Allergic Reaction and Milk Intolerance
V. F EDELE
CRA – Unità di Ricerca per la Zootecnia Estensiva, 85054 Bella Scalo, Potenza, Italy
Introduction
In human nutrition, milk has always been considered important for its balanced supply of proteins, fat, minerals and vitamins. Once milk turns into cheese, it is appreciated for many other features, such as taste and odour.
Numerous studies on the taste and smell of milk and cheese have been car- ried out in the past throughout the world, especially in Europe and New Zea- land. Such research has clearly shown that what our senses perceive is based on a multitude of molecules, each one being able to characterize a product.
Substances such as alcohols, esters, ketones and terpenes (and many others) are the basis of many of the odours that we perceive in a dairy product.
Research on nutritional quality has deeply changed in recent decades. The components taken into account are no longer only fat and protein, but also molecules previously almost unknown, such as the conjugated linoleic acids (CLA),n–3 fatty acids (FAs),b-carotenes, tocopherols and phenols, which are compounds with defence functions in the human organism. These substances are present in the herbage grazed by animals and can subsequently be found in dairy products.
From popular beliefs to science
In Italy, in the past it was common for cultured men to praise the qualities of a milk or cheese in this way:
They are famous for being delicious, for being only second to the ones of Pollino [mountain]. Their good quality is due to sweet herbs forming the animal pasture . . . the milk of the province . . . with no difficulty it is a high quality milk. Rich and fertile pastures, made of sweet plants, make milk sweet-smelling and savoury.
(Demarco, 1988)
CAB International 2008.Dairy Goats Feeding and Nutrition
(eds A. Cannas and G. Pulina) 95
The fact that the taste and flavour of milk or cheese are a direct consequence of what the animal eats has always been rooted in the mind of people who, being careful observers, used to take note of the repeating phenomena and their causes. Thanks to this spirit of observation, they were able to notice that the tastes and flavours of a cheese changed with seasons. Based on that, the best time to produce cheese could be chosen:
cheese should not be made in every season, but only in March, April, August and September. The worst cheese is made in May, that is why the good cheese is called
‘formaggio’, i.e. ‘fuori di maggio’, which meant it was made ‘out of May’. . . In May herbs are unripe, their juice is nasty and not yet elaborated and savory.
(Demarco, 1988) Therefore, the kind of pasture and the season were the elements that made the difference in milk and thus cheese quality. As indoor intensive systems spread, the traditional pastoral civilization and, consequently, the perception and the importance of ‘quality’ faded. Old evaluation parameters have been replaced gradually by high production, hygiene, dairy yield, technology and fat content parameters, without taking into consideration what a careful and sensitive writer such as Italo Calvino had already understood: ‘Behind each cheese, there is a different pasture of a different green under a different sky’ (Italo Calvino – from the poetry Palomar).
Only in the last decades have these issues been studied scientifically, dem- onstrating that each kind of pasture, or plant grouping, can represent a factor of qualitative differentiation of a product.
Gas chromatographic techniques have allowed the characterization of the compounds responsible for smell and taste. They are specific molecules or mole- cule groupings that are often defined as secondary metabolites. Only recently has the origin or the formation pathway of some of these secondary metabolites been understood. For many others no information is available today.
There have been many hypotheses regarding the origin of the compounds which produce taste and smell. For years, smells and tastes were considered the main expression, if not the only one, of microbiological activity (Urbach, 1977;
Keen and Wilson, 1993). Later, it was supposed that, at least to a certain extent, enzymatic activity and some oxidation processes of substances were involved (Forss, 1979; Molinard and Spinnler, 1996). Only recently the importance of molecules transferred from the feed eaten by the animals to the milk was acknowledged (Mariacaet al., 1997; Viallonet al., 1999; Fedeleet al., 2000).
Sensorial and instrumental assessment of taste and smell
In the assessment of the aromatic qualities of a product, the borderline between certain and uncertain is extremely narrow, as there is no clear discriminator between cause and effect or, better, between what is perceived and the cause of that perception.
Most assessments are made either at the sensory level or at the instrument level, for various reasons.
The sensorial assessment is purely subjective and gives information about:
(i) the perception of the difference between two or more products (e.g. ISO- 5495, 2005); (ii) the classification and arrangement of products on the basis of a quality attribute (ISO-6658, 2005); and (iii) the differences between two or more products in terms of well-defined sensory parameters. Therefore, it is not based on objective measures and neglects the study of causes and mechanisms.
The instrumental assessment is based on measurements made by a device whose reliability is mostly based on the validity of the analytical methods used.
Instruments provide data that do not always correspond to the actual differences in taste perceived by humans. This happens because a single product can con- tain many different molecules in variable concentrations and each compound can affect sensory perception at different concentration thresholds. Some mole- cules are able to transmit their characteristics at infinitesimal concentrations, while others, on the contrary, are not able to influence sensory perceptions even at very high concentrations. In addition, each aromatic note is the result of a combination of several flavours. For all of these reasons, the instrumental assess- ment does not represent a complete means for evaluating the sensorial quality of a product.
The combined assessment based on instrumental and sensorial techniques is undoubtedly best, because it enables one to: (i) study similarities and differ- ences between instrumental and sensorial results; and (ii) understand causes, effects and mechanisms involved in the aromatic qualities of a dairy product.
Despite the limited research in this field, some indications on the aromatic qual- ity of dairy products are given in this chapter.
Aromatic quality
Flavour is the sensory impression (smell and taste combined) that can be attrib- uted to all substances present in food and feed of plant (herbs, preserved forage, grains) or animal (milk, cheese, meat) origin and that can be pleasant or unpleasant to the consumer. Since the mid-1970s, much research has been car- ried out on flavours. Many studies showed that what was perceived by the senses, by smelling and/or tasting a milk or cheese, was the direct or indirect con- sequence of what the animal ate (Urbachet al., 1972; Keen and Wilson, 1993;
Bossetet al., 1997; Coulonet al., 2000). With the advance of studies, it became clear that at least part of the secondary metabolites contained in food are not subjected to any change. O’Sullivan (1967) suggested that when the feed reaches the animal’s mouth, the most volatile metabolites (many terpenes, alco- hols and ketones) volatilize and, through the breathed-in air, reach the lungs, where they are absorbed into the blood, and then transferred to the milk. The least volatile of these metabolites (short-chain terpenes) are freed only after a partial digestion of the feed and, through belched gases, reach the lungs following the same pathway just described.
All of these aspects have been extensively studied. Here only those closely linked to the feeding system, rather than those associated with each specific feed, are discussed.
Pasture and flavours
In everyday language, aromatic plants are associated with those used for cook- ing or for flavouring alcoholic drinks (e.g. thyme, mint, marjoram, oregano and rhubarb). However, all plants contain aromatic substances, even though only some of them supply flavours that can be easily perceived by our senses. In a pasture, some spontaneous plants can predominate over the others, depending on the environment and season, thus characterizing the flavour of dairy prod- ucts. Many natural chemical compounds present in feeds are able to increase these kinds of perceptions. Among them, terpenes, alcohols and ketones are the most studied in milk and cheese.