Printer Friendly

Physio-chemical and structural characteristics of "espresso" coffee brew.

Physio-chemical and structural characteristics of "espresso" coffee brew

In this work we study some physical, chemical and structural properties of the beverage known as "espresso coffee" in order to evaluate the concrete elements that could explain the success met by the "Italian style" brewing method even on markets of different traditions.

It can be seen that the "tazzina" (Italian for small cup) is becoming a wider and wider accepted method to enjoy the virtues of tastefulness and stimulation of coffee in a dynamic society who is replacing new lively and quicker "rites," compatible with modern life rhythm, to the traditional forms of socialization before a nervine drink that is typical of more relaxed civilizations.

It has been noticed for some time that in nearly all cultures the forms of meeting together at ease, discoursing and digressing with socialization (briefly: the equivalent of well known "coffee break") are psychological rituals that happen preferably before a cup of hot and fragrant beverage, normally a caffeine-containing one: from an economist's point of view the most important one is certainly coffee, followed by tea, cocoa, mate and other less widespread ones (guarana, cola, cassina).

These rituals usually need time and comfort that are not always available or enjoyable. More and more infrequently can an occidental worker devote during his busy working day a half-hour to sit down and sip a fine coffee cup, better if accompanied by a cake!

The need of a rite, or "break" is still existing: let's squeeze it in a shorter time, business-compatible, let's rather repeat it several times during the day, but let it leave a pleasant and protracting memory.

Italian-style "ristretto" espresso cup do fulfill these needs by means of its characteristics, because:

* it consists of a small volume of beverage, typically not more than 40 ml, which is consumed generally in not more than three draughts

* it is a fast energy carrier, thanks mainly to added sugar, a consumer's wish

* it is an optimal nerve stimulation carrier, thanks to its limited caffeine content

* it keeps all the digestive qualities commonly appreciated in coffee infusions

* it has a sheerly high sensorial impact, which acts in a concentrated way on:

- smell-taste - flavor - mouthfeel

* it leaves a long memory of the taste (after-taste), which may last a quarter of an hour.

In the face of these characters, well known by Italian and more and more by European and North-American consumers, we noticed a lack of objective scientifically systematized data trying to explain the reasons.

We engaged therefore in the study of physical, chemical and structural properties of the beverage, focalizing at first our efforts in trying to explicate the attribute which most often is related to the espresso image, that is mouthfed, or body.

The first obstacle encountered by anyone undertaking the study of the drink "espresso coffee" lies at the very beginning: in order to speak about characterization, indeed, he must rely upon some kind of definition of the material to be characterized.

Traditional Preparation Method

This is not so easy as one could think, for we are dealing with a traditional preparation method which has been evolving during decades, thanks to the application of consecutive approximations suggested by the power of observation of people employed in brewing cups. Those personages are craftsmen who in Italy have their own professional rank.

Non-commercial literature is meagre even about simple descriptions of espresso brewing procedure: after the historic chronology of Ukers where pioneers Bezzara and Pavoni are cited.

Following the most widespread meaning we may speak correctly of espresso only when we examine a freshly brewed cup (compare Italian "espressamente" = on purpose), in single or double dose, by means of a percolation device applying a considerable hydrostatic pressure (compare Latin "ex pressus" = pressed out).

Checking craftsmen's practice, that has settled as a "golden rule" at least in Italy, we can extract the following data:

min max weight of coffee
(roasted & ground) [g/cup] 5 8
beverage volume [ml/cup] 25 60
extraction time [s] 15 35
extraction pressure [bar] 7 12
extraction temperature [degree C] 86 92

Some beverage samples have been prepared (using Illycaffe' pure Arabica blend, Italian roasting) by means of espresso machines of both professional and home type, varying parameters values: a panel of trained assessors was evaluating its mouthfeel.

A couple of samples, strongly different according to assessor's judgement, has been obtained varying the geometry of the cake made up of ground and compacted coffee until the percolation time was halved. These two cups, that we shall name Hi-body and Lo-body, have been submitted to physical and rheological tests, to chemical proximate analysis by families and to structural inspection under optical and electronic, both scanning and transmission, microscopes.

The observation that mouthfeel increases as total extraction volume decreases generated an interest to analyse the consecutive time fractions of a Hi-body brew as well. At this aim a rack of test tubes has been used, which was shifted at regular time intervals under the percolating beverage gush.

Obviously the multiple iteration of the procedure has been needed, in order to collect a sufficient mean sample quantity for the tests.

The following physical properties have been examined:

* refractive index (and its evolution with temperature)

* density

* viscosity (and its evolution with temperature)

* presence and granulometry of dispersed phases

* pH (and its evolution with temperature)

* dry residue

As reported in table 1 below, the differences of density, refractive index and pH are not specially meaningful.

More interesting show up those of viscosity (and its dependance from temperatures), and further those of presence of particulate matter: both seem to us logically related to the dry matter difference, which may well denote the presence, besides solutes, of a dispersed phase, modifier of rheologic properties. No non-newtonian behavior has on the other hand been observed.

Such results advised a structural and histochemical microscopical study, hoping to clear up further the essence of this dispersed phase. [Tabular Data Omitted]

Structural Study

Some drops of freshly brewed beverage have been examined under an optical microscope at various magnifications.

Both Hi- and Lo-body specimens show the presence of a high number of small particles of varied shapes suspended in water (fig. 1).

Some of them may be identified as cellular wall fragments of roasted coffee; others, of spherical shape and of varied size, are present in much higher number.

In order to determine the nature of the latter, two histochemical staining techniques have been used: addition of Sudan IV dye and exposure to osmium tetroxide vapors.

Both techniques are claimed effective for lipids demonstration.

Almost the totality of the spherical particles has been recognized as lipids droplets, forming emulsion, whereas few have been interpreted as gaseous bubbles surviving the manipulation time which is inevitably not short. The presence of such structures has been confirmed by scanning electron microscopy (SEM), where both particle types have been put in evidence after removal of the aqueous matrix by ultrafiltration on porous membrane (fig. 2). The fine structure of the particles have been investigated by transmission electron microscopy (TEM), where surface details appear, maybe related to lipid droplet (fig. 3) or gaseous bubble (fig. 4) nature.

A higher optical magnification (x 1000) shows up the enormous number of very small lipid droplets: down to 1 [micro]m, and an evident difference between Hi-and Lo-body specimens may be appraised, embracing both number and average size (fig. 5).

The application of computerized image analysis allowed to quantify those quantities (fig. 6), leading to observe how Hi-body specimens have: higher number of droplets per unit surface, smaller mean diameter (about 3 [micro]m, and narrower dimension distribution.

This result, that seems to us the most important so far, suggests that the rheological properties of the beverage are due mainly to emulsion effects, and are related to both total quantity (see next point) and dimensional characteristic of this dispersed phase.

Chemical Analysis

The following chemical properties have been investigated: UV spectrum, lipids content, Kjeldhal nitrogen content, and caffeine content.

Lipids content (see table 2) shows a meaningful difference between Hi-and Lo-body, and that strengthens what recently hypothesized after the microscopical study on emulsion effects.

Neglecting some other logical results (higher UV absorption, higher nitrogen containing substances content, among which caffeine) little further discrimination has been obtained from the rest of this oversimplified analytical session: it proved nevertheless useful in order to program a fuller survey for the time fraction analysis. [Tabular Data Omitted]

As what's concerning the fractioned extraction, the above mentioned tests have been made on every fraction, with the addition of the dry matter determined on the limpid liquid after supercentrifugation, and obviously of the collected volumes.

The variability of these volumes (fig. 7) comes to remind us that the hydraulic process of dynamic extraction under pressure is strongly influenced by coffee cake morphology, and hence by its grinding degree homogeneity and by compaction eveness.

Density and refractive index trends (see table 3) are quite logical, thinking of the higher dilution of the subsequent fractions, while some perplexity is arising from the increasing viscosity (at room temperature) of the very last fractions: though not dramatic, we might think about a contribution from high molecular mass substances, difficult to be extracted.

Let's note also that the first fraction, after dilution to same dry matter content as the last, shows same viscosity.

While the contribution to refractive index of the insoluble (floating and settled) parts seems negligible, the interesting decreasing trend of truly soluble matter percentage in dry residue as the extraction proceeds might make us think at first about an increase in the emulsified lipids fractions. This hypothesis is yet confuted by the extraction lipids analysis where on the contrary a clear decrease after the very first fractions is evident.

As a possibility remains thence that the increasing insolubles are essentially suspended solids, which are dragged into cup in a fairly constant way.

The granulometrical analysis of the dispersed phase shows a sharp decrease of the median particles (droplets) diameter after the second fraction: this seems to be related to the sharp decrease in lipids content. [Tabular Data Omitted]
COPYRIGHT 1989 Lockwood Trade Journal Co., Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:part 1
Author:Petracco, M.
Publication:Tea & Coffee Trade Journal
Date:Nov 1, 1989
Previous Article:Technical coffee conference in Paipa, Colombia.
Next Article:On the market.

Related Articles
Espresso and institutional sales.
Physio-chemical and structural characteristics of "espresso" coffee brew.
Espresso as drama: the Americanization of cappuccino.
Espresso in restaurants: old concerns, new products.
(Re)interpreting the West Coast landscape through espresso-colored glasses.
Starbucks: at the helm of Seattle's espresso adulation.
In search of the perfect brew.
So where's the boom in pods? There may be an ESE answer.
Is espresso good for coffee?
The craft of espresso roasting: part 1.

Terms of use | Copyright © 2016 Farlex, Inc. | Feedback | For webmasters