The Book of Cheese Part 20

_CHEDDAR CHEESE RIPENING_

Freshly made Cheddar cheese is hard, tough and elastic and lacks characteristic cheese flavor. In this condition it is called "green,"

unripe or not cured. Before the cheese is ready to be eaten, it pa.s.ses through a complex series of changes which are collectively known as ripening. In the ripening process the texture becomes soft and mellow and the characteristic cheese flavors develop. Cheese ripening must be considered from two view-points, first, the changes taking place inside the cheese and secondly the outside conditions necessary for ripening.

Some of the chemical changes during ripening are known, while others are not understood. The different agents causing ripening, and the const.i.tuents of the milk, will be discussed.

+231. Fat.+--Numerous investigations have been made to ascertain what chemical changes the fat undergoes in the ripening process. Suzuki,[103]

in studying the fat, found no enzyme capable of producing lactic acid or volatile fatty acids. However, these acids were found in increasing amounts during the ripening process and after the lactose had disappeared. Acetic and propionic acids reached a maximum at three months and then decreased, while butyric and caproic acids continually increased during the experimental period covered. Formic acid was detected in the whole-milk cheese only at the five and one-half month stage. In the judgment of the experimenter the princ.i.p.al source of acetic and propionic acids was probably lactates. Traces of these acids may have had their origin in protein decomposition or further fermentation of glycerine. The princ.i.p.al sources indicated for butyric and caproic acids were fats and proteins.

The distillate from the experimental cheese was designated "flavor solution" and contained alcohols and esters, giving a close resemblance to the cheese aroma. The "flavor solution" from the mild whole-milk cheese contained esters made up largely of ethyl alcohol and acetic acid, while from the more pungent skimmed-milk cheese the esters were largely compounds of ethyl alcohol and caproic and butyric acids. The alcohol may have come from the lactose fermentation. It appears to be an important factor in flavor production. The agencies operative in the production of volatile acids and syntheses of esters are as yet undefined.

+232. Milk-sugar.+--The milk-sugar (lactose) is changed into lactic acid by the lactic acid-forming organisms, within the first few days after the cheese is made. This acid is combined with the other const.i.tuents as fast as it is formed. After a few days, the milk-sugar will have entirely disappeared from the cheese.[104] The relation between the milk-sugar and lactic acid is very close. It is necessary that milk-sugar be present in order later to have the lactic acid develop.

+233. The salts.+--Just what changes the salts[105] undergo or how they combine with the other compounds is not definitely known. It is supposed that the calcium salts first combine with the phosphates and later, as the lactic acid is formed, they combine with the lactic acid, forming a calcium lactate.

+234. Gases.+--In the process of cheese ripening, gases are formed, the commonest being carbon dioxide.[106] Exactly how this gas is formed is not known. It may be due to the formation of lactic acid from the milk-sugar or to the living organisms in the cheese.

+235. Casein or proteins.+--Complex ripening changes in the cheese take place in the casein compounds or proteins. Because of the complex chemical nature of the proteins and the various agents acting on them, it is difficult to follow these changes. This has led to different opinions regarding the ripening process. The various compounds thought to be formed from the casein or proteins are as follows:[107]

_Paracasein_ (formed by the action of the rennet on the casein).

Insoluble in brine and warm 5 per cent salt brine.

_Protein._ Soluble in warm 5 per cent salt brine.

_Protein._ Insoluble in warm salt brine or water.

_Paranuclein._ A protein soluble in water and precipitable by dilute hydrochloric acid.

_Caseoses and proteoses._ Protein derivations soluble in water and not coagulated by heat.

_Peptones._ Protein derivations simpler than the proteoses, soluble in water and not coagulated by heat.

_Amido acids._ Protein derivations soluble in water, least complex except ammonia.

_Ammonia._ The simplest protein derivations.

From the discussion of the const.i.tuents in the milk and cheese, it is evident that practically all the princ.i.p.al ripening changes are concerned with those taking place in the proteins.

+236. Causes of ripening changes.+--Authorities disagree as to the exact agents which cause the ripening changes. Some think they are due to the action of the enzymes in the rennet and those secreted in the milk.

Others hold that these changes are due entirely to bacterial action. A combination of the two seems probable. The action of the rennet extract renders the casein insoluble and in the ripening process the proteins become soluble, the degree depending on the length of time the cheese is ripened. The amount of water-soluble proteins and protein derivatives is used as a measure of the extent of cheese ripening, considered from a chemical standpoint.

+237. Action of the rennet extract.+--Some authorities hold that rennet extract contains two enzymes, rennin and pepsin, while others think it is a single peptic ferment. These enzymes produce effects[108] closely related to, if not identical with, those of pepsin in the following particulars: neither the rennet enzyme nor pepsin causes much, if any, proteolytic change except in the presence of acid; the quant.i.tative results of proteolysis furnished by the rennet enzyme and pepsin agree closely, when working on the same material under comparable conditions; the cla.s.ses of soluble nitrogen compounds formed by the two enzymes are the same, both quant.i.tatively and qualitatively; neither enzyme forms any considerable amount of amido compounds and neither produces any ammonia; the soluble nitrogen compounds formed by both enzymes are confined to the group of compounds known as paranuclein, caseoses and peptones.

Rennet exerts a digestive effect on the casein[109] which is intensified by the development of acid in the curd. The soluble nitrogenous products formed in Cheddar cheese by the rennet enzymes are the alb.u.moses and the higher peptones. Experiments show that no flavor develops until the amido acids and ammonia are formed. When the rennet enzymes were the only digesting ferments in the cheese, there was no trace of cheese flavor. This is probably due to the fact that the rennet enzyme changed the casein into caseoses and peptones but did not form amido acids and ammonia. Some authorities[110] think that the enzyme galactase carries the ripening of the protein from this stage. The question arises whether these intermediate compounds must be found before other agents can form the amido acids and ammonia.

TABLE XV[111]

SHOWING THE EFFECT OF DIFFERENT AMOUNTS OF RENEET EXTRACTS ON THE RATE OF FORMATION OF SOLUBLE NITROGEN COMPOUNDS IN CHEESE RIPENING

========================================================= PER CENT OF WATER SOLUBLE NITROGEN QUANt.i.tY OF COMPOUNDS IN THE CHEESE RENNET ADDED PER +--------------------------------------- 1000 LB. OF MILK Initial 32 days 80 days 270 days -----------------+---------+---------+---------+--------- 2 oz. 0.14 0.47 0.68 1.30 4 oz. 0.16 0.75 1.13 1.74 8 oz. 0.16 0.90 1.50 1.97 16 oz. 0.14 1.26 1.70 2.04 =========================================================

The above table shows that the more rennet extract used the faster the cheese cures, measured by the amount of water-soluble nitrogen compounds formed in the cheese.

+238. The action of the bacteria.+--Authorities[112] disagree as to the groups of bacteria found in Cheddar cheese. This may be due to lack of proper cla.s.sification. Some of the groups are: _Bacterium lactis acidi_, _B. coli communis_, _B. lactis aerogenes_, _B. casei_, Streptococci, _B.[113] Bulgaric.u.m_ and Micrococci. Authorities agree that the _B.

lactis acidi_ group is the most prominent. This group makes up 90 per cent or more of the total bacteria flora of the cheese in the early stages of ripening. In the course of a few weeks, however, this group is largely replaced by the _B. casei_ group[114].

TABLE XVI

SHOWING THE NUMBER OF BACTERIA TO A GRAM IN CHEDDAR CHEESE AS DETERMINED BY LACTOSE-AGAR PLATE CULTURES

==================================================================== TIME OF CHEESE NUMBER PLATING +-------------+---------------+-------------+-------------- 580 581 582 583 ---------+-------------+---------------+-------------+-------------- Milk 8,000,000 500,000 700,000 500,000 ---------+-------------+---------------+-------------+-------------- Curd at salting 160,000,000 326,000,000 912,000,000 839,000,000 time ---------+-------------+---------------+-------------+-------------- 12 hours 332,000,000 1,048,000,000 623,000,000 965,000,000 ---------+-------------+---------------+-------------+-------------- 1 day 586,000,000 736,000,000 709,000,000 569,000,000 ---------+-------------+---------------+-------------+-------------- 2 days 235,000,000 405,000,000 848,000,000 580,000,000 ---------+-------------+---------------+-------------+-------------- 4 days 145,000,000 684,000,000 522,000,000 1,025,000,000 ---------+-------------+---------------+-------------+-------------- 6 days 165,000,000 184,000,000 853,000,000 184,000,000 ---------+-------------+---------------+-------------+-------------- 14 days 51,000,000 211,000,000 369,000,000 401,000,000 ---------+-------------+---------------+-------------+-------------- 21 days 284,000,000 290,000,000 348,000,000 319,000,000 ---------+-------------+---------------+-------------+-------------- 28 days 285,000,000 453,000,000 314,000,000 144,000,000 ---------+-------------+---------------+-------------+-------------- 35 days 104,000,000 261,000,000 326,000,000 504,000,000 ---------+-------------+---------------+-------------+-------------- 49 days 132,000,000 228,000,000 436,000,000 661,000,000 ---------+-------------+---------------+-------------+-------------- 70 days 128,000,000 291,000,000 193,000,000 168,000,000 ---------+-------------+---------------+-------------+-------------- 98 days 114,000,000 212,000,000 45,000,000 55,000,000 ====================================================================

From Wis. Bul. 150.

The large number of bacteria in the cheese is very striking. The number as given in the accompanying table is not that actually in the cheese, as it is very difficult to obtain the sample in suitable condition for plating.[115]

The princ.i.p.al action of the lactic acid-forming bacteria in the cheese ripening is the changing of the milk-sugar or lactose into lactic acid and the formation of small amounts of other substances, such as acetic, succinic and formic acids, alcohol, aldehydes and esters and some gases, carbon dioxide and hydrogen. While the amount of these substances other than lactic acid is small, it is thought that the effect of these on the cheese may be important. Heinemann shows[116] that lactic acid exists in two optical modifications, the levorotatory and dextrorotary acids. In cheese they are usually found in the inactive or racemic form, the levorotatory and dextrorotary acids being present in equal amounts. What importance the question of optical activity of the lactic acid may a.s.sume is not definitely known. Just as some groups of bacteria have a specific effect on the lactose, producing only one modification of lactic acid, so bacteria attacking lactic acid may exercise a selective action and use only one or the other optically active modification. In other words, the early flora of cheese-ripening bacteria may determine the later flora by the production of a form of lactic acid attacked by one group of bacteria and not by another, and the effect on the flavor will differ accordingly. The amount of lactic acid in the cheese increases for a time, then decreases.

The errors in determining lactic acid are considerable. It seems that the tendency is toward an increase of lactic acid in the cheese long after the lactose has disappeared. Two explanations are offered: one, that in the lactic acid fermentation an intermediate compound or compounds are formed which exist for some time, the conversion into lactic acid being complete at about three months; the other is that lactic acid is formed as a product of paracasein proteolysis.

The lactic acid formed in cheese ripening does not exist in a free state but reacts with the calcium salts in the cheese and forms calcium lactates. It is thought that there is sufficient of these salts to neutralize all the acid formed, and therefore the acid does not enter into combination with the paracasein salts. It has been found that lactates are the princ.i.p.al source of acetic and propionic acids. These are supposed to have some effect on the flavor of the cheese.

The effect of lactic acid as a determinant of bacterial and enzymic changes is very important. Early in the ripening process, lactic acid suppresses the growth of undesirable micro-organisms. It also furnishes the acid medium necessary for the best action of both the coagulating and peptic enzymes.

The importance of the lactic acid bacteria in cheese ripening has been summed up by Hastings[117] as follows: "The functions of this group of bacteria in Cheddar cheese are through their by-product lactic acid as follows: (_a_) To favor the curdling of milk by rennet. (_b_) The bacteria of the milk are held in great part in the curd. Through the acid they influence the shrinkage of the curd and expulsion of the whey, (_c_) The acid so changes the nature of the curd as to cause 'matting,'

or 'cheddaring' of the curd, (_d_) The acid activates the pepsin of the rennet extract, (_e_) The acid prevents the growth of putrefactive bacteria in the cheese. (_f_) It has been shown that _Bacterium lactis acidi_ is able to form acid in the absence of the living cell. (_g_) The development of _Bacterium lactis acidi_ is followed by the growth of another group of acid-forming bacteria, the _Bacillus Bulgaricus_ group.

They reach numbers comparable with those of the first group, reaching their maximum number within the first month of ripening. Since they develop after the fermentation of the milk-sugar, they must have some other source of carbon and of energy than milk-sugar." It is also probable that other groups constantly present contribute to the changes.

From the preceding discussion it is evident that each of the ripening agents has its important part to play in the ripening process and a normal ripening of the cheese is a composite result of these various agencies.

+239. Conditions affecting the rate of cheese ripening.+--The rate at which these agents cause ripening of the cheese depends on several factors.[118] Most of these factors are within the control of man. They are as follows: the length of time; temperature of the curing-room; moisture-content of the cheese; size of the cheese; the quant.i.ty of salt used; the amount of rennet; the influence of acid.

+240. The length of time.+--The water-soluble nitrogen compounds increase as the cheese ages, other conditions being uniform. The rate of increase is not uniform; it is much more rapid in the early than in the succeeding stages of ripening.

+241. The temperature of the curing-room.+--Very few cheese factories have made any provision for regulating the temperature of the curing-room. Without such provision the temperature follows closely that of the outside air. In some cases the curing-room is located over the boiler-room and hence becomes very hot. In the cheese warehouses, provision has been made to control the temperature very closely.

Experiments show that the soluble nitrogen compounds increase, on the average, closely in proportion to an increase of temperature, when the other conditions are uniform.

The temperature of the curing-room has a material effect on the quality of the cheese. Cheese made from the same day's milk, and part cured at 40 F., part at 50 F., part at 60 F. show considerable differences, the greatest seeming to be in the flavor and texture. Those kept at the low temperature cure more slowly and develop a milder flavor, those at the higher temperature cure faster and develop undesirable flavors. At the higher temperature the undesirable organisms seem to be more active.

Some very skillful makers and judges of cheese have always contended that if Cheddar is properly made, firmed to the body and texture of a high-cla.s.s cheese, ripening at 55 to 60 F. gives a higher quality. Such a cheese must be low in moisture, perhaps 3 to 5 per cent lower than one cured successfully by the cold process.

The following tables[119] XVII, XVIII show the effect of different temperatures of curing cheese on the total score and on the points of the flavor, body and texture:

TABLE XVII

TABLE SHOWING THE RELATION OF TEMPERATURE OF CURING TO TOTAL SCORE