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Beynen AC, 2020. Pressed dog food

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Abstract

The top-selling dry dog foods are extruded kibbles, having an airy and crunchy texture and divergent shapes. Pressed food, or compact, rod-like pellets with smooth surface, has filled a niche in the market over the past half century. The product is now frequently touted as cold-pressed. That marketing phrase attempts to differentiate pressed from extruded food. It is claimed that cold pressing, by avoiding high temperature, preserves natural flavors and essential nutrients (Note 1). Maximum temperature during the processing stage is lower for pressed than extruded food, but both foods generally contain heat-treated animal meal as ingredient. Pressed unlike most extruded kibbles can also contain pre-cooked starch sources. The ingredient statements of some cold-pressed foods list pre-cooked grains, potatoes or peas (Note 2). It could be suggested that the use of pre-cooked starch sources weakens "cold pressed" as advertising slogan. With regard to grains as ingredients, there are pressed foods labeled either grain-free or wheat-free. Other products, according to their ingredient lists, do contain wheat or have rice as the sole type of grain. Quite some pressed foods claim high digestibility as based on their gentle way of production. Some pressed-food makers assert that their products reduce the risk of bloated and twisted stomach in dogs, but such link is unsubstantiated (Note 3). The high-digestibility claim for pressed food may be interpreted as higher net digestion compared to extruded food, due to the difference in production. That inference can only be evaluated by digestion tests with dogs fed the same ingredient mixture, but either pressed or extruded. Two published studies have done just that, but the outcomes for net protein digestibility were opposite. Thus, ingredient mixture and processing may affect one another. A high-digestibility claim for any dog food should be backed by a digestion test. Composition Out of 15 differently branded, complete pressed foods for adult dogs (Note 4), 11 products are presented as cold-pressed. Four and three foods are labeled grain-free and wheat-free. The ingredient lists of four foods declare wheat, whereas three foods contain rice as the sole source of grain. One product uses "low grain" as catch words. The energy composition (% of total dietary metabolizable energy) is as follows: protein, 29/25-39 (mean, n=15/range), fat, 26/20-37, carbohydrates, 45/33-52. The shape of the food pellets mostly is cylindrical (Note 5). Petfood can be evaluated by using its ingredient and analysis statements (1, 2). For pet owners with preferences or dislikes for food items, the ingredient list is helpful. The analysis panel provides limited information. On the basis of its label, complete petfood can be disqualified, but not approved unconditionally. The food labels of non-disqualified, complete pressed and extruded foods do not allow further (comparing and contrasting) nutritional validation. Only (comparative) feeding trials with dogs can do so.
Bonny Canteen 2020; 1: 16-22.
Anton C. Beynen
Pressed dog food
The top-selling dry dog foods are extruded kibbles, having an airy and crunchy texture and
divergent shapes. Pressed food, or compact, rod-like pellets with smooth surface, has filled a niche
in the market over the past half century. The product is now frequently touted as cold-pressed.
That marketing phrase attempts to differentiate pressed from extruded food. It is claimed that
cold pressing, by avoiding high temperature, preserves natural flavors and essential nutrients
(Note 1).
Maximum temperature during the processing stage is lower for pressed than extruded food, but
both foods generally contain heat-treated animal meal as ingredient. Pressed unlike most
extruded kibbles can also contain pre-cooked starch sources. The ingredient statements of some
cold-pressed foods list pre-cooked grains, potatoes or peas (Note 2). It could be suggested that the
use of pre-cooked starch sources weakens “cold pressed” as advertising slogan.
With regard to grains as ingredients, there are pressed foods labeled either grain-free or wheat-
free. Other products, according to their ingredient lists, do contain wheat or have rice as the sole
type of grain. Quite some pressed foods claim high digestibility as based on their gentle way of
production. Some pressed-food makers assert that their products reduce the risk of bloated and
twisted stomach in dogs, but such link is unsubstantiated (Note 3).
The high-digestibility claim for pressed food may be interpreted as higher net digestion compared
to extruded food, due to the difference in production. That inference can only be evaluated by
digestion tests with dogs fed the same ingredient mixture, but either pressed or extruded. Two
published studies have done just that, but the outcomes for net protein digestibility were opposite.
Thus, ingredient mixture and processing may affect one another. A high-digestibility claim for any
dog food should be backed by a digestion test.
Composition
Out of 15 differently branded, complete pressed foods for adult dogs (Note 4), 11 products are
presented as cold-pressed. Four and three foods are labeled grain-free and wheat-free. The
ingredient lists of four foods declare wheat, whereas three foods contain rice as the sole source of
grain. One product uses “low grain” as catch words. The energy composition (% of total dietary
metabolizable energy) is as follows: protein, 29/25-39 (mean, n=15/range), fat, 26/20-37,
carbohydrates, 45/33-52. The shape of the food pellets mostly is cylindrical (Note 5).
Petfood can be evaluated by using its ingredient and analysis statements (1, 2). For pet owners with
preferences or dislikes for food items, the ingredient list is helpful. The analysis panel provides
limited information. On the basis of its label, complete petfood can be disqualified, but not approved
unconditionally. The food labels of non-disqualified, complete pressed and extruded foods do not
allow further (comparing and contrasting) nutritional validation. Only (comparative) feeding trials
with dogs can do so.
Pressing versus extrusion
“Cold pressed” as an advertising phrase is linked to preservation of the key ingredients and aims at
disassociation from “extruded”. During the pressing/pelleting process, the temperature rises within
a short space of time to about 80 oC, whereas that is 130 oC for extrusion.
Pelleting involves the compacting and forcing of an ingredient mixture through die openings. The
strands leaving the die are cut at desired or maximum feasible length. During extrusion, the
moistened, heated ingredient mixture is first brought under high pressure and higher temperature in
the extruder. The mixture is then pushed through the die perforations. As a result of expansion, the
proportioned strands turn into airy, porous kibbles.
Protein digestion
An ingredient mixture containing 20% animal meal, 5% herring meal, 15% corn gluten meal, 25%
gelatinized corn starch and 20% dextrin was either extruded or pelleted (3). In adult dogs (n=6),
mean apparent, total-tract digestibility of crude protein was 78.4 and 72.4% of intake for the
pressed and extruded diet. The pressed versus extruded diet slightly increased dry-matter content
(26.3 versus 25.1%) and pH of feces (7.0 versus 6.6). The increase in pH suggests that the pressed
food reduced fermentation in the hindgut, leading to less bacterial protein excretion. The latter
effect in itself raises apparent protein digestibility.
Another comparative study (4) used an ingredient mixture with 15% poultry meal, 13% corn gluten
meal, 27% corn, 15% rice, 10% barley and 11% corn starch. The mixture with native grains was milled
to pass through a 0.4 mm sieve. In adult dogs (n=7), mean crude protein digestibilities were 76.1 and
79.6% for the pressed and extruded kibbles. The grains in the pressed food likely were less
digestible, thus increasing fecal bacterial protein and lowering apparent protein digestion. More
generally, the opposite outcomes of the two digestibility studies (3, 4) suggest that interactions
between ingredient composition and kibble-production method can affect apparent protein
digestion.
Pre-cooked starch
For the production of pressed dog food, pre-cooked starch sources are used normally. Uncooked
starch in the carrier ingredients is less accessible by pancreatic amylase and thus poorly digestible in
the small intestine. As a result, the energy provided by dietary starch is underutilized, while colonic
bacterial fermentation is stimulated by increased supply of substrate as undigested starch. Bacterial
overstimulation may lead to osmotic diarrhea as a result of excessive production of short-chain fatty
acids.
The pelleted and extruded kibbles based on the same recipe with three grain species contained 51%
starch (4). So-called damaged starch was specifically hydrolysed by a fungal α-amylase and then
quantified as glucose released by amyloglucosidase. The damaged-starch levels were 5 and 18% for
the pressed and extruded foods. Thus, extrusion versus pelleting of the milled, native grains likely
increased the amount of available starch. The pressed food did not negatively affect feces quality
(4), but it probably provided less metabolizable energy in the form of starch-derived glucose.
Gelatinization of starch by moist heat causes water adsorption and swelling, and enhances ileal
digestibility. The degree of starch gelatinization (SGD) in dog food can be quantified as
amyloglucosidase-hydrolysable starch expressed as percentage of acid-hydrolysable starch. For four
commercial, pressed and extruded adult dog foods, the mean SGD’s were 86 and 78% (5). The high
SGD of pressed foods points to the use of pre-cooked starch sources.
Corn as starch source
The feeding of a pressed food containing 60% uncooked, ground corn produced diarrhea in 5 out of
9 dogs within one day (6). After consumption of the food for 5 days, only one dog had diarrhea.
When 30% pre-cooked and 30% uncooked corn were incorporated into the pressed food (7), the
incidence of diarrhea was 3/10 after one day and 0/10 days after 5 days. Some kind of intestinal
adaptation appeared to bring about tolerance towards uncooked corn.
Dogs (n = 10) were fed diets containing 54.5% cooked or uncooked corn, 30% soybean meal, 10%
tallow, 0.5% vitamins and 5% mineral premix (8). The diets were moistened with water for increased
palatability. Cooked versus uncooked corn raised both fecal dry matter and apparent, fecal starch
digestibility, from 23.5 to 25.4% and from 94.3 to 95.7%. When the dogs were fed uncooked corn,
they did not develop diarrhea, but the slightly moister feces alerts to more colonic fermentation.
Ileal digestibility
Ileal (n =2) and cecal (n = 1) cannulated dogs were fed diets with 45% corn, 24% animal meal, 24%
soybean oil, 5% minerals, and 2% sawdust (9). The corn component was either only rolled or
moistened, infrared-treated and then rolled. The unheated corn did not induce diarrhea. Heated
versus unheated corn lowered the amount of fecal starch from 8.5% of dry feces to zero. Infrared
treatment also increased fecal dry matter (from 30.2 to 31.7%), fecal pH (from 6.4 to 6.9) and
apparent ileal starch digestibility (from 95.0 to 98.4%). It is clear that infrared-treated corn was more
digestible than unheated corn, thereby reducing fermentation.
Wheat as starch source
A mixture consisting of 68% highly extracted wheat meal, 8% animal meal, 8% casein, 6% soybean
meal, 6% cooking oil and 4% minerals was mixed with water, kneaded and dried, and then fed to
three dogs (10). All animals had diarrhea within a few days.
The three cannulated dogs (9) were also fed diets with 40% unheated or heated wheat. The diets did
not induce diarrhea. Heated versus unheated wheat lowered the amount of fecal starch from 4 to
1% of dry feces. Infrared treatment also reduced fecal dry matter (from 29.9 to 27.6%). Surprisingly,
it left both fecal pH (6.6) and apparent ileal starch digestibility unchanged (100%). Taken the data (9,
10) together, it is safe to assume that heat treatment of wheat improves starch utilization.
Rice and oats
The 10 dogs (8) were also fed rice or oats in uncooked or cooked form; the dietary levels were
54.5%. Cooked versus uncooked oats raised apparent fecal starch digestibility (from 93.8 to 95.8%),
but also slightly lowered group-mean fecal dry matter (from 18.8 to 17.6%). Cooked versus
uncooked rice increased fecal dry matter (from 19.3 to 23.1%), but left apparent fecal starch
digestibility essentially unchanged (98.0 versus 98.6%). When the dogs were fed either uncooked
rice or oats they did not develop diarrhea.
Pre-cooking assessed
There are limited data on the impact of cooked versus uncooked grains on starch digestibility in
dogs. Nevertheless, it can be concluded that pre-cooking of corn as ingredient of pressed dog food is
useful. Cooking gelatinizes the starch component, which increases its availability for ileal digestion
and reduces the risk of diarrhea. The higher availability increases the metabolic utilization of starch-
derived glucose and diminishes fermentation of undigested starch.
It is easy to see that feeding a pressed food containing 30 or 60% uncooked corn caused diarrhea in
dogs (6, 7), whereas pre-cooked corn does not. The measured effects for infrared treatment of
wheat (9) are conflicting. The observed diarrhea in dogs fed a dry food with 68% wheat meal (10)
probably was due to the combination of uncooked nature and high intake of the starch component.
Purified starches
Purified starch preparations are not normally applied as ingredients of pressed dog food, in any case
not at high inclusion levels. Nevertheless, the available research information is noteworthy. Studies
with cannulated dogs (11, 12) showed that different types of starch can have divergent apparent
ileal digestibilities. Preparations of cooked rice starch and uncooked corn starch were digested at
rates of 96 and 94%. The digestibility of uncooked tapioca starch was 47%, whereas uncooked
potato starch was not digested at all. In another study with cannulated dogs (13), ileal starch
digestibilities for uncooked wheat and tapioca were 98 and 58%. The values were 99 and 97% for
gelatinized wheat and tapioca starch.
Note 1.
Lysine is an essential nutrient. During petfood processing, the cross-link, lysinoalanine, and the
Maillard-reaction products, fructoselysine and carboxymethyllysine, may be formed. Lysine in the
three compounds cannot be utilized so that their formation renders the essential amino acid
unavailable. A comparison between 21 commercial extruded and 10 pressed dog foods shows that
the group-mean contents of the three lysine adducts were all higher for the pressed foods (14).
Note 2
Out of 15 brands of pressed dog foods, the ingredient list of one product declares gelatinized corn,
wheat and rice (Note 4, b). Another product states gelatinized corn and rice (Note 4, f). Three
products mention the use of thermally pre-treated (combinations of) starch sources: brown rice
(Note 4, h), sweet potato and peas (Note 4, i) or brown, whole-grain rice and peas (Note 4, j).
Note 3
One manufacturer (Note 4, c) says that pressed food causes less burdening of the stomach. Two
manufacturers (Note 4, h, m) claim that pressed food reduces the risk of canine bloat, also called
gastric dilatation, which can be followed by torsion and obstruction (volvulus). The higher density
and lower air content of pressed kibbles would cause less expansion in the stomach, thus reducing
the problems of bloating. There is no evidence that bloat is less common in dogs fed pressed in place
of extruded food. Furthermore, bloat is the result of bacterial gas accumulation in the stomach,
rather than expansion of kibbles (15).
Note 4
It seems that most brands of pressed dog foods are produced in Europe, mainly in the UK. Pressed
dog food probably is most prevalent on the European market. The 15 brands referred to are as
follows.
a. Impress Your Dog, Vobra Special Petfoods, impressyourdog.com
b. Farm Food HE Classic. https://www.farmfood.nl/product/farm-food-he-classic/
c. Prins Pro Care Super Active. https://www.prinspetfoods.nl/hond/prins-procare-protection/prins-
procare-protection-super-active
d. Biofood Adult. https://www.brekz.de/biofood-hundefutter/biofood-adult-gepresstes-
hundefutter.html
e. Rangers 5631. https://www.ecofut.de/hunde/rangers-gepresstes-hundefutter
f. Markus-Mühle Naturnah. https://markus-muehle.de/de/hundetrockenfutter-
kaltgepresst/naturnah/
g. Real Nature Adult. https://www.real-nature.com/en-ir/products/p/real-nature-real-nature-adult-
pasture-beef%2C-barbary-duck-and-north-sea-herring-dogs-dry-food
h. Gentle Cold Pressed Food. http://www.coldpresseddogfood.co.uk/
i. Forthglade Chicken Grain Free Cold Pressed Natural Dry Dog Food. https://forthglade.com/shop-
dog/dry-dog-food/chicken-grain-free-cold-pressed-natural-dry-dog-food
j. Guru Tasty Tucker Cold Pressed Dog Food. https://www.gurupetfood.com/product/5kg-tasty-
tucker/
k. Bellfor, Forstbach-Schmaus – Wild Salmon & Brook Trout – Grain-Free Cold-Pressed.
https://uk.bellfor.info/country-feast-cold-pressed/forstabach_schmaus_salmon_trout_10kg.html
l. Hayes Brothers, Farm House Dog Food. https://hayesbrothers.nl/product/farm-food/
m. Tribal, Fresh Turkey Adult. https://www.tribalpetfoods.co.uk/tribal-adult-all-breed-fresh-
turkey.html
n. Lukullus Dog Food Chicken & Norwegian Sea Salmon.
https://www.zooplus.co.uk/shop/dogs/dry_dog_food/lukullus/adult/156066
o. Natural Cornish Pet Shop, Aflora Gwithian Surf & Turf Cold Pressed dog food.
https://naturalcornishpet.co.uk/products/aflora-gwithian-surf-turf-cold-pressed-dog-food
Note 5
Pressed dog-food pellets frequently have a cylindrical shape (Note 4, e, f, h, i, k, n). The diameter
may be 6-10 mm and length 10-20 mm. Deviating shapes included double, side-by side pellets (Note
4, g) and triple, side-by-side pellets stacked in triangle form (Note 4, c). Occasionally, pressed foods
consisting of square pellets may be encountered.
Note 6
The pressed and extruded foods with identical ingredient composition were subjected to a two-bowl
preference test in 30 dogs (4). For the pressed versus extruded food, the intake ratio was 34:66,
indicating that the dogs preferred the extruded food.
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2. Beynen AC. Pet food label: analysis panel. Creature Companion 2014; May: 62-63.
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7. Vobra Special Petfoods, Internal report VOD96, 2019.
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https://www.researchgate.net/publication/332462109_Diet_and_canine_gastric_dilatation
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English summary. Thesis (doctoral)--Tierarztliche Hochschule Hannover, 1989. Includes bibliographical references (p. 94-100).
Zur Konzentration einiger wichtiger Stoffwechselabbauprodukte in den Faeces des Hundes nach Zufuhr hoher Mengen nativer Stärke mit dem Futter
  • W Drochner
Drochner W. Zur Konzentration einiger wichtiger Stoffwechselabbauprodukte in den Faeces des Hundes nach Zufuhr hoher Mengen nativer Stärke mit dem Futter. Kleintier-Praxis 1977; 22: 177-220.