Freshness and Shelf Life of Foods - American Chemical Society

Chapter 3

Flavor Issues in Maintaining Freshness G . A . Reineccius

Downloaded by UNIV LAVAL on May 17, 2016 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0836.ch003

Department of Food Science and Nutrition, 1334 Eckles Avenue, University of Minnesota, St. Paul, M N 55108

This paper discusses causes of flavor deterioration, i.e. a loss of freshness, in foods during storage. Freshness may be lost for several reasons including: a loss of specific aroma compounds that are 'fresh' in character; the loss of characterizing aroma compounds (e.g. bready, beany, etc); or the masking of a fresh flavor due to the development of offflavors. The hypothesis that "fresh" aroma compounds are lost during storage may be true but is difficult to prove since there are few, if any, aroma compounds identified to date that are characterized as being "fresh". The validity of each of these hypotheses is discussed and evidence from the literature is presented in support of the discussion.

One of the first questions that needs to be addressed is a definition of the term "freshness". According to Webster's Collegiate Dictionary, a "fresh" food is a food that is "free from taint" or "having its original qualities unimpaired". We may want to be slightly more liberal in the "free from taint" definition and add free from "off-flavors" as well since we generally consider that a "taint" is an external contamination while an "off-flavor" occurs due to the deterioration of the food itself (e.g. lipid oxidation, enzymatic action, or non-enzymatic browning). The concept that a food is free from "taint or off-flavors" vs. having its original qualities unimpaired can be viewed as two different considerations. The first is that the food has not developed off notes that mask or overwhelm

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© 2003 American Chemical Society

Cadwallader and Weenen; Freshness and Shelf Life of Foods ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

43 the desirable flavor of the food. We assume that the desirable flavor of a food is present and unimpaired but not sensed due to the occurrence of off notes. The second consideration is that freshness is lost due to the loss of the desirable qualities (desirable aroma components). This is a very important distinction and will form the basis of much of the discussion that follows.


Definition of Freshness I would like to start with a discussion of what is responsible for freshness in a food. Are there aroma components in a food that define "fresh" or does freshness result from specific characterizing aroma components? A look at the literature shows that there are very few aroma compounds characterized as being fresh. That is understandable since it is hard to imagine what fresh is out of context. It is easy to visualize a "bread-like" aroma or a "fishy" aroma but what is a fresh aroma? If one goes to the Flavornet web site established by Terry Acree (1), only ethyl-3-hydroxyhexanoate, α-phellandrene, benzyl acetate, pentadeeanal, and isopropyl hexanoate have been defined as fresh (or having fresh as one of the descriptors). This site lists the sensory properties of 313 aroma compounds. If one checks one of the well established references in the flavor creation field, ethyl-3-hydroxyhexanoate is described as being fruity, α-phellandrene as being citrus, fresh, pleasant, and minty, benzyl acetate as being jasmine-like, pentadecanal is not listed and isopropyl hexanoate as having a sweet, delicate, fruity odor of pineapple with a fresh, berry-like taste (2). When fresh is mentioned, it is not the single or a primary attribute but is a part of the overall aroma character. A more comprehensive listing of odor descriptors (approximately 1500 aroma compounds are described) is provided on a web site organized by Mottram (3). In this extensive listing, only 3-hydroxy-2-butanone (acetoin) has fresh as a component of its description. Acetoin is described as having a butter, fresh, fruity, green, moldy, slightly rancid sensory character. Fresh is again a minor descriptor as opposed to being a primary note. Burdock (2) describes acetoin as having a "pleasant butter-like odor and flavor". There is no mention of fresh in this description. A final source of information is a professional flavorist i.e. a person who deals daily with creating flavors and therefore, must be intimately familiar with the creation of specific flavor notes. Frank Fischetti (4) defined freshness as "a nondescript lightness added to a flavor. I say nondescript (never-the-less contributing) as opposed to a definite character because these notes add the front, initial part of a flavor, the first thing perceived, which is then followed by the character (those compounds that make the flavor taste like bread or meat, for



44 example) items. These notes must be pleasant, immediately perceived and then disappear. I think freshness is often confused with impact, which I define as the initial impression of a flavor, apperceived indication of the strength and character of a flavor." It is relevant and appropriate (in my opinion) that Fischetti uses the term "nondescript" in his definition. When asked for examples of fresh aroma compounds, he said that the first thing he thinks of is esters, at least for all fruit flavors. He emphasized that especially the lower boiling esters contribute freshness e.g., ethylacetate, ethylbutyrate etc. Fischetti also mentioned acetaldehyde as a compound that imparts freshness to many flavors especially citrus flavors. A final example was cis-3 hexenol which he feels adds a fresh note to many fruit flavors, especially berry flavors. It is worthwhile to note that a flavorist considers "fresh" components to be "nondescript" and offers very volatile non-characterizing components as examples. Fischetti also notes that fresh notes are product dependent. One may consider that the contributors to freshness are different from food to food.

Research on "Fresh" Aroma Compounds The limited number of compounds in the literature characterized as being "fresh" is understandable considering how these compounds are being identified today. Most of our work in identifying and characterizing the sensory properties of aroma compounds is done by aroma isolation, gas chromatography and then gas chromatography/olfactometry (GC/O, sniffing the G C effluent). If there are aroma compounds that impart freshness through sensory character, one would expect these components to be quite volatile. This would make isolation by many traditional methodologies problematic. Aroma compounds might be lost during distillation, or concentration, or not adequately retained on adsorbent traps and thus be present in the aroma isolate at extremely low concentrations. Furthermore, the selection of potent aroma compounds is done by a G C sniffing methodology (e.g. a dilution, detection or intensity method). The ability of a panelist to detect an aroma compound is enhanced by his/her recognition of it. Thus, compounds that are not recognizable (e.g. fresh) would tend to be missed or underestimated in a sniffing experiment. It is possible that there are a group of chemicals, albeit unique for different classes of food products, that impart freshness and have not been characterized to date. To find these compounds, an alternative methodology may have to be used.


45 A n approach for finding fresh aroma compounds could involve combining instrumental and sensory methodologies. One could produce an aroma isolate that is judged to be "fresh" bread aroma, for example. The aroma isolate could be subjected to fractionation by G C and then by recombining fractions, one could determine i f there is a fraction that imparts freshness to the aroma profile. If there is such a fraction, then through refined fractionation, one could focus on the aroma compounds that are fresh.


Off-Flavors vs. Loss of Desirable Characterizing Flavor Two additional alternatives remain for explaining a loss of freshness: one is the loss of desirable characterizing aroma and the other the formation of offflavors that will mask the desirable flavor. In one case, we are considering that a loss of freshness is due to the loss of characterizing aroma components (e.g. 2acetyl-l-pyrroline for bread). This loss can occur due to chemical reactions (e.g. between aroma compounds or between aroma compounds and the food matrix), chemical binding to major food constituents (e.g. linear starch fractions binding volatiles in bread) or through volatilization (including scalping by the package). In this scenario, we are assuming that there are no aroma components that are fresh but that freshness comes from characterizing aroma compounds. The remaining alternative is that a loss of freshness is due to the formation of off-flavors which will mask the desirable flavors. The intensity of the offflavor may be too low to be recognizable but high enough to mask desirable notes. Lipid oxidation and the Maillard reaction come to mind as mechanisms for the production of off-flavors in most foods. There are few studies in the literature to resolve this issue. However, bread aroma has been the topic of considerable research and some of the data are helpful in considering this question.

Staling of Bread While there is considerable data published on flavor changes in bread during storage, two articles from Prof. Grosch's laboratory are quite relevant (5,6). Schieberle and Grosch (5) determined the characterizing (key) aroma compounds in freshly baked white bread and then monitored the FD-factors (potential contribution to odor) during storage. A tabulation of the changes in FD-factors of the key aroma contributors is presented in Table 1. These data show that the characterizing aroma


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Table I. Changes in the FD-factors of Crust Odorants During Storage of White Bread FDFDFactof at Factor at 0-hr 96-hr 16 128 64 16 8 64 4 32 16 32 32 16 32 32 2 16 16 16

Freshness and Shelf Life of Foods - American Chemical Society

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