Because of its vacuum-packed format, instant active dry yeast requires little more of its storage area than clean, dry conditions, but compressed yeast must be kept refrigerated at 2 to 7°C (36 to 45°F) until use. It has a high level of moisture (68 to 71%), and its activity diminishes over time. Compressed yeast, as well as its crumbled form, is delivered to the bakery in 50-lb, multiple-walled, poly-lined bags, stacked 50 bags to the pallet. Because this form of bakers yeast is short-lived, deliveries are generally scheduled two to three times a week.
Cream yeast systems consist of a minimum of two large insulated storage tanks, ranging in size from 2,600 to 6,000 gal and equipped with refrigerated jackets and clean-in-place (CIP) capacity, plus insulated, stainless-steel, ringmain-style piping that constantly circulates the yeast between the holding tanks and the air-actuated, sanitary dispensing valves at each mixer (Boge 1994). Glycol cooling of the storage tanks is typical because many bakeries also use glycol to cool mixer jackets and refrigerated and freezer storage areas. Two centrifugal pumps move the yeast through the ringmain, and the size of the pump motors depends on the number of dispensing points and the length of the ringmain. Sanitary design of cream yeast equipment is essential and should follow standards such as 3A, BISSC, NEMA or ANSI Z50 for all components.
Flow meters regulate the delivery of cream yeast to use points, and two strategies can be employed. One is to put a single meter in the yeast loop (ringmain). When yeast is required at a given drop point, the control system locks out all the other drop points. It closes a valve on the return side of the recirculating loop downstream of the drop points and dead-heads the entire system. When the required drop point opens, the single flow meter measures the movement of yeast and, thus, controls dispensing. A second strategy is to install a flow meter at each drop point, dedicated to that mixer or use point. Although requiring a greater capital investment, this second method avoids compression errors due to gas generation by the yeast in the ringmain. It also prevents system shut-down should a flow meter fail.
Like all liquid ingredient storage tanks, the cream yeast system must be equipped with a means of drainage that complies with environmental regulatory standards. Tanks can be installed either in vertical or horizontal position, depending on the space available. In suitable climates, the tanks can be installed outside under cover, but most bakeries prefer to locate them inside the plant to best control hygiene as well as security.
Computers or PLCs govern the operation of cream yeast systems, displaying status and operating conditions such as amounts in tanks and pipelines, usage rates, alarms, transfers, deliveries, product temperatures, CIP functions and so forth. The hardware and software must provide failsafe operation of the system to prevent contamination or loss of yeast. The bakery’s electronic batch controllers interface with the cream yeast system to govern the sequence in which the yeast reaches the mixers.
For more than a decade now, bulk cream yeast systems have been included in the layout of every new large wholesale bakery built in the US. Many bakeries in Europe use cream yeast, delivered in bulk, 100-liter drums and even 20-liter bag-in-box packages.
Cream yeast comes into the plant at 78 to 80% moisture and 5.6 to 5.8 pH. Yeast shipping temperature should be between 0.5 and 7°C (33 and 45°F), and storage temperature in the bakery should not exceed 7°C (45°F). Frequency of delivery can be held to once a week, although a bakery should not let its cream yeast supply age beyond 10 days. As Boge (1994) noted, bulk delivery of cream yeast qualifies as environmentally friendly because it does not require disposal of packaging materials.
Because of the sensitive biological nature of cream yeast, the baker should insist on the manufacturer’s assurance of a uniform, consistent product with a clean microbiological profile. For each shipment, the yeast manufacturer should provide the baker with a tank truck inspection form. Thermally insulated tanker trucks can maintain the temperature of their contents ±1 C° (±2 F°) and should be inspected by the shipper before loading. A list of the three prior hauls should also be presented at the loading site and reviewed by the loading personnel to make certain that previous contents were food-grade only.
Yeast manufacturers can also be asked to provide a certificate of analysis (COA) for each load. According to Zimmerman (1999), the three most important items on the certificate are the batch or lot code, the loading temperature of the yeast and the gassing activity test results. Although Risograph testing is typical, other measurements of gassing power are used, and the baker must be familiar with the method chosen. The baker should request that the certificate arrive before the delivery of the yeast, being faxed or e-mailed ahead of time. Zimmerman (1999) strongly urged use of a certified thermometer when testing yeast both at delivery and in the bakery. He also stressed that delivery meters be standardized and calibrated to assure accuracy.
Because cream yeast can be vulnerable to bacterial spoilage, the equipment that stores and handles it must be kept in peak sanitary condition. A clean-in-place (CIP) system is an absolute requirement, and it must encompass the receiving line into the plant, both tanks and the ringmain piping. If tanks are cleaned immediately after they empty, they will be ready to receive new supplies in hygienic fashion. The ringmain should be cleaned at least once a week or whenever yeast circulation stops. Zimmerman (1999) recommended thorough sanitation of the whole system every 6 months.
A separate 200-gal CIP tank allows the sanitizing chemicals to be measured and mixed before cleaning operations start. The 1 to 1.5 hours necessary to sanitize the ringmain is usually scheduled for a down day, but the receiving line and the idle storage tank can be cleaned while the plant is in full operation, with at least 20 minutes of wash time allowed. Good Manufacturing Practices (GMPs) for sanitation of this system must be followed. Automation of this sanitation activity allows repeatable uniformity and collects data that document its operations for the plant sanitarian.
The cleaning cycle should use a pre-rinse caustic, wash water of at least 60°C (140°F) and a wash time of no less than 20 minutes. Avoid using any compounds containing chlorine. Zimmerman (1999) noted that these same standards are followed by yeast manufacturers to clean their systems.
Boge, T.C. 1994. Cream yeast. Proc. Am. Soc. Bakery Engrs. 70: 123.
Zimmerman, B. 1999. Cream yeast standards. Proc. Am. Soc. Baking 75: 147.
More on this topic can be found in “Baking Science & Technology, 4th ed., Vol. II,” Page 390, by E.J. Pyler and L.A. Gorton. Details are in our store.