Nose Dive - Harold McGee

Introduction: A Sense for the Essential

Our sense of smell is the bridge between our experience of foods and our experience of the larger world.

• “Smell is such a powerful and revealing sense because it detects actual little pieces of things in the world.”
• Taste and smell = most direct, molecular senses
  • Around 50 taste receptors → five taste sensations
  • Around 400 odor receptors to detect volatile molecules (tending to evaporate) → smell is much more versatile, specific, sensitive than taste
• Things have volatile compounds, and we have smell/taste perceptions of them based on neural processing, including other sensory input
  • Gas chromatography-olfactometry → separate volatiles and send, one at a time, to be sniffed and identified by a person → break down the individual molecules making up scents
  • Still, very subjective and relative, bc our descriptors depend on experience

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Part 1: Simplest Smells

1. Among the Stars

• Balance of gravity and fusion → protons combine into helium = fusion, which releases energy & counteracts gravity until it takes over, fusing into larger atoms
  • 26 layers of diff atoms form in the star, until gravity takes over for the final time → collapse and subsequent explosion in a supernova → release of 90+ more atoms!
• Formation of molecules in certain cloudy interstellar regions → interstellar dust → “seeds” for eventual formation of larger molecules, including aromatic volatiles
• Smells from space dust…
  • Hydrogen sulfide, sulfur dioxide→ “eggy,” sulfuric smell, somewhat irritating
  • Ammonia → unscented cleaning products
  • Ozone → fresh and pungent, usually only smelled after things like a lightning strike
  • Small carbon chains like methane, ethyne (found also from ripening fruit)
• Some larger compounds— benzene/carbon chains, alcohols, ketones, aldehydes (ranging from solvent smells to almond extract to fruity)

2. Planet Earth, Early Life, Stinking Sulfur

• Sulfurous scents from hot springs, volcanic eruptions give us an idea of what early Earth may have smelled like
• Building organic molecules requires source of energy → first forms of life (and life now) harnesses natural flow of electrons from donors → acceptors
  • Byproducts often volatile, have smells
  • Sulfur in many forms (hydrogen sulfide, sulfates, sulfuric acid) was abundant & able to act as donor and acceptor → microbes probably existed in networks, regenerating compounds the others depended on
• Next leap = harnessing energy in sunlight & oxygen inbound in water with photosynthesis → Great Oxygenation/Oxidation of atmosphere
  • Also led to oxidation of many minerals

3. Life’s Starter Set

• ”Starter set” of volatiles = small/simple carbon chains: acids, alcohols, aldehydes, hydrocarbons
  • Families of molecules generally smell similarly, specific scent often from context in which we smell them
  • Shorter chains → more likely to be unpleasant bc metabolic byproducts, longer chains → more pleasant bc likely generated “intentionally” as signals by plants/animals
• Alcohols = vodka (LOL), fruity, fresh, waxy, floral, ethereal
• Short-chain fatty acids = vinegar, cheesy, fatty, sharp/biting
• Hydrocarbons → fuels; aldehydes → wide-ranging
• We developed sensitivity to these molecules → tell us something about our environment

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Part 2: Animals

4. Animal Bodies

• Animal smells distinguished bc rich in proteins (routinely include N and S) and purines (ATP)
  • Breakdown of these → smells of rotting and fresh vegetables, sulfides, cheese, tar
• Amines from urea, uric acid → ammonia, putrid, fishy
  • Certain set of amines emitted from dying/dead animals have special receptors— “death detectors”; we don’t actually have much experience with this smell
• Anaerobic metabolism from microbiome → main source of volatiles in excrement— unpleasantness is likely learned, as many animals eat shit or are comforted by it!
  • Smell can vary by diet and digestive system: horses digest fast and incompletely, so more sweet smelling manure vs cattle, which have many stomachs and produce more volatile byproducts
  • Ammonia and amine emissions from urine amplified as microbes feed on the urea and uric acid once excreted
  • Volatiles produced from CAFOs actually dangerous, not composted into manure
• Externally, animals have pretty mild pure scent from oils and such; strengthens when microbes start breaking things down

5. Animal Signals

• Urine volatiles = direct result of metabolism, signals general status update on the animal via scent-marking
  • Animals synthesize nonvolatile molecules that are broken down by enzymes in the kidney and even more so once urine has been peed— very complex! Pheromones!
• Secretions from other glands
• Goats and sheep → particularly and distinctively smelly
• Insects → very diverse set of volatiles serving as pheromones and other signals; sometimes overlapping with plants— evidence of their coevolution

6. The Human Animal

• Volatiles we release at any moment → lots of information about our body, can serve as diagnostics
• Diet heavy in carbohydrates/fiber vs protein/meat → different gut microbiome composition → different volatiles released → different smell of excrement, and different effects on colon health
• Volatiles exhaled in breath: mainly rubbery isoprene (also major volatile released by plants!) and solventy acetone
• Once food and drink are in your mouth, microbial enzymes help release even more volatiles → enhance flavor
• Smells from our skin
  • Secretions of sebums (waxy, water-resistant material) and sweat (eccrine from physical exertion, and apocrine, from fewer glands but with lots more molecules)
  • On their own, all pretty odorless, maybe lightly waxy or citrusy; may be influenced by diet (garlic, fenugreek)
  • Release smells when metabolized by microbes
• Feet often smell like rind-washed cheeses
• Specific set of smells composed for armpit, through apocrine glands— volatile part released by microbial enzymes once secreted

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Part 3: Land Plants— Independence, Immobility, Virtuosity

7. Sweet Smells of Success

• Volatiles from plant act both as attractants and warnings signals
  • As defense system, very effective— while most of plant biomass in ocean is consumed, only 1/5 of that on land is
• Back to the evolution of plants:
  • Lignin → building block of decorated benzene rings, allowed plants to have structure/height and get more light
  • Most plants have a lot of genes, produce many more molecules than we do
  • First evolved volatiles to mimic chemical language of insects to influence their behavior
• Fruity and floral = distinct categories of smells— the former to attract animals to spread seeds, the latter to attract pollinators
• Single molecules can remind us of multiple smells— usually sensed in combination with others, dependent on our experience
• Chemesthesis → when plant volatiles trigger sensations other than smell or taste, such as cooling, warming, irritation
  • May react w receptors for temperature, etc.
  • Menthol, thymol
• Why do human animals find its anti-animal weapons sweet and blissful?
  • Plant volatiles much larger, more complex than animal and microbial ones → interesting and different

8. Plant Volatile Families: Green, Fruity, Flowery, Spicy

• Plant volatiles usually formed in side branches of major metabolic pathways
• Skin highway
  • Green-leaf volatiles (GLVs) = 6-carbon chains released as warning signal by live leaves → other plants produce them too; green and fresh
  • Esters = combination of an acid and alcohol; most fruity smells = mixture of several smaller esters in different ratios
  • Lactones = special class of esters → more coconut, dairy scent
  • Jasminoids: signals, defenses, floral
• Light-collecting metabolic pathway → terpenes → kinked, 5 carbon chain; named for resin from trees initially found in and defining smell of many conifers/pines
  • Monoterpenoids (combo of 2 terpenes → 10 carbons → woody, herbal, floral, fruity; some are warming/produce pain (camphor), others cooling/minty (menthol)
    • Floral terpenoids = very common— citronellol, geraniol, nerol, linalool (last is significant, very widespread, role as pesticide?)
  • Sesquiterterpenoids → 15 carbons — larger, a little less volatile — woody smells
• Carbohydrate/cell-wall pathways: carbon-oxygen rings → caramel
• Amino acid pathway:
  • Benzenoids — generally meant as deterrents, but lots of pleasant scents!
    • Phenyl rings → common floral & honey scents
    • Cinnamic acid and derivatives → resin, almond, balsamic
    • Coumaric acid and derivatives → hay, anise, clove, vanilla, other spices
  • Nitrogen and sulfur additions → more volatiles to deter animals — variety of earthy, fruity smells; mustard/wasabi spice, garlic

9. Mosses, Trees, Grasses, Weeds

• Isoprene = most common volatile from most plants except conifers → function is actually to manage damage from solar radiation, harmful byproducts of photosynthesis, cools leaves when it evaporates
  • Conifers use terpenoids for same effect
  • Both produce “blue” or “smoky” haze; encourage cloud formation
• Resins (from conifer trees) = very effective defensive mechanism vs insects, fungal infection; main volatiles = turpenoids

10. Flowers

• Why are humans so enamored by flowers? No direct benefits from their smell, but pleasant feeling from being around them is likely healthful
• Two volatiles common to so many flowers → generic “flowery” smell: terpenoid alcohol linalool and benzanoid alcohol phenylalcohol (esp roses)
  • Along with others, these serve to both attract pollinators and repel other insects from eating the flowers or staying around too long
• Some volatiles from flowers alone = very unpleasant, but nice when in combination— perhaps act to capture attention?
• Sweetbrier and moss rose → leaves that smell sweet!

11. Edible Greens and Herbs

• Focus here = fresh greens (vs dried or cooked, discussed later)
• Again, volatiles meant as defense system vs insects; for us, add interest to our food/tastes
  • Wild varieties thus have have stronger aromas, bc actually need the defense!
• Mint family (lavender, basil, rosemary, thyme, etc. too) → native to Mediterranean, pack leaves with terpenoids as chemical defense vs herbivores — when they drop into the soil, also inhibit growth of competitors!
  • Store terpenoids in glands on surface of leaves (gives them a fuzzy look— in surface hairs?) → first line defense, released when just brushed
  • Bartenders can infuse flavor by slapping a leaf to release terpenoids w/o generic, leafy GLVs— but this leaves a lot on your hands, so better to rub underside of two leaves together and drop into glass
  • Basil → composition of volatiles changes with age; true Genovese pesto made from young plant
• Celery family → celery, dill, cilantro, parsley
• Daisy family → tarragon, other uncommon things
• All of above = only three families! And primarily Eurasian. Several other interesting herbs…
  • Tomato leaf is actually nontoxic and may be beneficial, can add at the end to tomato dishes

12. Edible Roots and Seeds: Staples and Spices

• Most spices actually = small, dried fruits, but some actual seeds (peppercorns); others = roots, bark, parts of flowers, etc.

13. Fruits

• Most evolved to be eaten by birds and mammals, and birds attracted by sight → smells evolved by selection from our ancestors
• Some fruit volatiles similar to those from vegetables and herbs → dual purpose to repel insects and pests
• Esters = important to almost all fruity aromas
  • Common alcohol-acid esters, as well as lactones and furanones (include rings)
• Most fruits also contain trace amts of sulfide volatiles → interest to aroma
• Wild yet large fruits (ex. apples) → “evidence of the power of animal need to direct plant creativity”
• Forests of Tian Shan, China → teeming with all varieties of fruits— many from rose family, in two groups: pome fruits (apple, pear) and stone fruits; similarity detectable in taste of seeds
• Aroma of wine actually from metabolites of yeast instead of grapes— happens to be fruity as well, evolved in conjunction with fruit flies, which became tolerant to alcohol and feed off fermented juice, protecting the yeast from competing fungi
• Most fruits → stronger aroma as they ripen, but in peppers (capsicums) most volatiles decrease
• Orange to describe the fruit, derived from Sanskrit narang, relating to fragrance, came long before orange the color (16th c)

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Part 4: Land, Waters, After-Life

14. The Land: Soil, Fungi, Stone

• Fungi = decomposers, responsible for soil
  • Unlike animals which take in food and break it down, fungi secrete digestive enzymes and absorb products
  • Characterized by hyphae, spores
  • Streptomyces = bacteria that are very fungus-like
  • In process of decomposing, volatiles released from organic material, which can be consumed as food by other fungi!
  • Characteristic “mushroomy” smell from eight-carbon molecules that act as defenses and signals (vs six-carbon for plants— parallel communication systems?)
  • Truffles = any of 200 species of genus Tuber; nodes of interdependent web of species; volatiles responsible for their iconic smell actually produced by symbiotic microbes!
  • Yeast (esp Saccharomyces) → most important fungi to us bc produces alcohol (also other fruity volatiles)
• Smell after rain → CO2 released when dry soil is wetted, which carries all the volatiles left in the ground
  • Smell from inorganic rocks left from volatiles left by other life forms that have been modified by sunlight, atmosphere, each other (petrichor, blood of the rocks— or, more accurately, gaiaichor, exhalations of the earth)
• Wetlands, marshes smell bad bc volatiles produces from anaerobic metabolism

15. The Waters: Plankton, Seaweeds, Shellfish, Fish

• Inorganic components of water (besides water) = halogens that form salts (chlorine, iodine, bromine)
• Life in the water → responsible for most of the volatiles/smell
  • Algae = photosynthesizers → tiny phytoplankton (cyanobacteria, diatoms, dinoflagellates, coccolithophores) and seaweed (red, green, brown)
    • Produce lots of dimethyl sulfide (DMS); byproduct of DMPS, which is important compatible solute that balances osmotic pressures in cells
  • Plankton = tiny animals that feed on algae, very efficient cf land creatures
  • Emission of DMS, halogen volatiles → hazy, can impact cloud cover, reflection/absorption of sunlight

16. After-life: Smoke, Asphalt, Industry

• Smells of synthetic organic materials, plastics not themselves unnatural (same as those of afterlife), but their prominence is
• Fire → near constant once atmosphere was oxygenated, = combustion (destroys volatiles— breakdown of fuel into CO2 and H2O) and pyrolysis (creative process— heating of organic molecules without oxygen, breaks down into smaller carbon chains including volatiles, some of which stick together)
  • Together, leads to smoke: eg. for wood, combustion at surface creates heat → warms material below enough to start pyrolysis, which leads to release of volatiles, some of which are combusted as they leave → positive feedback loop
  • Different phases of the fire → different smells
  • Burning of lignin → coincidental release of some very specific spice smells, like vanilla and clove, produced very intentionally by the plants!
• Tar, pitch, turpentine = derived from burned wood, widely used for many purposes across Europe (generic hydrocarbon)
• Fossil organically (petroleum, coal) = similar compounds, but pyrolyzed by the planet— turns original complex molecules into a complex mix of simple molecules
  • First used in Middle East, China
• Isolation of single volatiles from fossil organic → foundations of identifying all the smells of molecules discussed here
• Volatiles from synthetic materials like plastics not from the polymer itself but residues from manufacturing and solvents— omnipresent these days, responsible for “new car” smell, big contributor to air pollution
  • Many of these xenobiotics = harmful to human health— carcinogenic, ex.
  • Why do we find some of them pleasing to smell?

Part 5: Chosen Smells

17. Fragrances

• Incense: smoldering wood at specific temp → releases heavy, usually 15-carbon volatiles without destroying them
  • Includes tobacco, marijuana
• Perfumery: freeing and capturing volatiles from aromatics to be able to mix and use at will
  • Initially extracted volatiles by pressing juices/essential oil from flowers
  • Soon developed methods of distillation (capturing steam from steeped petals and water), infusions into alcohol (resulting from distillation)
  • New method of solvent extraction— soak an aromatic in ether, let evaporate → solid?, waxy part left with aromatics, which can be washed w alcohols to purify
    • Basis of extraction methods used today, which use lots of solvents to isolate individual volatiles
• Plant materials in fragrances… flowers (most popular = rose, jasmine, neroli/orange blossom), leaves, wood/bark, roots/rhizomes (underground stems— some very unique molecules!)
• Animal materials in fragrances: seem unpleasant alone, but add different dimension, accentuate floral notes
  • Also a class of animal components much larger and less volatile— musk— draw the smeller in, act as fixative to make scent last longer
  • Rarely used today though
  • Ambergris = from build-up of undigested squid remnants in sperm whales; accumulates other food, microbes, bile → eventually whale dies and huge mass ends up on the beach?
• Synthetic fragrances— sometimes later found to be naturally-occurring!
• Actual perfumes → first “invented” in 1800s France; comprise head note (lasts for minutes), heart note (an hour), base note (hours)
• Worth it to take the time to listen to smells!

18. Cooked Foods

• Process of cooking transforms aroma of many of the previously discussed ingredients… this chapter = pyro-alchemy
• Only 320 molecules → majority of volatiles in most foods; complexity from combinatorial code
• Breakdown of sugars, proteins, starches, lipids → four main sets of volatiles = four bouquets
  1. Fatty → from breakdown of lipids— fried, coconut, creamy
  2. Sulfurous → this and the rest from Maillard rxns at higher temps— sulfur associated with “boiled” smell
  3. Sweet → at temps hot enough to lightly brown foods— caramel, maple, generic sweet
  4. Nutty → temps hot enough to turn foods darker brown— toasted, nutty
• Different methods of cooking → different sets of aromas (dry vs wet, higher vs lower temps, in oil)
• Heme → make blood taste “metallic” metal atoms reacting with other molecules (also reason metals smell that way)
• Grass- vs grain-fed meat → different levels of saturation of fats → different volatiles
• Terpenoid pigments in tomatoes and carrots → fruity and floral fragrances when cooked
• Garlic, onions, other alliums → complex flavor profiles dependent on method of preparation
• Many greens/vegetables lose green leafy volatile flavor snd gain fatty/sulfurous bouquet when cooked— spinach, potatoes, etc.
• Fruits by default have nice aromas, modified when cooked by us to preserve → basically get sweeter… similar process to insects making honey from nectar, which does preserve some of original floral fragrance
• Coffee: robusta beans generate more smoky-spicy molecules, arabica → sweet & sulfurous; aroma strongest in freshly ground coffee and quickly diminishes once brewed (not very water soluble)
• Cocoa/chocolate → much more lasting, stable smell— roasted much more gently than coffee, but still develops complex & deep flavor bc fermented slightly before roasting which begins enzymatic breakdown

19. Cured and Fermented Food

• Fermentation = method of preparing food relying on microbes and their enzymes
• Well-managed curing, hanging, aging of meats and such → proteins and lipids began to break down into new flavors, some tenderness
• Tea → all made from same plant, but different treatments/processes (handling the plants more → different breakdown of enzymes → different flavors, or grown in shade → more chlorophyll machinery, fermented before drying, etc.)
• Important fermentation bacteria = lactic acid bacteria (LAB → soy sauce, pickled veg, sourdough), acetic acid bacteria (→ kombucha, vinegar)
• Many important yeasts, molds (→ blue cheese, Brie, soy pastes), koji (pure culture of Aspergillus)
• Cheeses: flavor deriving from starting milk, wild microbes, rennet, starter culture of LAB, added cultures for flavor enhancement
• Wine: fermented by Saccharomyces yeasts; aromas contain solventy-fruity and sour microbial bouquets
  • Fermentation enhances many flavors/volatiles contained in the grapes that you can’t smell in juice
  • Flavor can also derive from wood barrels/chips
• Beer → yeast-fermented barley
  • Grain must first be malted- moistened so that they begin to sprout and break down some of their own starches into sugar
  • Mix malted grain, raw grain, and water; heat to speed up enzymatic reactions; add hops (female flowers/seeds of vine related to cannabis → add bitter volatiles, other flavor notes); boil together; ferment for a few weeks
  • Can be bottom-fermented (ex. lagers), where yeast sinks to bottom, subdued blend of volatiles; or too-fermented (ales) → more diverse volatiles from more active microbes, very fruity and little sulfurousness
• Asian alcohols → rice wines
  • Sake— made from milled rice → blank slate of starches for yeast to work on, fermented by single strain of Aspergillus, koji, and specific strains of yeast
• Distilled spirits
  • Get best smell of aroma by diluting with equal volume water, which forces out the volatiles
  • Chinese baijus— from sorghum, involve solid-state processing