FLAVOR DEVELOPMENT AND MICROBIOLOGY OF SWISS CHEESE-A REVIEW: III. RIPENING AND FLAVOR PRODUCTION

This paper, the third of four, dismtsses the ripening and flavor produotion processes of Swiss cheese. First, general ripening changes are reviewed; then, a discussion of eye formation and body and texture changes follows. The important flavor compounds found in Swiss cheese are examined in detail beginning with consideration of the analytical methods used to isolate and measure fatty acids. The different volatile and nonvolatile fatty acids and their importance in Swiss cheese are considered, as well as production of propionic and acetic acids from lactate, production of keto acids, and lipolysis. Then, analyses for and significance of carbonyl compounds, alcohols, esters, lactones, hydrocarbons, and diaeetyl are presented. Possibilities for production of these compounds by microorganisms are given. Subsequent sections deal with nitrogeneous componnds, degradation of caseins to peptides and amino acids and their importance to flavor, and end with a review of sulfur compounds present. After Swiss cheese has been removed from the press, brined, and prestored for 7 to 14 days, it is transferred to the hot room where the temperature is 21 to 25 C (70 to 77 F). During the period in the hot room cheese becomes sufficiently elastic and the principal ripening of the cheese, with active growth of propionibacteria, takes place, forming the typical eyes of Swiss cheese ( 430 ). When the eyes are large enough, the cheese is moved to the curing cellar or "finished cooler" and kept there at 2 to 5 C ( 35.6 to 41 F) for 2 to 9 mon•ths. Ripening for at least 6 months should develop the fine, full flavor characteristic of a well-ripened Swiss cheese (116); however, most commercial Swiss cheese sold in the United States today is much younger when sold (345). GENERAL ASPECTS OF S'WISS CHEESE RIPENING During the time in the press, high-temperature lactic-acid bacteria rapidly hydrolyze lactose in the young cheese to glucose and galactose (338), which, by the Embden-Meyerhof pa:thway, is oxidized to 'Journal Paper No. J-7505 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 1839. 'This review is appearing in four parts: I. Milk Quality and Treatments and II. Starters, Manufacturing Processes and Procedures appeared in earlier issues; IV. Defects will appear in a subsequent issue. Literature citations will follow part IV. lactic acid by the same bacteria (B). Production of lactic acid in the cheese produces a readily assimilable substrate for the propionibacteria. Propionibacteria were first found by Sherman (393) to be essential for eye formation as well as production of the characteristic sweet flavor of Swiss cheese. A large part of the lactic acid in Swiss cheese is produced by homofermentative lactic-acid fermentation, but a small portion also is produced through the heterofermentative pathway. Moderate amounts of acetic acid are found in the cheese after pressing (218). Because of this heterofermentative activity, carbon dioxide, which induces growth of propionibacteria, also has been found (150). Lactic-acid bacteria also reduce the redox potential of the Swiss cheese to -200 mv, which is favorable for growth of propionibac,"l:eria (199 ). Another important factor for growth induction of propionibacteria is the pH-value of the cheese out of the press. The pH must be between 5.0 and 5.3 to produce good Swiss cheese. If the pH is lower than 5.0, growth of propionibacteria will not take place (431). During the hot room and curing room treatment of Swiss cheese, the number of S. therrrwphilus and L. helveticus decline (79 ), even though it has been stated that mixed cultures of S. thermophilus and L. helveticus may grow at temperatures as low as 15 C (59 F) (171). The primary reason for this numerical decline is the elimination of lactose as an energy source and the formation of some metabolic products in the cheese that may have an adverse effect on growth, especially as the cheeses are ripened near their minimum growth temperature. Lactobacillus casei, however, grows during ripening, and SGme investigators consider it an important ripening organism