrate breakdown as a major characteristics (lactose lactate)• Preservation: production of acids and alcohol (by “beneficial” bacteria) to inhibit spoilage bacteria and pathogens
Cheese:- product made from the curd of the milk of cows (or other animals)- casein coagulated by rennin and acid- subsequent heating, salting, pressing, aging
Classification of Cheeses (Potter, 1995)• Soft - unripened: cottage cheese, cream cheese - ripened: Brie, Camembert• Semisoft - Munster, Limburger, Blue• Hard - cheddar, swiss• Very hard (grating) - Parmesan, Asiago• whey cheeses (ricotta)• processed cheese
Cheddar Cheese Making Process pasteurized milk Setting the milk Cutting the curd Cooking the curd Draining the whey Milling and salting Pressing Ripening
Cheddar cheese making process• starting ingredient: pasteurized whole milk• setting the milk - while stirring heat to 31°C - add lactic-acid producing starter cultures - (add natural color) - add rennin to coagulate caseins and form curds - stop stirring and let set• Cut the curd - increase surface area - release the whey• Cooking (38°C for 30 minutes) - removes more whey - increases growth and acid production of cultures
Cheddar cheese making (cont.)• Draining whey and matting the curd - remove excess whey - form curds into a slab - cheddaring: cutting curd slab into blocks to allow excess whey to drain, and allow acidity to increase• Milling and salting - cut curds into small pieces -2.5% salt is added: drains whey, inhibits spoilage organisms and adds flavor• Pressing to remove more whey - moisture content will affect bacterial growth and texture• Ripening: bacteria develop flavor and texture over time
Ripening: flavor and texture development• Primary proteolysis - 60 days; residual chymosin - caseins broken down into medium molec. wt. peptides• secondary proteolysis - starter cultures break down peptides to lower molec. wts.• Temperature: 5-7°C• pH: 5.0 - 4.7 - inhibits growth of spoilage organisms - inhibits enzyme activity
Cheese flavor development• A complex, dynamic process• Nature of the flavor evolves• Proteolysis essential for full flavor development - Proteolytic enzymes • Allow LAB to utilize proteins present in milk to obtain essential amino acids necessary for growth - Generates peptides and amino acids • Impart flavor directly or serve as flavor precursors
Whey• 100 lb of milk => 10 lb cheese + 90 lb whey (NYS produces 3.6 billion lb/year)• low solids, high lactose• highly perishable (contains starter organisms)• Acid whey: drained from cheese curd acidified to 4.6 by cultures (or acid); ex. Cottage cheese• sweet whey: drained from curd formed by rennet coagulation ex. Cheddar
Whey Products• concentrated and spray dried• whey powder• whey protein concentrates - different % purity - food ingredient• lactose - food ingredient - fermented into alchohol• whey cheeses
References:• Boor, K. J., 2001, ADSA Foundation Scholar Award; fluid dairy product quality and safety: looking to the future. Jornal of Dairy Science, 84: 1-11• Champagne, C. P., Laing, R.R., Roy, Dennis, Mafu, Akier Assanta, Griffiths, Mansel W. 1994. Psychrotrophs in Dairy Products: Their Effects and Their Control. Critical Reviews in Food Science and Nutrition, 34: 1-30.• Department of Agriculture and Markets Division of milk Control and Dairy Services New York State Dairy Statistics, 2001 Annual Summary., 1 Winners Circle, Albany NY 12235• Lewis, M., Heppell, N., 2000. Continuous Thermal Processing of Foods; Psteurization and UHT Sterilization. Aspen Publishers, Gaithersburg, MD• Muir DD, 1996. The shelf-life of dairy products .1. Factors influencing raw milk and fresh products Journal of the Society of Dairy Technology 49 (1): 24-32• Pasteurized Milk Ordinance (PMO) http://vm.cfsan.fda.gov/~ear/p-nci.htm#pmo96Potter, N., Hotchkiss, J. H., 1995 Milk and milk products. In: Food Science, 5th Edition, Chapman and Hall, New York